Merge tag 'hwmon-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/groeck...

[~andy/linux] / arch / x86 / mm / init.c

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index fdc5dca14fb35de5537765acca869ccfb447262d..eaac1743def7ea70cccda0926e53fc2e343f7f06 100644 (file)
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -359,7 +359,17 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
 }
 
 /*
- * would have hole in the middle or ends, and only ram parts will be mapped.
+ * We need to iterate through the E820 memory map and create direct mappings
+ * for only E820_RAM and E820_KERN_RESERVED regions. We cannot simply
+ * create direct mappings for all pfns from [0 to max_low_pfn) and
+ * [4GB to max_pfn) because of possible memory holes in high addresses
+ * that cannot be marked as UC by fixed/variable range MTRRs.
+ * Depending on the alignment of E820 ranges, this may possibly result
+ * in using smaller size (i.e. 4K instead of 2M or 1G) page tables.
+ *
+ * init_mem_mapping() calls init_range_memory_mapping() with big range.
+ * That range would have hole in the middle or ends, and only ram parts
+ * will be mapped in init_range_memory_mapping().
  */
 static unsigned long __init init_range_memory_mapping(
                                           unsigned long r_start,
@@ -419,6 +429,13 @@ void __init init_mem_mapping(void)
        max_pfn_mapped = 0; /* will get exact value next */
        min_pfn_mapped = real_end >> PAGE_SHIFT;
        last_start = start = real_end;
+
+       /*
+        * We start from the top (end of memory) and go to the bottom.
+        * The memblock_find_in_range() gets us a block of RAM from the
+        * end of RAM in [min_pfn_mapped, max_pfn_mapped) used as new pages
+        * for page table.
+        */
        while (last_start > ISA_END_ADDRESS) {
                if (last_start > step_size) {
                        start = round_down(last_start - 1, step_size);