The patch titled
Subject: mm: thp: fix SMP race condition between THP page fault and MADV_DONTNEED
has been added to the -mm tree. Its filename is
mm-thp-fix-smp-race-condition-between-thp-page-fault-and-madv_dontneed.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-thp-fix-smp-race-condition-between-thp-page-fault-and-madv_dontneed.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-thp-fix-smp-race-condition-between-thp-page-fault-and-madv_dontneed.patch
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------------------------------------------------------
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Subject: 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.
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Reported-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memory.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff -puN mm/memory.c~mm-thp-fix-smp-race-condition-between-thp-page-fault-and-madv_dontneed mm/memory.c
--- a/mm/memory.c~mm-thp-fix-smp-race-condition-between-thp-page-fault-and-madv_dontneed
+++ a/mm/memory.c
@@ -3404,8 +3404,19 @@ static int __handle_mm_fault(struct mm_s
if (unlikely(pmd_none(*pmd)) &&
unlikely(__pte_alloc(mm, vma, pmd, address)))
return VM_FAULT_OOM;
- /* if an huge pmd materialized from under us just retry later */
- if (unlikely(pmd_trans_huge(*pmd) || pmd_devmap(*pmd)))
+ /*
+ * If an huge pmd materialized from under us just retry later.
+ * Use pmd_trans_unstable() instead of pmd_trans_huge() to
+ * ensure the pmd didn't become pmd_trans_huge from under us
+ * and then back to pmd_none, as result of MADV_DONTNEED
+ * running immediately after a huge pmd fault of a different
+ * thread of this mm, in turn leading to a misleading
+ * pmd_trans_huge() retval. All we have to ensure is that it
+ * is a regular pmd that we can walk with pte_offset_map() and
+ * we can do that through an atomic read in C, which is what
+ * pmd_trans_unstable() is provided for.
+ */
+ if (unlikely(pmd_trans_unstable(pmd) || pmd_devmap(*pmd)))
return 0;
/*
* A regular pmd is established and it can't morph into a huge pmd
_
Patches currently in -mm which might be from aarcange@xxxxxxxxxx are
mm-thp-fix-smp-race-condition-between-thp-page-fault-and-madv_dontneed.patch
ksm-introduce-ksm_max_page_sharing-per-page-deduplication-limit.patch
ksm-introduce-ksm_max_page_sharing-per-page-deduplication-limit-fix-3.patch
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