[~andy/linux] / fs / btrfs / scrub.c

/*
 * Copyright (C) 2011, 2012 STRATO.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */

#include <linux/blkdev.h>
#include <linux/ratelimit.h>
#include "ctree.h"
#include "volumes.h"
#include "disk-io.h"
#include "ordered-data.h"
#include "transaction.h"
#include "backref.h"
#include "extent_io.h"
#include "check-integrity.h"
#include "rcu-string.h"

/*
 * This is only the first step towards a full-features scrub. It reads all
 * extent and super block and verifies the checksums. In case a bad checksum
 * is found or the extent cannot be read, good data will be written back if
 * any can be found.
 *
 * Future enhancements:
 *  - In case an unrepairable extent is encountered, track which files are
 *    affected and report them
 *  - track and record media errors, throw out bad devices
 *  - add a mode to also read unallocated space
 */

struct scrub_block;
struct scrub_ctx;

#define SCRUB_PAGES_PER_BIO     16      /* 64k per bio */
#define SCRUB_BIOS_PER_CTX      16      /* 1 MB per device in flight */

/*
 * the following value times PAGE_SIZE needs to be large enough to match the
 * largest node/leaf/sector size that shall be supported.
 * Values larger than BTRFS_STRIPE_LEN are not supported.
 */
#define SCRUB_MAX_PAGES_PER_BLOCK       16      /* 64k per node/leaf/sector */

struct scrub_page {
        struct scrub_block      *sblock;
        struct page             *page;
        struct btrfs_device     *dev;
        u64                     flags;  /* extent flags */
        u64                     generation;
        u64                     logical;
        u64                     physical;
        atomic_t                ref_count;
        struct {
                unsigned int    mirror_num:8;
                unsigned int    have_csum:1;
                unsigned int    io_error:1;
        };
        u8                      csum[BTRFS_CSUM_SIZE];
};

struct scrub_bio {
        int                     index;
        struct scrub_ctx        *sctx;
        struct btrfs_device     *dev;
        struct bio              *bio;
        int                     err;
        u64                     logical;
        u64                     physical;
        struct scrub_page       *pagev[SCRUB_PAGES_PER_BIO];
        int                     page_count;
        int                     next_free;
        struct btrfs_work       work;
};

struct scrub_block {
        struct scrub_page       *pagev[SCRUB_MAX_PAGES_PER_BLOCK];
        int                     page_count;
        atomic_t                outstanding_pages;
        atomic_t                ref_count; /* free mem on transition to zero */
        struct scrub_ctx        *sctx;
        struct {
                unsigned int    header_error:1;
                unsigned int    checksum_error:1;
                unsigned int    no_io_error_seen:1;
                unsigned int    generation_error:1; /* also sets header_error */
        };
};

struct scrub_ctx {
        struct scrub_bio        *bios[SCRUB_BIOS_PER_CTX];
        struct btrfs_root       *dev_root;
        int                     first_free;
        int                     curr;
        atomic_t                bios_in_flight;
        atomic_t                workers_pending;
        spinlock_t              list_lock;
        wait_queue_head_t       list_wait;
        u16                     csum_size;
        struct list_head        csum_list;
        atomic_t                cancel_req;
        int                     readonly;
        int                     pages_per_bio; /* <= SCRUB_PAGES_PER_BIO */
        u32                     sectorsize;
        u32                     nodesize;
        u32                     leafsize;
        /*
         * statistics
         */
        struct btrfs_scrub_progress stat;
        spinlock_t              stat_lock;
};

struct scrub_fixup_nodatasum {
        struct scrub_ctx        *sctx;
        struct btrfs_device     *dev;
        u64                     logical;
        struct btrfs_root       *root;
        struct btrfs_work       work;
        int                     mirror_num;
};

struct scrub_warning {
        struct btrfs_path       *path;
        u64                     extent_item_size;
        char                    *scratch_buf;
        char                    *msg_buf;
        const char              *errstr;
        sector_t                sector;
        u64                     logical;
        struct btrfs_device     *dev;
        int                     msg_bufsize;
        int                     scratch_bufsize;
};


static void scrub_pending_bio_inc(struct scrub_ctx *sctx);
static void scrub_pending_bio_dec(struct scrub_ctx *sctx);
static void scrub_pending_trans_workers_inc(struct scrub_ctx *sctx);
static void scrub_pending_trans_workers_dec(struct scrub_ctx *sctx);
static int scrub_handle_errored_block(struct scrub_block *sblock_to_check);
static int scrub_setup_recheck_block(struct scrub_ctx *sctx,
                                     struct btrfs_fs_info *fs_info,
                                     u64 length, u64 logical,
                                     struct scrub_block *sblock);
static void scrub_recheck_block(struct btrfs_fs_info *fs_info,
                                struct scrub_block *sblock, int is_metadata,
                                int have_csum, u8 *csum, u64 generation,
                                u16 csum_size);
static void scrub_recheck_block_checksum(struct btrfs_fs_info *fs_info,
                                         struct scrub_block *sblock,
                                         int is_metadata, int have_csum,
                                         const u8 *csum, u64 generation,
                                         u16 csum_size);
static void scrub_complete_bio_end_io(struct bio *bio, int err);
static int scrub_repair_block_from_good_copy(struct scrub_block *sblock_bad,
                                             struct scrub_block *sblock_good,
                                             int force_write);
static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
                                            struct scrub_block *sblock_good,
                                            int page_num, int force_write);
static int scrub_checksum_data(struct scrub_block *sblock);
static int scrub_checksum_tree_block(struct scrub_block *sblock);
static int scrub_checksum_super(struct scrub_block *sblock);
static void scrub_block_get(struct scrub_block *sblock);
static void scrub_block_put(struct scrub_block *sblock);
static void scrub_page_get(struct scrub_page *spage);
static void scrub_page_put(struct scrub_page *spage);
static int scrub_add_page_to_bio(struct scrub_ctx *sctx,
                                 struct scrub_page *spage);
static int scrub_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
                       u64 physical, struct btrfs_device *dev, u64 flags,
                       u64 gen, int mirror_num, u8 *csum, int force);
static void scrub_bio_end_io(struct bio *bio, int err);
static void scrub_bio_end_io_worker(struct btrfs_work *work);
static void scrub_block_complete(struct scrub_block *sblock);


static void scrub_pending_bio_inc(struct scrub_ctx *sctx)
{
        atomic_inc(&sctx->bios_in_flight);
}

static void scrub_pending_bio_dec(struct scrub_ctx *sctx)
{
        atomic_dec(&sctx->bios_in_flight);
        wake_up(&sctx->list_wait);
}

/*
 * used for workers that require transaction commits (i.e., for the
 * NOCOW case)
 */
static void scrub_pending_trans_workers_inc(struct scrub_ctx *sctx)
{
        struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;

        /*
         * increment scrubs_running to prevent cancel requests from
         * completing as long as a worker is running. we must also
         * increment scrubs_paused to prevent deadlocking on pause
         * requests used for transactions commits (as the worker uses a
         * transaction context). it is safe to regard the worker
         * as paused for all matters practical. effectively, we only
         * avoid cancellation requests from completing.
         */
        mutex_lock(&fs_info->scrub_lock);
        atomic_inc(&fs_info->scrubs_running);
        atomic_inc(&fs_info->scrubs_paused);
        mutex_unlock(&fs_info->scrub_lock);
        atomic_inc(&sctx->workers_pending);
}

/* used for workers that require transaction commits */
static void scrub_pending_trans_workers_dec(struct scrub_ctx *sctx)
{
        struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;

        /*
         * see scrub_pending_trans_workers_inc() why we're pretending
         * to be paused in the scrub counters
         */
        mutex_lock(&fs_info->scrub_lock);
        atomic_dec(&fs_info->scrubs_running);
        atomic_dec(&fs_info->scrubs_paused);
        mutex_unlock(&fs_info->scrub_lock);
        atomic_dec(&sctx->workers_pending);
        wake_up(&fs_info->scrub_pause_wait);
        wake_up(&sctx->list_wait);
}

static void scrub_free_csums(struct scrub_ctx *sctx)
{
        while (!list_empty(&sctx->csum_list)) {
                struct btrfs_ordered_sum *sum;
                sum = list_first_entry(&sctx->csum_list,
                                       struct btrfs_ordered_sum, list);
                list_del(&sum->list);
                kfree(sum);
        }
}

static noinline_for_stack void scrub_free_ctx(struct scrub_ctx *sctx)
{
        int i;

        if (!sctx)
                return;

        /* this can happen when scrub is cancelled */
        if (sctx->curr != -1) {
                struct scrub_bio *sbio = sctx->bios[sctx->curr];

                for (i = 0; i < sbio->page_count; i++) {
                        BUG_ON(!sbio->pagev[i]);
                        BUG_ON(!sbio->pagev[i]->page);
                        scrub_block_put(sbio->pagev[i]->sblock);
                }
                bio_put(sbio->bio);
        }

        for (i = 0; i < SCRUB_BIOS_PER_CTX; ++i) {
                struct scrub_bio *sbio = sctx->bios[i];

                if (!sbio)
                        break;
                kfree(sbio);
        }

        scrub_free_csums(sctx);
        kfree(sctx);
}

static noinline_for_stack
struct scrub_ctx *scrub_setup_ctx(struct btrfs_device *dev)
{
        struct scrub_ctx *sctx;
        int             i;
        struct btrfs_fs_info *fs_info = dev->dev_root->fs_info;
        int pages_per_bio;

        pages_per_bio = min_t(int, SCRUB_PAGES_PER_BIO,
                              bio_get_nr_vecs(dev->bdev));
        sctx = kzalloc(sizeof(*sctx), GFP_NOFS);
        if (!sctx)
                goto nomem;
        sctx->pages_per_bio = pages_per_bio;
        sctx->curr = -1;
        sctx->dev_root = dev->dev_root;
        for (i = 0; i < SCRUB_BIOS_PER_CTX; ++i) {
                struct scrub_bio *sbio;

                sbio = kzalloc(sizeof(*sbio), GFP_NOFS);
                if (!sbio)
                        goto nomem;
                sctx->bios[i] = sbio;

                sbio->index = i;
                sbio->sctx = sctx;
                sbio->page_count = 0;
                sbio->work.func = scrub_bio_end_io_worker;

