mm: migrate: check page_count of THP before migrating
Hugh Dickins pointed out that migrate_misplaced_transhuge_page() does not check page_count before migrating like base page migration and khugepage. He could not see why this was safe and he is right. The potential impact of the bug is avoided due to the limitations of NUMA balancing. The page_mapcount() check ensures that only a single address space is using this page and as THPs are typically private it should not be possible for another address space to fault it in parallel. If the address space has one associated task then it's difficult to have both a GUP pin and be referencing the page at the same time. If there are multiple tasks then a buggy scenario requires that another thread be accessing the page while the direct IO is in flight. This is dodgy behaviour as there is a possibility of corruption with or without THP migration. It would be While we happen to be safe for the most part it is shoddy to depend on such "safety" so this patch checks the page count similar to anonymous pages. Note that this does not mean that the page_mapcount() check can go away. If we were to remove the page_mapcount() check the the THP would have to be unmapped from all referencing PTEs, replaced with migration PTEs and restored properly afterwards. Signed-off-by: NMel Gorman <mgorman@suse.de> Reported-by: NHugh Dickins <hughd@google.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Andrea Arcangeli <aarcange@redhat.com> Acked-by: NHugh Dickins <hughd@google.com> Signed-off-by: NAndrew Morton <akpm@linux-foundation.org> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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