[PATCH] readahead: backoff on I/O error

[~andy/linux] / mm / filemap.c

diff --git a/mm/filemap.c b/mm/filemap.c
index 1ed4be2a7654084743042058e6c9715aa80880b6..9c7334bafda8da5150f4a9f0a7556bda113b8a19 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -828,6 +828,32 @@ grab_cache_page_nowait(struct address_space *mapping, unsigned long index)
 }
 EXPORT_SYMBOL(grab_cache_page_nowait);
 
+/*
+ * CD/DVDs are error prone. When a medium error occurs, the driver may fail
+ * a _large_ part of the i/o request. Imagine the worst scenario:
+ *
+ *      ---R__________________________________________B__________
+ *         ^ reading here                             ^ bad block(assume 4k)
+ *
+ * read(R) => miss => readahead(R...B) => media error => frustrating retries
+ * => failing the whole request => read(R) => read(R+1) =>
+ * readahead(R+1...B+1) => bang => read(R+2) => read(R+3) =>
+ * readahead(R+3...B+2) => bang => read(R+3) => read(R+4) =>
+ * readahead(R+4...B+3) => bang => read(R+4) => read(R+5) => ......
+ *
+ * It is going insane. Fix it by quickly scaling down the readahead size.
+ */
+static void shrink_readahead_size_eio(struct file *filp,
+                                       struct file_ra_state *ra)
+{
+       if (!ra->ra_pages)
+               return;
+
+       ra->ra_pages /= 4;
+       printk(KERN_WARNING "Reducing readahead size to %luK\n",
+                       ra->ra_pages << (PAGE_CACHE_SHIFT - 10));
+}
+
 /**
  * do_generic_mapping_read - generic file read routine
  * @mapping:   address_space to be read
@@ -985,6 +1011,7 @@ readpage:
                                }
                                unlock_page(page);
                                error = -EIO;
+                               shrink_readahead_size_eio(filp, &ra);
                                goto readpage_error;
                        }
                        unlock_page(page);
@@ -1522,6 +1549,7 @@ page_not_uptodate:
         * Things didn't work out. Return zero to tell the
         * mm layer so, possibly freeing the page cache page first.
         */
+       shrink_readahead_size_eio(file, ra);
        page_cache_release(page);
        return NULL;
 }