                if (i != SCRUB_BIOS_PER_CTX - 1)
                        sctx->bios[i]->next_free = i + 1;
                else
                        sctx->bios[i]->next_free = -1;
        }
        sctx->first_free = 0;
        sctx->nodesize = dev->dev_root->nodesize;
        sctx->leafsize = dev->dev_root->leafsize;
        sctx->sectorsize = dev->dev_root->sectorsize;
        atomic_set(&sctx->bios_in_flight, 0);
        atomic_set(&sctx->workers_pending, 0);
        atomic_set(&sctx->cancel_req, 0);
        sctx->csum_size = btrfs_super_csum_size(fs_info->super_copy);
        INIT_LIST_HEAD(&sctx->csum_list);

        spin_lock_init(&sctx->list_lock);
        spin_lock_init(&sctx->stat_lock);
        init_waitqueue_head(&sctx->list_wait);
        return sctx;

nomem:
        scrub_free_ctx(sctx);
        return ERR_PTR(-ENOMEM);
}

static int scrub_print_warning_inode(u64 inum, u64 offset, u64 root, void *ctx)
{
        u64 isize;
        u32 nlink;
        int ret;
        int i;
        struct extent_buffer *eb;
        struct btrfs_inode_item *inode_item;
        struct scrub_warning *swarn = ctx;
        struct btrfs_fs_info *fs_info = swarn->dev->dev_root->fs_info;
        struct inode_fs_paths *ipath = NULL;
        struct btrfs_root *local_root;
        struct btrfs_key root_key;

        root_key.objectid = root;
        root_key.type = BTRFS_ROOT_ITEM_KEY;
        root_key.offset = (u64)-1;
        local_root = btrfs_read_fs_root_no_name(fs_info, &root_key);
        if (IS_ERR(local_root)) {
                ret = PTR_ERR(local_root);
                goto err;
        }

        ret = inode_item_info(inum, 0, local_root, swarn->path);
        if (ret) {
                btrfs_release_path(swarn->path);
                goto err;
        }

        eb = swarn->path->nodes[0];
        inode_item = btrfs_item_ptr(eb, swarn->path->slots[0],
                                        struct btrfs_inode_item);
        isize = btrfs_inode_size(eb, inode_item);
        nlink = btrfs_inode_nlink(eb, inode_item);
        btrfs_release_path(swarn->path);

        ipath = init_ipath(4096, local_root, swarn->path);
        if (IS_ERR(ipath)) {
                ret = PTR_ERR(ipath);
                ipath = NULL;
                goto err;
        }
        ret = paths_from_inode(inum, ipath);

        if (ret < 0)
                goto err;

        /*
         * we deliberately ignore the bit ipath might have been too small to
         * hold all of the paths here
         */
        for (i = 0; i < ipath->fspath->elem_cnt; ++i)
                printk_in_rcu(KERN_WARNING "btrfs: %s at logical %llu on dev "
                        "%s, sector %llu, root %llu, inode %llu, offset %llu, "
                        "length %llu, links %u (path: %s)\n", swarn->errstr,
                        swarn->logical, rcu_str_deref(swarn->dev->name),
                        (unsigned long long)swarn->sector, root, inum, offset,
                        min(isize - offset, (u64)PAGE_SIZE), nlink,
                        (char *)(unsigned long)ipath->fspath->val[i]);

        free_ipath(ipath);
        return 0;

err:
        printk_in_rcu(KERN_WARNING "btrfs: %s at logical %llu on dev "
                "%s, sector %llu, root %llu, inode %llu, offset %llu: path "
                "resolving failed with ret=%d\n", swarn->errstr,
                swarn->logical, rcu_str_deref(swarn->dev->name),
                (unsigned long long)swarn->sector, root, inum, offset, ret);

        free_ipath(ipath);
        return 0;
}

static void scrub_print_warning(const char *errstr, struct scrub_block *sblock)
{
        struct btrfs_device *dev;
        struct btrfs_fs_info *fs_info;
        struct btrfs_path *path;
        struct btrfs_key found_key;
        struct extent_buffer *eb;
        struct btrfs_extent_item *ei;
        struct scrub_warning swarn;
        unsigned long ptr = 0;
        u64 extent_item_pos;
        u64 flags = 0;
        u64 ref_root;
        u32 item_size;
        u8 ref_level;
        const int bufsize = 4096;
        int ret;

        WARN_ON(sblock->page_count < 1);
        dev = sblock->pagev[0]->dev;
        fs_info = sblock->sctx->dev_root->fs_info;

        path = btrfs_alloc_path();

        swarn.scratch_buf = kmalloc(bufsize, GFP_NOFS);
        swarn.msg_buf = kmalloc(bufsize, GFP_NOFS);
        swarn.sector = (sblock->pagev[0]->physical) >> 9;
        swarn.logical = sblock->pagev[0]->logical;
        swarn.errstr = errstr;
        swarn.dev = NULL;
        swarn.msg_bufsize = bufsize;
        swarn.scratch_bufsize = bufsize;

        if (!path || !swarn.scratch_buf || !swarn.msg_buf)
                goto out;

        ret = extent_from_logical(fs_info, swarn.logical, path, &found_key,
                                  &flags);
        if (ret < 0)
                goto out;

        extent_item_pos = swarn.logical - found_key.objectid;
        swarn.extent_item_size = found_key.offset;

        eb = path->nodes[0];
        ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
        item_size = btrfs_item_size_nr(eb, path->slots[0]);
        btrfs_release_path(path);

        if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                do {
                        ret = tree_backref_for_extent(&ptr, eb, ei, item_size,
                                                        &ref_root, &ref_level);
                        printk_in_rcu(KERN_WARNING
                                "btrfs: %s at logical %llu on dev %s, "
                                "sector %llu: metadata %s (level %d) in tree "
                                "%llu\n", errstr, swarn.logical,
                                rcu_str_deref(dev->name),
                                (unsigned long long)swarn.sector,
                                ref_level ? "node" : "leaf",
                                ret < 0 ? -1 : ref_level,
                                ret < 0 ? -1 : ref_root);
                } while (ret != 1);
        } else {
                swarn.path = path;
                swarn.dev = dev;
                iterate_extent_inodes(fs_info, found_key.objectid,
                                        extent_item_pos, 1,
                                        scrub_print_warning_inode, &swarn);
        }

out:
        btrfs_free_path(path);
        kfree(swarn.scratch_buf);
        kfree(swarn.msg_buf);
}

static int scrub_fixup_readpage(u64 inum, u64 offset, u64 root, void *ctx)
{
        struct page *page = NULL;
        unsigned long index;
        struct scrub_fixup_nodatasum *fixup = ctx;
        int ret;
        int corrected = 0;
        struct btrfs_key key;
        struct inode *inode = NULL;
        u64 end = offset + PAGE_SIZE - 1;
        struct btrfs_root *local_root;

        key.objectid = root;
        key.type = BTRFS_ROOT_ITEM_KEY;
        key.offset = (u64)-1;
        local_root = btrfs_read_fs_root_no_name(fixup->root->fs_info, &key);
        if (IS_ERR(local_root))
                return PTR_ERR(local_root);

        key.type = BTRFS_INODE_ITEM_KEY;
        key.objectid = inum;
        key.offset = 0;
        inode = btrfs_iget(fixup->root->fs_info->sb, &key, local_root, NULL);
        if (IS_ERR(inode))
                return PTR_ERR(inode);

        index = offset >> PAGE_CACHE_SHIFT;

        page = find_or_create_page(inode->i_mapping, index, GFP_NOFS);
        if (!page) {
                ret = -ENOMEM;
                goto out;
        }

        if (PageUptodate(page)) {
                struct btrfs_fs_info *fs_info;
                if (PageDirty(page)) {
                        /*
                         * we need to write the data to the defect sector. the
                         * data that was in that sector is not in memory,
                         * because the page was modified. we must not write the
                         * modified page to that sector.
                         *
                         * TODO: what could be done here: wait for the delalloc
                         *       runner to write out that page (might involve
                         *       COW) and see whether the sector is still
                         *       referenced afterwards.
                         *
                         * For the meantime, we'll treat this error
                         * incorrectable, although there is a chance that a
                         * later scrub will find the bad sector again and that
                         * there's no dirty page in memory, then.
                         */
                        ret = -EIO;
                        goto out;
                }
                fs_info = BTRFS_I(inode)->root->fs_info;
                ret = repair_io_failure(fs_info, offset, PAGE_SIZE,
                                        fixup->logical, page,
                                        fixup->mirror_num);
                unlock_page(page);
                corrected = !ret;
        } else {
                /*
                 * we need to get good data first. the general readpage path
                 * will call repair_io_failure for us, we just have to make
                 * sure we read the bad mirror.
                 */
                ret = set_extent_bits(&BTRFS_I(inode)->io_tree, offset, end,
                                        EXTENT_DAMAGED, GFP_NOFS);
                if (ret) {
                        /* set_extent_bits should give proper error */
                        WARN_ON(ret > 0);
                        if (ret > 0)
                                ret = -EFAULT;
                        goto out;
                }

                ret = extent_read_full_page(&BTRFS_I(inode)->io_tree, page,
                                                btrfs_get_extent,
                                                fixup->mirror_num);
                wait_on_page_locked(page);

                corrected = !test_range_bit(&BTRFS_I(inode)->io_tree, offset,
                                                end, EXTENT_DAMAGED, 0, NULL);
                if (!corrected)
                        clear_extent_bits(&BTRFS_I(inode)->io_tree, offset, end,
                                                EXTENT_DAMAGED, GFP_NOFS);
        }

out:
        if (page)
                put_page(page);
        if (inode)
                iput(inode);

        if (ret < 0)
                return ret;

        if (ret == 0 && corrected) {
                /*
                 * we only need to call readpage for one of the inodes belonging
                 * to this extent. so make iterate_extent_inodes stop
                 */
                return 1;
        }

        return -EIO;
}

static void scrub_fixup_nodatasum(struct btrfs_work *work)
{
        int ret;
        struct scrub_fixup_nodatasum *fixup;
        struct scrub_ctx *sctx;
        struct btrfs_trans_handle *trans = NULL;
        struct btrfs_fs_info *fs_info;
        struct btrfs_path *path;
        int uncorrectable = 0;

        fixup = container_of(work, struct scrub_fixup_nodatasum, work);
        sctx = fixup->sctx;
        fs_info = fixup->root->fs_info;

        path = btrfs_alloc_path();
        if (!path) {
                spin_lock(&sctx->stat_lock);
                ++sctx->stat.malloc_errors;
                spin_unlock(&sctx->stat_lock);
                uncorrectable = 1;
                goto out;
        }

        trans = btrfs_join_transaction(fixup->root);
        if (IS_ERR(trans)) {
                uncorrectable = 1;
                goto out;
        }

