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>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/Kconfig b/fs/Kconfig
index 6725f59..b8fcb41 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -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