Commit 348e967a authored May 12, 2016 by Jan Kara Committed by Ross Zwisler May 19, 2016

dax: Make huge page handling depend of CONFIG_BROKEN



Currently the handling of huge pages for DAX is racy. For example the
following can happen:

CPU0 (THP write fault)			CPU1 (normal read fault)

__dax_pmd_fault()			__dax_fault()
  get_block(inode, block, &bh, 0) -> not mapped
					get_block(inode, block, &bh, 0)
					  -> not mapped
  if (!buffer_mapped(&bh) && write)
    get_block(inode, block, &bh, 1) -> allocates blocks
  truncate_pagecache_range(inode, lstart, lend);
					dax_load_hole();

This results in data corruption since process on CPU1 won't see changes
into the file done by CPU0.

The race can happen even if two normal faults race however with THP the
situation is even worse because the two faults don't operate on the same
entries in the radix tree and we want to use these entries for
serialization. So make THP support in DAX code depend on CONFIG_BROKEN
for now.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>

parent b9953536

fs/Kconfig

+1 −0

Original line number	Diff line number	Diff line
		@@ -52,6 +52,7 @@ config FS_DAX_PMD
		depends on FS_DAX
		depends on ZONE_DEVICE
		depends on TRANSPARENT_HUGEPAGE
		depends on BROKEN

		endif # BLOCK

fs/dax.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -675,7 +675,7 @@ int dax_fault(struct vm_area_struct vma, struct vm_fault vmf,
		}
		EXPORT_SYMBOL_GPL(dax_fault);

		#ifdef CONFIG_TRANSPARENT_HUGEPAGE
		#if defined(CONFIG_TRANSPARENT_HUGEPAGE)
		/*
		* The 'colour' (ie low bits) within a PMD of a page offset. This comes up
		* more often than one might expect in the below function.

include/linux/dax.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -29,7 +29,7 @@ static inline int __dax_zero_page_range(struct block_device *bdev,
		}
		#endif

		#ifdef CONFIG_TRANSPARENT_HUGEPAGE
		#if defined(CONFIG_TRANSPARENT_HUGEPAGE)
		int dax_pmd_fault(struct vm_area_struct , unsigned long addr, pmd_t ,
		unsigned int flags, get_block_t);
		int __dax_pmd_fault(struct vm_area_struct , unsigned long addr, pmd_t ,

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