        /*
         * the idea is to trigger a regular read through the standard path. we
         * read a page from the (failed) logical address by specifying the
         * corresponding copynum of the failed sector. thus, that readpage is
         * expected to fail.
         * that is the point where on-the-fly error correction will kick in
         * (once it's finished) and rewrite the failed sector if a good copy
         * can be found.
         */
        ret = iterate_inodes_from_logical(fixup->logical, fixup->root->fs_info,
                                                path, scrub_fixup_readpage,
                                                fixup);
        if (ret < 0) {
                uncorrectable = 1;
                goto out;
        }
        WARN_ON(ret != 1);

        spin_lock(&sctx->stat_lock);
        ++sctx->stat.corrected_errors;
        spin_unlock(&sctx->stat_lock);

out:
        if (trans && !IS_ERR(trans))
                btrfs_end_transaction(trans, fixup->root);
        if (uncorrectable) {
                spin_lock(&sctx->stat_lock);
                ++sctx->stat.uncorrectable_errors;
                spin_unlock(&sctx->stat_lock);

                printk_ratelimited_in_rcu(KERN_ERR
                        "btrfs: unable to fixup (nodatasum) error at logical %llu on dev %s\n",
                        (unsigned long long)fixup->logical,
                        rcu_str_deref(fixup->dev->name));
        }

        btrfs_free_path(path);
        kfree(fixup);

        scrub_pending_trans_workers_dec(sctx);
}

/*
 * scrub_handle_errored_block gets called when either verification of the
 * pages failed or the bio failed to read, e.g. with EIO. In the latter
 * case, this function handles all pages in the bio, even though only one
 * may be bad.
 * The goal of this function is to repair the errored block by using the
 * contents of one of the mirrors.
 */
static int scrub_handle_errored_block(struct scrub_block *sblock_to_check)
{
        struct scrub_ctx *sctx = sblock_to_check->sctx;
        struct btrfs_device *dev;
        struct btrfs_fs_info *fs_info;
        u64 length;
        u64 logical;
        u64 generation;
        unsigned int failed_mirror_index;
        unsigned int is_metadata;
        unsigned int have_csum;
        u8 *csum;
        struct scrub_block *sblocks_for_recheck; /* holds one for each mirror */
        struct scrub_block *sblock_bad;
        int ret;
        int mirror_index;
        int page_num;
        int success;
        static DEFINE_RATELIMIT_STATE(_rs, DEFAULT_RATELIMIT_INTERVAL,
                                      DEFAULT_RATELIMIT_BURST);

        BUG_ON(sblock_to_check->page_count < 1);
        fs_info = sctx->dev_root->fs_info;
        length = sblock_to_check->page_count * PAGE_SIZE;
        logical = sblock_to_check->pagev[0]->logical;
        generation = sblock_to_check->pagev[0]->generation;
        BUG_ON(sblock_to_check->pagev[0]->mirror_num < 1);
        failed_mirror_index = sblock_to_check->pagev[0]->mirror_num - 1;
        is_metadata = !(sblock_to_check->pagev[0]->flags &
                        BTRFS_EXTENT_FLAG_DATA);
        have_csum = sblock_to_check->pagev[0]->have_csum;
        csum = sblock_to_check->pagev[0]->csum;
        dev = sblock_to_check->pagev[0]->dev;

        /*
         * read all mirrors one after the other. This includes to
         * re-read the extent or metadata block that failed (that was
         * the cause that this fixup code is called) another time,
         * page by page this time in order to know which pages
         * caused I/O errors and which ones are good (for all mirrors).
         * It is the goal to handle the situation when more than one
         * mirror contains I/O errors, but the errors do not
         * overlap, i.e. the data can be repaired by selecting the
         * pages from those mirrors without I/O error on the
         * particular pages. One example (with blocks >= 2 * PAGE_SIZE)
         * would be that mirror #1 has an I/O error on the first page,
         * the second page is good, and mirror #2 has an I/O error on
         * the second page, but the first page is good.
         * Then the first page of the first mirror can be repaired by
         * taking the first page of the second mirror, and the
         * second page of the second mirror can be repaired by
         * copying the contents of the 2nd page of the 1st mirror.
         * One more note: if the pages of one mirror contain I/O
         * errors, the checksum cannot be verified. In order to get
         * the best data for repairing, the first attempt is to find
         * a mirror without I/O errors and with a validated checksum.
         * Only if this is not possible, the pages are picked from
         * mirrors with I/O errors without considering the checksum.
         * If the latter is the case, at the end, the checksum of the
         * repaired area is verified in order to correctly maintain
         * the statistics.
         */

        sblocks_for_recheck = kzalloc(BTRFS_MAX_MIRRORS *
                                     sizeof(*sblocks_for_recheck),
                                     GFP_NOFS);
        if (!sblocks_for_recheck) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.malloc_errors++;
                sctx->stat.read_errors++;
                sctx->stat.uncorrectable_errors++;
                spin_unlock(&sctx->stat_lock);
                btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_READ_ERRS);
                goto out;
        }

        /* setup the context, map the logical blocks and alloc the pages */
        ret = scrub_setup_recheck_block(sctx, fs_info, length,
                                        logical, sblocks_for_recheck);
        if (ret) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.read_errors++;
                sctx->stat.uncorrectable_errors++;
                spin_unlock(&sctx->stat_lock);
                btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_READ_ERRS);
                goto out;
        }
        BUG_ON(failed_mirror_index >= BTRFS_MAX_MIRRORS);
        sblock_bad = sblocks_for_recheck + failed_mirror_index;

        /* build and submit the bios for the failed mirror, check checksums */
        scrub_recheck_block(fs_info, sblock_bad, is_metadata, have_csum,
                            csum, generation, sctx->csum_size);

        if (!sblock_bad->header_error && !sblock_bad->checksum_error &&
            sblock_bad->no_io_error_seen) {
                /*
                 * the error disappeared after reading page by page, or
                 * the area was part of a huge bio and other parts of the
                 * bio caused I/O errors, or the block layer merged several
                 * read requests into one and the error is caused by a
                 * different bio (usually one of the two latter cases is
                 * the cause)
                 */
                spin_lock(&sctx->stat_lock);
                sctx->stat.unverified_errors++;
                spin_unlock(&sctx->stat_lock);

                goto out;
        }

        if (!sblock_bad->no_io_error_seen) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.read_errors++;
                spin_unlock(&sctx->stat_lock);
                if (__ratelimit(&_rs))
                        scrub_print_warning("i/o error", sblock_to_check);
                btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_READ_ERRS);
        } else if (sblock_bad->checksum_error) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.csum_errors++;
                spin_unlock(&sctx->stat_lock);
                if (__ratelimit(&_rs))
                        scrub_print_warning("checksum error", sblock_to_check);
                btrfs_dev_stat_inc_and_print(dev,
                                             BTRFS_DEV_STAT_CORRUPTION_ERRS);
        } else if (sblock_bad->header_error) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.verify_errors++;
                spin_unlock(&sctx->stat_lock);
                if (__ratelimit(&_rs))
                        scrub_print_warning("checksum/header error",
                                            sblock_to_check);
                if (sblock_bad->generation_error)
                        btrfs_dev_stat_inc_and_print(dev,
                                BTRFS_DEV_STAT_GENERATION_ERRS);
                else
                        btrfs_dev_stat_inc_and_print(dev,
                                BTRFS_DEV_STAT_CORRUPTION_ERRS);
        }

        if (sctx->readonly)
                goto did_not_correct_error;

        if (!is_metadata && !have_csum) {
                struct scrub_fixup_nodatasum *fixup_nodatasum;

                /*
                 * !is_metadata and !have_csum, this means that the data
                 * might not be COW'ed, that it might be modified
                 * concurrently. The general strategy to work on the
                 * commit root does not help in the case when COW is not
                 * used.
                 */
                fixup_nodatasum = kzalloc(sizeof(*fixup_nodatasum), GFP_NOFS);
                if (!fixup_nodatasum)
                        goto did_not_correct_error;
                fixup_nodatasum->sctx = sctx;
                fixup_nodatasum->dev = dev;
                fixup_nodatasum->logical = logical;
                fixup_nodatasum->root = fs_info->extent_root;
                fixup_nodatasum->mirror_num = failed_mirror_index + 1;
                scrub_pending_trans_workers_inc(sctx);
                fixup_nodatasum->work.func = scrub_fixup_nodatasum;
                btrfs_queue_worker(&fs_info->scrub_workers,
                                   &fixup_nodatasum->work);
                goto out;
        }

        /*
         * now build and submit the bios for the other mirrors, check
         * checksums.
         * First try to pick the mirror which is completely without I/O
         * errors and also does not have a checksum error.
         * If one is found, and if a checksum is present, the full block
         * that is known to contain an error is rewritten. Afterwards
         * the block is known to be corrected.
         * If a mirror is found which is completely correct, and no
         * checksum is present, only those pages are rewritten that had
         * an I/O error in the block to be repaired, since it cannot be
         * determined, which copy of the other pages is better (and it
         * could happen otherwise that a correct page would be
         * overwritten by a bad one).
         */
        for (mirror_index = 0;
             mirror_index < BTRFS_MAX_MIRRORS &&
             sblocks_for_recheck[mirror_index].page_count > 0;
             mirror_index++) {
                struct scrub_block *sblock_other;

                if (mirror_index == failed_mirror_index)
                        continue;
                sblock_other = sblocks_for_recheck + mirror_index;

                /* build and submit the bios, check checksums */
                scrub_recheck_block(fs_info, sblock_other, is_metadata,
                                    have_csum, csum, generation,
                                    sctx->csum_size);

                if (!sblock_other->header_error &&
                    !sblock_other->checksum_error &&
                    sblock_other->no_io_error_seen) {
                        int force_write = is_metadata || have_csum;

                        ret = scrub_repair_block_from_good_copy(sblock_bad,
                                                                sblock_other,
                                                                force_write);
                        if (0 == ret)
                                goto corrected_error;
                }
        }

        /*
         * in case of I/O errors in the area that is supposed to be
         * repaired, continue by picking good copies of those pages.
         * Select the good pages from mirrors to rewrite bad pages from
         * the area to fix. Afterwards verify the checksum of the block
         * that is supposed to be repaired. This verification step is
         * only done for the purpose of statistic counting and for the
         * final scrub report, whether errors remain.
         * A perfect algorithm could make use of the checksum and try
         * all possible combinations of pages from the different mirrors
         * until the checksum verification succeeds. For example, when
         * the 2nd page of mirror #1 faces I/O errors, and the 2nd page
         * of mirror #2 is readable but the final checksum test fails,
         * then the 2nd page of mirror #3 could be tried, whether now
         * the final checksum succeedes. But this would be a rare
         * exception and is therefore not implemented. At least it is
         * avoided that the good copy is overwritten.
         * A more useful improvement would be to pick the sectors
         * without I/O error based on sector sizes (512 bytes on legacy
         * disks) instead of on PAGE_SIZE. Then maybe 512 byte of one
         * mirror could be repaired by taking 512 byte of a different
         * mirror, even if other 512 byte sectors in the same PAGE_SIZE
         * area are unreadable.
         */

