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authorAndrea Arcangeli <aarcange@redhat.com>2016-02-26 15:19:28 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2016-02-27 10:28:52 -0800
commitad33bb04b2a6cee6c1f99fabb15cddbf93ff0433 (patch)
treef38c333dcb92d8b558e48ab873d58e27f0d24057 /mm/migrate.c
parent29a9faa641857425af76e44f2fab22db53032401 (diff)
mm: thp: fix SMP race condition between THP page fault and MADV_DONTNEED
pmd_trans_unstable()/pmd_none_or_trans_huge_or_clear_bad() were introduced to locklessy (but atomically) detect when a pmd is a regular (stable) pmd or when the pmd is unstable and can infinitely transition from pmd_none() and pmd_trans_huge() from under us, while only holding the mmap_sem for reading (for writing not). While holding the mmap_sem only for reading, MADV_DONTNEED can run from under us and so before we can assume the pmd to be a regular stable pmd we need to compare it against pmd_none() and pmd_trans_huge() in an atomic way, with pmd_trans_unstable(). The old pmd_trans_huge() left a tiny window for a race. Useful applications are unlikely to notice the difference as doing MADV_DONTNEED concurrently with a page fault would lead to undefined behavior. [akpm@linux-foundation.org: tidy up comment grammar/layout] Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> Reported-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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