        /* can only fix I/O errors from here on */
        if (sblock_bad->no_io_error_seen)
                goto did_not_correct_error;

        success = 1;
        for (page_num = 0; page_num < sblock_bad->page_count; page_num++) {
                struct scrub_page *page_bad = sblock_bad->pagev[page_num];

                if (!page_bad->io_error)
                        continue;

                for (mirror_index = 0;
                     mirror_index < BTRFS_MAX_MIRRORS &&
                     sblocks_for_recheck[mirror_index].page_count > 0;
                     mirror_index++) {
                        struct scrub_block *sblock_other = sblocks_for_recheck +
                                                           mirror_index;
                        struct scrub_page *page_other = sblock_other->pagev[
                                                        page_num];

                        if (!page_other->io_error) {
                                ret = scrub_repair_page_from_good_copy(
                                        sblock_bad, sblock_other, page_num, 0);
                                if (0 == ret) {
                                        page_bad->io_error = 0;
                                        break; /* succeeded for this page */
                                }
                        }
                }

                if (page_bad->io_error) {
                        /* did not find a mirror to copy the page from */
                        success = 0;
                }
        }

        if (success) {
                if (is_metadata || have_csum) {
                        /*
                         * need to verify the checksum now that all
                         * sectors on disk are repaired (the write
                         * request for data to be repaired is on its way).
                         * Just be lazy and use scrub_recheck_block()
                         * which re-reads the data before the checksum
                         * is verified, but most likely the data comes out
                         * of the page cache.
                         */
                        scrub_recheck_block(fs_info, sblock_bad,
                                            is_metadata, have_csum, csum,
                                            generation, sctx->csum_size);
                        if (!sblock_bad->header_error &&
                            !sblock_bad->checksum_error &&
                            sblock_bad->no_io_error_seen)
                                goto corrected_error;
                        else
                                goto did_not_correct_error;
                } else {
corrected_error:
                        spin_lock(&sctx->stat_lock);
                        sctx->stat.corrected_errors++;
                        spin_unlock(&sctx->stat_lock);
                        printk_ratelimited_in_rcu(KERN_ERR
                                "btrfs: fixed up error at logical %llu on dev %s\n",
                                (unsigned long long)logical,
                                rcu_str_deref(dev->name));
                }
        } else {
did_not_correct_error:
                spin_lock(&sctx->stat_lock);
                sctx->stat.uncorrectable_errors++;
                spin_unlock(&sctx->stat_lock);
                printk_ratelimited_in_rcu(KERN_ERR
                        "btrfs: unable to fixup (regular) error at logical %llu on dev %s\n",
                        (unsigned long long)logical,
                        rcu_str_deref(dev->name));
        }

out:
        if (sblocks_for_recheck) {
                for (mirror_index = 0; mirror_index < BTRFS_MAX_MIRRORS;
                     mirror_index++) {
                        struct scrub_block *sblock = sblocks_for_recheck +
                                                     mirror_index;
                        int page_index;

                        for (page_index = 0; page_index < sblock->page_count;
                             page_index++) {
                                sblock->pagev[page_index]->sblock = NULL;
                                scrub_page_put(sblock->pagev[page_index]);
                        }
                }
                kfree(sblocks_for_recheck);
        }

        return 0;
}

static int scrub_setup_recheck_block(struct scrub_ctx *sctx,
                                     struct btrfs_fs_info *fs_info,
                                     u64 length, u64 logical,
                                     struct scrub_block *sblocks_for_recheck)
{
        int page_index;
        int mirror_index;
        int ret;

        /*
         * note: the two members ref_count and outstanding_pages
         * are not used (and not set) in the blocks that are used for
         * the recheck procedure
         */

        page_index = 0;
        while (length > 0) {
                u64 sublen = min_t(u64, length, PAGE_SIZE);
                u64 mapped_length = sublen;
                struct btrfs_bio *bbio = NULL;

                /*
                 * with a length of PAGE_SIZE, each returned stripe
                 * represents one mirror
                 */
                ret = btrfs_map_block(fs_info, WRITE, logical, &mapped_length,
                                      &bbio, 0);
                if (ret || !bbio || mapped_length < sublen) {
                        kfree(bbio);
                        return -EIO;
                }

                BUG_ON(page_index >= SCRUB_PAGES_PER_BIO);
                for (mirror_index = 0; mirror_index < (int)bbio->num_stripes;
                     mirror_index++) {
                        struct scrub_block *sblock;
                        struct scrub_page *page;

                        if (mirror_index >= BTRFS_MAX_MIRRORS)
                                continue;

                        sblock = sblocks_for_recheck + mirror_index;
                        sblock->sctx = sctx;
                        page = kzalloc(sizeof(*page), GFP_NOFS);
                        if (!page) {
leave_nomem:
                                spin_lock(&sctx->stat_lock);
                                sctx->stat.malloc_errors++;
                                spin_unlock(&sctx->stat_lock);
                                kfree(bbio);
                                return -ENOMEM;
                        }
                        scrub_page_get(page);
                        sblock->pagev[page_index] = page;
                        page->logical = logical;
                        page->physical = bbio->stripes[mirror_index].physical;
                        /* for missing devices, dev->bdev is NULL */
                        page->dev = bbio->stripes[mirror_index].dev;
                        page->mirror_num = mirror_index + 1;
                        sblock->page_count++;
                        page->page = alloc_page(GFP_NOFS);
                        if (!page->page)
                                goto leave_nomem;
                }
                kfree(bbio);
                length -= sublen;
                logical += sublen;
                page_index++;
        }

        return 0;
}

/*
 * this function will check the on disk data for checksum errors, header
 * errors and read I/O errors. If any I/O errors happen, the exact pages
 * which are errored are marked as being bad. The goal is to enable scrub
 * to take those pages that are not errored from all the mirrors so that
 * the pages that are errored in the just handled mirror can be repaired.
 */
static void scrub_recheck_block(struct btrfs_fs_info *fs_info,
                                struct scrub_block *sblock, int is_metadata,
                                int have_csum, u8 *csum, u64 generation,
                                u16 csum_size)
{
        int page_num;

        sblock->no_io_error_seen = 1;
        sblock->header_error = 0;
        sblock->checksum_error = 0;

        for (page_num = 0; page_num < sblock->page_count; page_num++) {
                struct bio *bio;
                struct scrub_page *page = sblock->pagev[page_num];
                DECLARE_COMPLETION_ONSTACK(complete);

                if (page->dev->bdev == NULL) {
                        page->io_error = 1;
                        sblock->no_io_error_seen = 0;
                        continue;
                }

                WARN_ON(!page->page);
                bio = bio_alloc(GFP_NOFS, 1);
                if (!bio) {
                        page->io_error = 1;
                        sblock->no_io_error_seen = 0;
                        continue;
                }
                bio->bi_bdev = page->dev->bdev;
                bio->bi_sector = page->physical >> 9;
                bio->bi_end_io = scrub_complete_bio_end_io;
                bio->bi_private = &complete;

                bio_add_page(bio, page->page, PAGE_SIZE, 0);
                btrfsic_submit_bio(READ, bio);

                /* this will also unplug the queue */
                wait_for_completion(&complete);

                page->io_error = !test_bit(BIO_UPTODATE, &bio->bi_flags);
                if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
                        sblock->no_io_error_seen = 0;
                bio_put(bio);
        }

        if (sblock->no_io_error_seen)
                scrub_recheck_block_checksum(fs_info, sblock, is_metadata,
                                             have_csum, csum, generation,
                                             csum_size);

        return;
}

static void scrub_recheck_block_checksum(struct btrfs_fs_info *fs_info,
                                         struct scrub_block *sblock,
                                         int is_metadata, int have_csum,
                                         const u8 *csum, u64 generation,
                                         u16 csum_size)
{
        int page_num;
        u8 calculated_csum[BTRFS_CSUM_SIZE];
        u32 crc = ~(u32)0;
        struct btrfs_root *root = fs_info->extent_root;
        void *mapped_buffer;

        WARN_ON(!sblock->pagev[0]->page);
        if (is_metadata) {
                struct btrfs_header *h;

                mapped_buffer = kmap_atomic(sblock->pagev[0]->page);
                h = (struct btrfs_header *)mapped_buffer;

                if (sblock->pagev[0]->logical != le64_to_cpu(h->bytenr) ||
                    memcmp(h->fsid, fs_info->fsid, BTRFS_UUID_SIZE) ||
                    memcmp(h->chunk_tree_uuid, fs_info->chunk_tree_uuid,
                           BTRFS_UUID_SIZE)) {
                        sblock->header_error = 1;
                } else if (generation != le64_to_cpu(h->generation)) {
                        sblock->header_error = 1;
                        sblock->generation_error = 1;
                }
                csum = h->csum;
        } else {
                if (!have_csum)
                        return;

                mapped_buffer = kmap_atomic(sblock->pagev[0]->page);
        }

        for (page_num = 0;;) {
                if (page_num == 0 && is_metadata)
                        crc = btrfs_csum_data(root,
                                ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE,
                                crc, PAGE_SIZE - BTRFS_CSUM_SIZE);
                else
                        crc = btrfs_csum_data(root, mapped_buffer, crc,
                                              PAGE_SIZE);

                kunmap_atomic(mapped_buffer);
                page_num++;
                if (page_num >= sblock->page_count)
                        break;
                WARN_ON(!sblock->pagev[page_num]->page);

                mapped_buffer = kmap_atomic(sblock->pagev[page_num]->page);
        }

        btrfs_csum_final(crc, calculated_csum);
        if (memcmp(calculated_csum, csum, csum_size))
                sblock->checksum_error = 1;
}

static void scrub_complete_bio_end_io(struct bio *bio, int err)
{
        complete((struct completion *)bio->bi_private);
}

static int scrub_repair_block_from_good_copy(struct scrub_block *sblock_bad,
                                             struct scrub_block *sblock_good,
                                             int force_write)
{
        int page_num;
        int ret = 0;

        for (page_num = 0; page_num < sblock_bad->page_count; page_num++) {
                int ret_sub;

                ret_sub = scrub_repair_page_from_good_copy(sblock_bad,
                                                           sblock_good,
                                                           page_num,
                                                           force_write);
                if (ret_sub)
                        ret = ret_sub;
        }

        return ret;
}

static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
                                            struct scrub_block *sblock_good,
                                            int page_num, int force_write)
{
        struct scrub_page *page_bad = sblock_bad->pagev[page_num];
        struct scrub_page *page_good = sblock_good->pagev[page_num];

        BUG_ON(page_bad->page == NULL);
        BUG_ON(page_good->page == NULL);
        if (force_write || sblock_bad->header_error ||
            sblock_bad->checksum_error || page_bad->io_error) {
                struct bio *bio;
                int ret;
                DECLARE_COMPLETION_ONSTACK(complete);

                bio = bio_alloc(GFP_NOFS, 1);
                if (!bio)
                        return -EIO;
                bio->bi_bdev = page_bad->dev->bdev;
                bio->bi_sector = page_bad->physical >> 9;
                bio->bi_end_io = scrub_complete_bio_end_io;
                bio->bi_private = &complete;

                ret = bio_add_page(bio, page_good->page, PAGE_SIZE, 0);
                if (PAGE_SIZE != ret) {
                        bio_put(bio);
                        return -EIO;
                }
                btrfsic_submit_bio(WRITE, bio);

                /* this will also unplug the queue */
                wait_for_completion(&complete);
                if (!bio_flagged(bio, BIO_UPTODATE)) {
                        btrfs_dev_stat_inc_and_print(page_bad->dev,
                                BTRFS_DEV_STAT_WRITE_ERRS);
                        bio_put(bio);
                        return -EIO;
                }
                bio_put(bio);
        }

        return 0;
}

static void scrub_checksum(struct scrub_block *sblock)
{
        u64 flags;
        int ret;

        WARN_ON(sblock->page_count < 1);
        flags = sblock->pagev[0]->flags;
        ret = 0;
        if (flags & BTRFS_EXTENT_FLAG_DATA)
                ret = scrub_checksum_data(sblock);
        else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
                ret = scrub_checksum_tree_block(sblock);
        else if (flags & BTRFS_EXTENT_FLAG_SUPER)
                (void)scrub_checksum_super(sblock);
        else
                WARN_ON(1);
        if (ret)
                scrub_handle_errored_block(sblock);
}

static int scrub_checksum_data(struct scrub_block *sblock)
{
        struct scrub_ctx *sctx = sblock->sctx;
        u8 csum[BTRFS_CSUM_SIZE];
        u8 *on_disk_csum;
        struct page *page;
        void *buffer;
        u32 crc = ~(u32)0;
        int fail = 0;
        struct btrfs_root *root = sctx->dev_root;
        u64 len;
        int index;

        BUG_ON(sblock->page_count < 1);
        if (!sblock->pagev[0]->have_csum)
                return 0;

        on_disk_csum = sblock->pagev[0]->csum;
        page = sblock->pagev[0]->page;
        buffer = kmap_atomic(page);

        len = sctx->sectorsize;
        index = 0;
        for (;;) {
                u64 l = min_t(u64, len, PAGE_SIZE);

                crc = btrfs_csum_data(root, buffer, crc, l);
                kunmap_atomic(buffer);
                len -= l;
                if (len == 0)
                        break;
                index++;
                BUG_ON(index >= sblock->page_count);
                BUG_ON(!sblock->pagev[index]->page);
                page = sblock->pagev[index]->page;
                buffer = kmap_atomic(page);
        }

        btrfs_csum_final(crc, csum);
        if (memcmp(csum, on_disk_csum, sctx->csum_size))
                fail = 1;

        return fail;
}

static int scrub_checksum_tree_block(struct scrub_block *sblock)
{
        struct scrub_ctx *sctx = sblock->sctx;
        struct btrfs_header *h;
        struct btrfs_root *root = sctx->dev_root;
        struct btrfs_fs_info *fs_info = root->fs_info;
        u8 calculated_csum[BTRFS_CSUM_SIZE];
        u8 on_disk_csum[BTRFS_CSUM_SIZE];
        struct page *page;
        void *mapped_buffer;
        u64 mapped_size;
        void *p;
        u32 crc = ~(u32)0;
        int fail = 0;
        int crc_fail = 0;
        u64 len;
        int index;

        BUG_ON(sblock->page_count < 1);
        page = sblock->pagev[0]->page;
        mapped_buffer = kmap_atomic(page);
        h = (struct btrfs_header *)mapped_buffer;
        memcpy(on_disk_csum, h->csum, sctx->csum_size);

        /*
         * we don't use the getter functions here, as we
         * a) don't have an extent buffer and
         * b) the page is already kmapped
         */

        if (sblock->pagev[0]->logical != le64_to_cpu(h->bytenr))
                ++fail;

        if (sblock->pagev[0]->generation != le64_to_cpu(h->generation))
                ++fail;

        if (memcmp(h->fsid, fs_info->fsid, BTRFS_UUID_SIZE))
                ++fail;

        if (memcmp(h->chunk_tree_uuid, fs_info->chunk_tree_uuid,
                   BTRFS_UUID_SIZE))
                ++fail;

        BUG_ON(sctx->nodesize != sctx->leafsize);
        len = sctx->nodesize - BTRFS_CSUM_SIZE;
        mapped_size = PAGE_SIZE - BTRFS_CSUM_SIZE;
        p = ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE;
        index = 0;
        for (;;) {
                u64 l = min_t(u64, len, mapped_size);

                crc = btrfs_csum_data(root, p, crc, l);
                kunmap_atomic(mapped_buffer);
                len -= l;
                if (len == 0)
                        break;
                index++;
                BUG_ON(index >= sblock->page_count);
                BUG_ON(!sblock->pagev[index]->page);
                page = sblock->pagev[index]->page;
                mapped_buffer = kmap_atomic(page);
                mapped_size = PAGE_SIZE;
                p = mapped_buffer;
        }

        btrfs_csum_final(crc, calculated_csum);
        if (memcmp(calculated_csum, on_disk_csum, sctx->csum_size))
                ++crc_fail;

        return fail || crc_fail;
}

static int scrub_checksum_super(struct scrub_block *sblock)
{
        struct btrfs_super_block *s;
        struct scrub_ctx *sctx = sblock->sctx;
        struct btrfs_root *root = sctx->dev_root;
        struct btrfs_fs_info *fs_info = root->fs_info;
        u8 calculated_csum[BTRFS_CSUM_SIZE];
        u8 on_disk_csum[BTRFS_CSUM_SIZE];
        struct page *page;
        void *mapped_buffer;
        u64 mapped_size;
        void *p;
        u32 crc = ~(u32)0;
        int fail_gen = 0;
        int fail_cor = 0;
        u64 len;
        int index;

        BUG_ON(sblock->page_count < 1);
        page = sblock->pagev[0]->page;
        mapped_buffer = kmap_atomic(page);
        s = (struct btrfs_super_block *)mapped_buffer;
        memcpy(on_disk_csum, s->csum, sctx->csum_size);

        if (sblock->pagev[0]->logical != le64_to_cpu(s->bytenr))
                ++fail_cor;

        if (sblock->pagev[0]->generation != le64_to_cpu(s->generation))
                ++fail_gen;

        if (memcmp(s->fsid, fs_info->fsid, BTRFS_UUID_SIZE))
                ++fail_cor;

        len = BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE;
        mapped_size = PAGE_SIZE - BTRFS_CSUM_SIZE;
        p = ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE;
        index = 0;
        for (;;) {
                u64 l = min_t(u64, len, mapped_size);

                crc = btrfs_csum_data(root, p, crc, l);
                kunmap_atomic(mapped_buffer);
                len -= l;
                if (len == 0)
                        break;
                index++;
                BUG_ON(index >= sblock->page_count);
                BUG_ON(!sblock->pagev[index]->page);
                page = sblock->pagev[index]->page;
                mapped_buffer = kmap_atomic(page);
                mapped_size = PAGE_SIZE;
                p = mapped_buffer;
        }

        btrfs_csum_final(crc, calculated_csum);
        if (memcmp(calculated_csum, on_disk_csum, sctx->csum_size))
                ++fail_cor;

        if (fail_cor + fail_gen) {
                /*
                 * if we find an error in a super block, we just report it.
                 * They will get written with the next transaction commit
                 * anyway
                 */
                spin_lock(&sctx->stat_lock);
                ++sctx->stat.super_errors;
                spin_unlock(&sctx->stat_lock);
                if (fail_cor)
                        btrfs_dev_stat_inc_and_print(sblock->pagev[0]->dev,
                                BTRFS_DEV_STAT_CORRUPTION_ERRS);
                else
                        btrfs_dev_stat_inc_and_print(sblock->pagev[0]->dev,
                                BTRFS_DEV_STAT_GENERATION_ERRS);
        }

        return fail_cor + fail_gen;
}

static void scrub_block_get(struct scrub_block *sblock)
{
        atomic_inc(&sblock->ref_count);
}

static void scrub_block_put(struct scrub_block *sblock)
{
        if (atomic_dec_and_test(&sblock->ref_count)) {
                int i;

                for (i = 0; i < sblock->page_count; i++)
                        scrub_page_put(sblock->pagev[i]);
                kfree(sblock);
        }
}

static void scrub_page_get(struct scrub_page *spage)
{
        atomic_inc(&spage->ref_count);
}

static void scrub_page_put(struct scrub_page *spage)
{
        if (atomic_dec_and_test(&spage->ref_count)) {
                if (spage->page)
                        __free_page(spage->page);
                kfree(spage);
        }
}

static void scrub_submit(struct scrub_ctx *sctx)
{
        struct scrub_bio *sbio;

        if (sctx->curr == -1)
                return;

        sbio = sctx->bios[sctx->curr];
        sctx->curr = -1;
        scrub_pending_bio_inc(sctx);

        btrfsic_submit_bio(READ, sbio->bio);
}

static int scrub_add_page_to_bio(struct scrub_ctx *sctx,
                                 struct scrub_page *spage)
{
        struct scrub_block *sblock = spage->sblock;
        struct scrub_bio *sbio;
        int ret;

again:
        /*
         * grab a fresh bio or wait for one to become available
         */
        while (sctx->curr == -1) {
                spin_lock(&sctx->list_lock);
                sctx->curr = sctx->first_free;
                if (sctx->curr != -1) {
                        sctx->first_free = sctx->bios[sctx->curr]->next_free;
                        sctx->bios[sctx->curr]->next_free = -1;
                        sctx->bios[sctx->curr]->page_count = 0;
                        spin_unlock(&sctx->list_lock);
                } else {
                        spin_unlock(&sctx->list_lock);
                        wait_event(sctx->list_wait, sctx->first_free != -1);
                }
        }
        sbio = sctx->bios[sctx->curr];
        if (sbio->page_count == 0) {
                struct bio *bio;

                sbio->physical = spage->physical;
                sbio->logical = spage->logical;
                sbio->dev = spage->dev;
                bio = sbio->bio;
                if (!bio) {
                        bio = bio_alloc(GFP_NOFS, sctx->pages_per_bio);
                        if (!bio)
                                return -ENOMEM;
                        sbio->bio = bio;
                }

                bio->bi_private = sbio;
                bio->bi_end_io = scrub_bio_end_io;
                bio->bi_bdev = sbio->dev->bdev;
                bio->bi_sector = sbio->physical >> 9;
                sbio->err = 0;
        } else if (sbio->physical + sbio->page_count * PAGE_SIZE !=
                   spage->physical ||
                   sbio->logical + sbio->page_count * PAGE_SIZE !=
                   spage->logical ||
                   sbio->dev != spage->dev) {
                scrub_submit(sctx);
                goto again;
        }

        sbio->pagev[sbio->page_count] = spage;
        ret = bio_add_page(sbio->bio, spage->page, PAGE_SIZE, 0);
        if (ret != PAGE_SIZE) {
                if (sbio->page_count < 1) {
                        bio_put(sbio->bio);
                        sbio->bio = NULL;
                        return -EIO;
                }
                scrub_submit(sctx);
                goto again;
        }

        scrub_block_get(sblock); /* one for the added page */
        atomic_inc(&sblock->outstanding_pages);
        sbio->page_count++;
        if (sbio->page_count == sctx->pages_per_bio)
                scrub_submit(sctx);

        return 0;
}

static int scrub_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
                       u64 physical, struct btrfs_device *dev, u64 flags,
                       u64 gen, int mirror_num, u8 *csum, int force)
{
        struct scrub_block *sblock;
        int index;

        sblock = kzalloc(sizeof(*sblock), GFP_NOFS);
        if (!sblock) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.malloc_errors++;
                spin_unlock(&sctx->stat_lock);
                return -ENOMEM;
        }

        /* one ref inside this function, plus one for each page added to
         * a bio later on */
        atomic_set(&sblock->ref_count, 1);
        sblock->sctx = sctx;
        sblock->no_io_error_seen = 1;

        for (index = 0; len > 0; index++) {
                struct scrub_page *spage;
                u64 l = min_t(u64, len, PAGE_SIZE);

                spage = kzalloc(sizeof(*spage), GFP_NOFS);
                if (!spage) {
leave_nomem:
                        spin_lock(&sctx->stat_lock);
                        sctx->stat.malloc_errors++;
                        spin_unlock(&sctx->stat_lock);
                        scrub_block_put(sblock);
                        return -ENOMEM;
                }
                BUG_ON(index >= SCRUB_MAX_PAGES_PER_BLOCK);
                scrub_page_get(spage);
                sblock->pagev[index] = spage;
                spage->sblock = sblock;
                spage->dev = dev;
                spage->flags = flags;
                spage->generation = gen;
                spage->logical = logical;
                spage->physical = physical;
                spage->mirror_num = mirror_num;
                if (csum) {
                        spage->have_csum = 1;
                        memcpy(spage->csum, csum, sctx->csum_size);
                } else {
                        spage->have_csum = 0;
                }
                sblock->page_count++;
                spage->page = alloc_page(GFP_NOFS);
                if (!spage->page)
                        goto leave_nomem;
                len -= l;
                logical += l;
                physical += l;
        }

        WARN_ON(sblock->page_count == 0);
        for (index = 0; index < sblock->page_count; index++) {
                struct scrub_page *spage = sblock->pagev[index];
                int ret;

                ret = scrub_add_page_to_bio(sctx, spage);
                if (ret) {
                        scrub_block_put(sblock);
                        return ret;
                }
        }

        if (force)
                scrub_submit(sctx);

        /* last one frees, either here or in bio completion for last page */
        scrub_block_put(sblock);
        return 0;
}

static void scrub_bio_end_io(struct bio *bio, int err)
{
        struct scrub_bio *sbio = bio->bi_private;
        struct btrfs_fs_info *fs_info = sbio->dev->dev_root->fs_info;

        sbio->err = err;
        sbio->bio = bio;

        btrfs_queue_worker(&fs_info->scrub_workers, &sbio->work);
}

static void scrub_bio_end_io_worker(struct btrfs_work *work)
{
        struct scrub_bio *sbio = container_of(work, struct scrub_bio, work);
        struct scrub_ctx *sctx = sbio->sctx;
        int i;

        BUG_ON(sbio->page_count > SCRUB_PAGES_PER_BIO);
        if (sbio->err) {
                for (i = 0; i < sbio->page_count; i++) {
                        struct scrub_page *spage = sbio->pagev[i];

                        spage->io_error = 1;
                        spage->sblock->no_io_error_seen = 0;
                }
        }

        /* now complete the scrub_block items that have all pages completed */
        for (i = 0; i < sbio->page_count; i++) {
                struct scrub_page *spage = sbio->pagev[i];
                struct scrub_block *sblock = spage->sblock;

                if (atomic_dec_and_test(&sblock->outstanding_pages))
                        scrub_block_complete(sblock);
                scrub_block_put(sblock);
        }

        bio_put(sbio->bio);
        sbio->bio = NULL;
        spin_lock(&sctx->list_lock);
        sbio->next_free = sctx->first_free;
        sctx->first_free = sbio->index;
        spin_unlock(&sctx->list_lock);
        scrub_pending_bio_dec(sctx);
}

static void scrub_block_complete(struct scrub_block *sblock)
{
        if (!sblock->no_io_error_seen)
                scrub_handle_errored_block(sblock);
        else
                scrub_checksum(sblock);
}

static int scrub_find_csum(struct scrub_ctx *sctx, u64 logical, u64 len,
                           u8 *csum)
{
        struct btrfs_ordered_sum *sum = NULL;
        int ret = 0;
        unsigned long i;
        unsigned long num_sectors;

        while (!list_empty(&sctx->csum_list)) {
                sum = list_first_entry(&sctx->csum_list,
                                       struct btrfs_ordered_sum, list);
                if (sum->bytenr > logical)
                        return 0;
                if (sum->bytenr + sum->len > logical)
                        break;

                ++sctx->stat.csum_discards;
                list_del(&sum->list);
                kfree(sum);
                sum = NULL;
        }
        if (!sum)
                return 0;

        num_sectors = sum->len / sctx->sectorsize;
        for (i = 0; i < num_sectors; ++i) {
                if (sum->sums[i].bytenr == logical) {
                        memcpy(csum, &sum->sums[i].sum, sctx->csum_size);
                        ret = 1;
                        break;
                }
        }
        if (ret && i == num_sectors - 1) {
                list_del(&sum->list);
                kfree(sum);
        }
        return ret;
}

/* scrub extent tries to collect up to 64 kB for each bio */
static int scrub_extent(struct scrub_ctx *sctx, u64 logical, u64 len,
                        u64 physical, struct btrfs_device *dev, u64 flags,
                        u64 gen, int mirror_num)
{
        int ret;
        u8 csum[BTRFS_CSUM_SIZE];
        u32 blocksize;

        if (flags & BTRFS_EXTENT_FLAG_DATA) {
                blocksize = sctx->sectorsize;
                spin_lock(&sctx->stat_lock);
                sctx->stat.data_extents_scrubbed++;
                sctx->stat.data_bytes_scrubbed += len;
                spin_unlock(&sctx->stat_lock);
        } else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                BUG_ON(sctx->nodesize != sctx->leafsize);
                blocksize = sctx->nodesize;
                spin_lock(&sctx->stat_lock);
                sctx->stat.tree_extents_scrubbed++;
                sctx->stat.tree_bytes_scrubbed += len;
                spin_unlock(&sctx->stat_lock);
        } else {
                blocksize = sctx->sectorsize;
                BUG_ON(1);
        }

        while (len) {
                u64 l = min_t(u64, len, blocksize);
                int have_csum = 0;

                if (flags & BTRFS_EXTENT_FLAG_DATA) {
                        /* push csums to sbio */
                        have_csum = scrub_find_csum(sctx, logical, l, csum);
                        if (have_csum == 0)
                                ++sctx->stat.no_csum;
                }
                ret = scrub_pages(sctx, logical, l, physical, dev, flags, gen,
                                  mirror_num, have_csum ? csum : NULL, 0);
                if (ret)
                        return ret;
                len -= l;
                logical += l;
                physical += l;
        }
        return 0;
}

static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
                                           struct map_lookup *map,
                                           struct btrfs_device *scrub_dev,
                                           int num, u64 base, u64 length)
{
        struct btrfs_path *path;
        struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;
        struct btrfs_root *root = fs_info->extent_root;
        struct btrfs_root *csum_root = fs_info->csum_root;
        struct btrfs_extent_item *extent;
        struct blk_plug plug;
        u64 flags;
        int ret;
        int slot;
        int i;
        u64 nstripes;
        struct extent_buffer *l;
        struct btrfs_key key;
        u64 physical;
        u64 logical;
        u64 generation;
        int mirror_num;
        struct reada_control *reada1;
        struct reada_control *reada2;
        struct btrfs_key key_start;
        struct btrfs_key key_end;
        u64 increment = map->stripe_len;
        u64 offset;

        nstripes = length;
        offset = 0;
        do_div(nstripes, map->stripe_len);
        if (map->type & BTRFS_BLOCK_GROUP_RAID0) {
                offset = map->stripe_len * num;
                increment = map->stripe_len * map->num_stripes;
                mirror_num = 1;
        } else if (map->type & BTRFS_BLOCK_GROUP_RAID10) {
                int factor = map->num_stripes / map->sub_stripes;
                offset = map->stripe_len * (num / map->sub_stripes);
                increment = map->stripe_len * factor;
                mirror_num = num % map->sub_stripes + 1;
        } else if (map->type & BTRFS_BLOCK_GROUP_RAID1) {
                increment = map->stripe_len;
                mirror_num = num % map->num_stripes + 1;
        } else if (map->type & BTRFS_BLOCK_GROUP_DUP) {
                increment = map->stripe_len;
                mirror_num = num % map->num_stripes + 1;
        } else {
                increment = map->stripe_len;
                mirror_num = 1;
        }

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;

        /*
         * work on commit root. The related disk blocks are static as
         * long as COW is applied. This means, it is save to rewrite
         * them to repair disk errors without any race conditions
         */
        path->search_commit_root = 1;
        path->skip_locking = 1;

        /*
         * trigger the readahead for extent tree csum tree and wait for
         * completion. During readahead, the scrub is officially paused
         * to not hold off transaction commits
         */
        logical = base + offset;

        wait_event(sctx->list_wait,
                   atomic_read(&sctx->bios_in_flight) == 0);
        atomic_inc(&fs_info->scrubs_paused);
        wake_up(&fs_info->scrub_pause_wait);

        /* FIXME it might be better to start readahead at commit root */
        key_start.objectid = logical;
        key_start.type = BTRFS_EXTENT_ITEM_KEY;
        key_start.offset = (u64)0;
        key_end.objectid = base + offset + nstripes * increment;
        key_end.type = BTRFS_EXTENT_ITEM_KEY;
        key_end.offset = (u64)0;
        reada1 = btrfs_reada_add(root, &key_start, &key_end);

        key_start.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
        key_start.type = BTRFS_EXTENT_CSUM_KEY;
        key_start.offset = logical;
        key_end.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
        key_end.type = BTRFS_EXTENT_CSUM_KEY;
        key_end.offset = base + offset + nstripes * increment;
        reada2 = btrfs_reada_add(csum_root, &key_start, &key_end);

        if (!IS_ERR(reada1))
                btrfs_reada_wait(reada1);
        if (!IS_ERR(reada2))
                btrfs_reada_wait(reada2);

        mutex_lock(&fs_info->scrub_lock);
        while (atomic_read(&fs_info->scrub_pause_req)) {
                mutex_unlock(&fs_info->scrub_lock);
                wait_event(fs_info->scrub_pause_wait,
                   atomic_read(&fs_info->scrub_pause_req) == 0);
                mutex_lock(&fs_info->scrub_lock);
        }
        atomic_dec(&fs_info->scrubs_paused);
        mutex_unlock(&fs_info->scrub_lock);
        wake_up(&fs_info->scrub_pause_wait);

        /*
         * collect all data csums for the stripe to avoid seeking during
         * the scrub. This might currently (crc32) end up to be about 1MB
         */
        blk_start_plug(&plug);

        /*
         * now find all extents for each stripe and scrub them
         */
        logical = base + offset;
        physical = map->stripes[num].physical;
        ret = 0;
        for (i = 0; i < nstripes; ++i) {
                /*
                 * canceled?
                 */
                if (atomic_read(&fs_info->scrub_cancel_req) ||
                    atomic_read(&sctx->cancel_req)) {
                        ret = -ECANCELED;
                        goto out;
                }
                /*
                 * check to see if we have to pause
                 */
                if (atomic_read(&fs_info->scrub_pause_req)) {
                        /* push queued extents */
                        scrub_submit(sctx);
                        wait_event(sctx->list_wait,
                                   atomic_read(&sctx->bios_in_flight) == 0);
                        atomic_inc(&fs_info->scrubs_paused);
                        wake_up(&fs_info->scrub_pause_wait);
                        mutex_lock(&fs_info->scrub_lock);
                        while (atomic_read(&fs_info->scrub_pause_req)) {
                                mutex_unlock(&fs_info->scrub_lock);
                                wait_event(fs_info->scrub_pause_wait,
                                   atomic_read(&fs_info->scrub_pause_req) == 0);
                                mutex_lock(&fs_info->scrub_lock);
                        }
                        atomic_dec(&fs_info->scrubs_paused);
                        mutex_unlock(&fs_info->scrub_lock);
                        wake_up(&fs_info->scrub_pause_wait);
                }

                ret = btrfs_lookup_csums_range(csum_root, logical,
                                               logical + map->stripe_len - 1,
                                               &sctx->csum_list, 1);
                if (ret)
                        goto out;

                key.objectid = logical;
                key.type = BTRFS_EXTENT_ITEM_KEY;
                key.offset = (u64)0;

                ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
                if (ret < 0)
                        goto out;
                if (ret > 0) {
                        ret = btrfs_previous_item(root, path, 0,
                                                  BTRFS_EXTENT_ITEM_KEY);
                        if (ret < 0)
                                goto out;
                        if (ret > 0) {
                                /* there's no smaller item, so stick with the
                                 * larger one */
                                btrfs_release_path(path);
                                ret = btrfs_search_slot(NULL, root, &key,
                                                        path, 0, 0);
                                if (ret < 0)
                                        goto out;
                        }
                }

                while (1) {
                        l = path->nodes[0];
                        slot = path->slots[0];
                        if (slot >= btrfs_header_nritems(l)) {
                                ret = btrfs_next_leaf(root, path);
                                if (ret == 0)
                                        continue;
                                if (ret < 0)
                                        goto out;

                                break;
                        }
                        btrfs_item_key_to_cpu(l, &key, slot);

                        if (key.objectid + key.offset <= logical)
                                goto next;

                        if (key.objectid >= logical + map->stripe_len)
                                break;

                        if (btrfs_key_type(&key) != BTRFS_EXTENT_ITEM_KEY)
                                goto next;

                        extent = btrfs_item_ptr(l, slot,
                                                struct btrfs_extent_item);
                        flags = btrfs_extent_flags(l, extent);
                        generation = btrfs_extent_generation(l, extent);

                        if (key.objectid < logical &&
                            (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)) {
                                printk(KERN_ERR
                                       "btrfs scrub: tree block %llu spanning "
                                       "stripes, ignored. logical=%llu\n",
                                       (unsigned long long)key.objectid,
                                       (unsigned long long)logical);
                                goto next;
                        }

                        /*
                         * trim extent to this stripe
                         */
                        if (key.objectid < logical) {
                                key.offset -= logical - key.objectid;
                                key.objectid = logical;
                        }
                        if (key.objectid + key.offset >
                            logical + map->stripe_len) {
                                key.offset = logical + map->stripe_len -
                                             key.objectid;
                        }

                        ret = scrub_extent(sctx, key.objectid, key.offset,
                                           key.objectid - logical + physical,
                                           scrub_dev, flags, generation,
                                           mirror_num);
                        if (ret)
                                goto out;

next:
                        path->slots[0]++;
                }
                btrfs_release_path(path);
                logical += increment;
                physical += map->stripe_len;
                spin_lock(&sctx->stat_lock);
                sctx->stat.last_physical = physical;
                spin_unlock(&sctx->stat_lock);
        }
        /* push queued extents */
        scrub_submit(sctx);

out:
        blk_finish_plug(&plug);
        btrfs_free_path(path);
        return ret < 0 ? ret : 0;
}

static noinline_for_stack int scrub_chunk(struct scrub_ctx *sctx,
                                          struct btrfs_device *scrub_dev,
                                          u64 chunk_tree, u64 chunk_objectid,
                                          u64 chunk_offset, u64 length,
                                          u64 dev_offset)
{
        struct btrfs_mapping_tree *map_tree =
                &sctx->dev_root->fs_info->mapping_tree;
        struct map_lookup *map;
        struct extent_map *em;
        int i;
        int ret = -EINVAL;

        read_lock(&map_tree->map_tree.lock);
        em = lookup_extent_mapping(&map_tree->map_tree, chunk_offset, 1);
        read_unlock(&map_tree->map_tree.lock);

        if (!em)
                return -EINVAL;

        map = (struct map_lookup *)em->bdev;
        if (em->start != chunk_offset)
                goto out;

        if (em->len < length)
                goto out;

        for (i = 0; i < map->num_stripes; ++i) {
                if (map->stripes[i].dev->bdev == scrub_dev->bdev &&
                    map->stripes[i].physical == dev_offset) {
                        ret = scrub_stripe(sctx, map, scrub_dev, i,
                                           chunk_offset, length);
                        if (ret)
                                goto out;
                }
        }
out:
        free_extent_map(em);

        return ret;
}

static noinline_for_stack
int scrub_enumerate_chunks(struct scrub_ctx *sctx,
                           struct btrfs_device *scrub_dev, u64 start, u64 end)
{
        struct btrfs_dev_extent *dev_extent = NULL;
        struct btrfs_path *path;
        struct btrfs_root *root = sctx->dev_root;
        struct btrfs_fs_info *fs_info = root->fs_info;
        u64 length;
        u64 chunk_tree;
        u64 chunk_objectid;
        u64 chunk_offset;
        int ret;
        int slot;
        struct extent_buffer *l;
        struct btrfs_key key;
        struct btrfs_key found_key;
        struct btrfs_block_group_cache *cache;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;

        path->reada = 2;
        path->search_commit_root = 1;
        path->skip_locking = 1;

        key.objectid = scrub_dev->devid;
        key.offset = 0ull;
        key.type = BTRFS_DEV_EXTENT_KEY;

        while (1) {
                ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
                if (ret < 0)
                        break;
                if (ret > 0) {
                        if (path->slots[0] >=
                            btrfs_header_nritems(path->nodes[0])) {
                                ret = btrfs_next_leaf(root, path);
                                if (ret)
                                        break;
                        }
                }

                l = path->nodes[0];
                slot = path->slots[0];

                btrfs_item_key_to_cpu(l, &found_key, slot);

                if (found_key.objectid != scrub_dev->devid)
                        break;

                if (btrfs_key_type(&found_key) != BTRFS_DEV_EXTENT_KEY)
                        break;

                if (found_key.offset >= end)
                        break;

                if (found_key.offset < key.offset)
                        break;

                dev_extent = btrfs_item_ptr(l, slot, struct btrfs_dev_extent);
                length = btrfs_dev_extent_length(l, dev_extent);

                if (found_key.offset + length <= start) {
                        key.offset = found_key.offset + length;
                        btrfs_release_path(path);
                        continue;
                }

                chunk_tree = btrfs_dev_extent_chunk_tree(l, dev_extent);
                chunk_objectid = btrfs_dev_extent_chunk_objectid(l, dev_extent);
                chunk_offset = btrfs_dev_extent_chunk_offset(l, dev_extent);

                /*
                 * get a reference on the corresponding block group to prevent
                 * the chunk from going away while we scrub it
                 */
                cache = btrfs_lookup_block_group(fs_info, chunk_offset);
                if (!cache) {
                        ret = -ENOENT;
                        break;
                }
                ret = scrub_chunk(sctx, scrub_dev, chunk_tree, chunk_objectid,
                                  chunk_offset, length, found_key.offset);
                btrfs_put_block_group(cache);
                if (ret)
                        break;

                key.offset = found_key.offset + length;
                btrfs_release_path(path);
        }

        btrfs_free_path(path);

        /*
         * ret can still be 1 from search_slot or next_leaf,
         * that's not an error
         */
        return ret < 0 ? ret : 0;
}

static noinline_for_stack int scrub_supers(struct scrub_ctx *sctx,
                                           struct btrfs_device *scrub_dev)
{
        int     i;
        u64     bytenr;
        u64     gen;
        int     ret;
        struct btrfs_root *root = sctx->dev_root;

        if (root->fs_info->fs_state & BTRFS_SUPER_FLAG_ERROR)
                return -EIO;

        gen = root->fs_info->last_trans_committed;

        for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; i++) {
                bytenr = btrfs_sb_offset(i);
                if (bytenr + BTRFS_SUPER_INFO_SIZE > scrub_dev->total_bytes)
                        break;

                ret = scrub_pages(sctx, bytenr, BTRFS_SUPER_INFO_SIZE, bytenr,
                                  scrub_dev, BTRFS_EXTENT_FLAG_SUPER, gen, i,
                                  NULL, 1);
                if (ret)
                        return ret;
        }
        wait_event(sctx->list_wait, atomic_read(&sctx->bios_in_flight) == 0);

        return 0;
}

/*
 * get a reference count on fs_info->scrub_workers. start worker if necessary
 */
static noinline_for_stack int scrub_workers_get(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        int ret = 0;

        mutex_lock(&fs_info->scrub_lock);
        if (fs_info->scrub_workers_refcnt == 0) {
                btrfs_init_workers(&fs_info->scrub_workers, "scrub",
                           fs_info->thread_pool_size, &fs_info->generic_worker);
                fs_info->scrub_workers.idle_thresh = 4;
                ret = btrfs_start_workers(&fs_info->scrub_workers);
                if (ret)
                        goto out;
        }
        ++fs_info->scrub_workers_refcnt;
out:
        mutex_unlock(&fs_info->scrub_lock);

        return ret;
}

static noinline_for_stack void scrub_workers_put(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;

        mutex_lock(&fs_info->scrub_lock);
        if (--fs_info->scrub_workers_refcnt == 0)
                btrfs_stop_workers(&fs_info->scrub_workers);
        WARN_ON(fs_info->scrub_workers_refcnt < 0);
        mutex_unlock(&fs_info->scrub_lock);
}


int btrfs_scrub_dev(struct btrfs_root *root, u64 devid, u64 start, u64 end,
                    struct btrfs_scrub_progress *progress, int readonly)
{
        struct scrub_ctx *sctx;
        struct btrfs_fs_info *fs_info = root->fs_info;
        int ret;
        struct btrfs_device *dev;

        if (btrfs_fs_closing(root->fs_info))
                return -EINVAL;

        /*
         * check some assumptions
         */
        if (root->nodesize != root->leafsize) {
                printk(KERN_ERR
                       "btrfs_scrub: size assumption nodesize == leafsize (%d == %d) fails\n",
                       root->nodesize, root->leafsize);
                return -EINVAL;
        }

        if (root->nodesize > BTRFS_STRIPE_LEN) {
                /*
                 * in this case scrub is unable to calculate the checksum
                 * the way scrub is implemented. Do not handle this
                 * situation at all because it won't ever happen.
                 */
                printk(KERN_ERR
                       "btrfs_scrub: size assumption nodesize <= BTRFS_STRIPE_LEN (%d <= %d) fails\n",
                       root->nodesize, BTRFS_STRIPE_LEN);
                return -EINVAL;
        }

        if (root->sectorsize != PAGE_SIZE) {
                /* not supported for data w/o checksums */
                printk(KERN_ERR
                       "btrfs_scrub: size assumption sectorsize != PAGE_SIZE (%d != %lld) fails\n",
                       root->sectorsize, (unsigned long long)PAGE_SIZE);
                return -EINVAL;
        }

        if (fs_info->chunk_root->nodesize >
            PAGE_SIZE * SCRUB_MAX_PAGES_PER_BLOCK ||
            fs_info->chunk_root->sectorsize >
            PAGE_SIZE * SCRUB_MAX_PAGES_PER_BLOCK) {
                /*
                 * would exhaust the array bounds of pagev member in
                 * struct scrub_block
                 */
                pr_err("btrfs_scrub: size assumption nodesize and sectorsize <= SCRUB_MAX_PAGES_PER_BLOCK (%d <= %d && %d <= %d) fails\n",
                       fs_info->chunk_root->nodesize,
                       SCRUB_MAX_PAGES_PER_BLOCK,
                       fs_info->chunk_root->sectorsize,
                       SCRUB_MAX_PAGES_PER_BLOCK);
                return -EINVAL;
        }

        ret = scrub_workers_get(root);
        if (ret)
                return ret;

        mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
        dev = btrfs_find_device(root, devid, NULL, NULL);
        if (!dev || dev->missing) {
                mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
                scrub_workers_put(root);
                return -ENODEV;
        }
        mutex_lock(&fs_info->scrub_lock);

        if (!dev->in_fs_metadata) {
                mutex_unlock(&fs_info->scrub_lock);
                mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
                scrub_workers_put(root);
                return -ENODEV;
        }

        if (dev->scrub_device) {
                mutex_unlock(&fs_info->scrub_lock);
                mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
                scrub_workers_put(root);
                return -EINPROGRESS;
        }
        sctx = scrub_setup_ctx(dev);
        if (IS_ERR(sctx)) {
                mutex_unlock(&fs_info->scrub_lock);
                mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
                scrub_workers_put(root);
                return PTR_ERR(sctx);
        }
        sctx->readonly = readonly;
        dev->scrub_device = sctx;

        atomic_inc(&fs_info->scrubs_running);
        mutex_unlock(&fs_info->scrub_lock);
        mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);

        down_read(&fs_info->scrub_super_lock);
        ret = scrub_supers(sctx, dev);
        up_read(&fs_info->scrub_super_lock);

        if (!ret)
                ret = scrub_enumerate_chunks(sctx, dev, start, end);

        wait_event(sctx->list_wait, atomic_read(&sctx->bios_in_flight) == 0);
        atomic_dec(&fs_info->scrubs_running);
        wake_up(&fs_info->scrub_pause_wait);

        wait_event(sctx->list_wait, atomic_read(&sctx->workers_pending) == 0);

        if (progress)
                memcpy(progress, &sctx->stat, sizeof(*progress));

        mutex_lock(&fs_info->scrub_lock);
        dev->scrub_device = NULL;
        mutex_unlock(&fs_info->scrub_lock);

        scrub_free_ctx(sctx);
        scrub_workers_put(root);

        return ret;
}

void btrfs_scrub_pause(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;

        mutex_lock(&fs_info->scrub_lock);
        atomic_inc(&fs_info->scrub_pause_req);
        while (atomic_read(&fs_info->scrubs_paused) !=
               atomic_read(&fs_info->scrubs_running)) {
                mutex_unlock(&fs_info->scrub_lock);
                wait_event(fs_info->scrub_pause_wait,
                           atomic_read(&fs_info->scrubs_paused) ==
                           atomic_read(&fs_info->scrubs_running));
                mutex_lock(&fs_info->scrub_lock);
        }
        mutex_unlock(&fs_info->scrub_lock);
}

void btrfs_scrub_continue(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;

        atomic_dec(&fs_info->scrub_pause_req);
        wake_up(&fs_info->scrub_pause_wait);
}

void btrfs_scrub_pause_super(struct btrfs_root *root)
{
        down_write(&root->fs_info->scrub_super_lock);
}

void btrfs_scrub_continue_super(struct btrfs_root *root)
{
        up_write(&root->fs_info->scrub_super_lock);
}

int __btrfs_scrub_cancel(struct btrfs_fs_info *fs_info)
{

        mutex_lock(&fs_info->scrub_lock);
        if (!atomic_read(&fs_info->scrubs_running)) {
                mutex_unlock(&fs_info->scrub_lock);
                return -ENOTCONN;
        }

        atomic_inc(&fs_info->scrub_cancel_req);
        while (atomic_read(&fs_info->scrubs_running)) {
                mutex_unlock(&fs_info->scrub_lock);
                wait_event(fs_info->scrub_pause_wait,
                           atomic_read(&fs_info->scrubs_running) == 0);
                mutex_lock(&fs_info->scrub_lock);
        }
        atomic_dec(&fs_info->scrub_cancel_req);
        mutex_unlock(&fs_info->scrub_lock);

        return 0;
}

int btrfs_scrub_cancel(struct btrfs_root *root)
{
        return __btrfs_scrub_cancel(root->fs_info);
}

int btrfs_scrub_cancel_dev(struct btrfs_root *root, struct btrfs_device *dev)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct scrub_ctx *sctx;

        mutex_lock(&fs_info->scrub_lock);
        sctx = dev->scrub_device;
        if (!sctx) {
                mutex_unlock(&fs_info->scrub_lock);
                return -ENOTCONN;
        }
        atomic_inc(&sctx->cancel_req);
        while (dev->scrub_device) {
                mutex_unlock(&fs_info->scrub_lock);
                wait_event(fs_info->scrub_pause_wait,
                           dev->scrub_device == NULL);
                mutex_lock(&fs_info->scrub_lock);
        }
        mutex_unlock(&fs_info->scrub_lock);

        return 0;
}

int btrfs_scrub_cancel_devid(struct btrfs_root *root, u64 devid)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_device *dev;
        int ret;

        /*
         * we have to hold the device_list_mutex here so the device
         * does not go away in cancel_dev. FIXME: find a better solution
         */
        mutex_lock(&fs_info->fs_devices->device_list_mutex);
        dev = btrfs_find_device(root, devid, NULL, NULL);
        if (!dev) {
                mutex_unlock(&fs_info->fs_devices->device_list_mutex);
                return -ENODEV;
        }
        ret = btrfs_scrub_cancel_dev(root, dev);
        mutex_unlock(&fs_info->fs_devices->device_list_mutex);

        return ret;
}

int btrfs_scrub_progress(struct btrfs_root *root, u64 devid,
                         struct btrfs_scrub_progress *progress)
{
        struct btrfs_device *dev;
        struct scrub_ctx *sctx = NULL;

        mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
        dev = btrfs_find_device(root, devid, NULL, NULL);
        if (dev)
                sctx = dev->scrub_device;
        if (sctx)
                memcpy(progress, &sctx->stat, sizeof(*progress));
        mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);

        return dev ? (sctx ? 0 : -ENOTCONN) : -ENODEV;
}