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    Subject[PATCH 3.8 81/81] mm: fix BUG in __split_huge_page_pmd
    Date
    3.8.13.12 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Hugh Dickins <hughd@google.com>

    commit 750e8165f5e87b6a142be953640eabb13a9d350a upstream.

    Occasionally we hit the BUG_ON(pmd_trans_huge(*pmd)) at the end of
    __split_huge_page_pmd(): seen when doing madvise(,,MADV_DONTNEED).

    It's invalid: we don't always have down_write of mmap_sem there: a racing
    do_huge_pmd_wp_page() might have copied-on-write to another huge page
    before our split_huge_page() got the anon_vma lock.

    Forget the BUG_ON, just go back and try again if this happens.

    Signed-off-by: Hugh Dickins <hughd@google.com>
    Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    Cc: David Rientjes <rientjes@google.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    mm/huge_memory.c | 10 +++++++++-
    1 file changed, 9 insertions(+), 1 deletion(-)

    diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    index e9308d3..a057a7d 100644
    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -2740,6 +2740,7 @@ void __split_huge_page_pmd(struct vm_area_struct *vma, unsigned long address,

    mmun_start = haddr;
    mmun_end = haddr + HPAGE_PMD_SIZE;
    +again:
    mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
    spin_lock(&mm->page_table_lock);
    if (unlikely(!pmd_trans_huge(*pmd))) {
    @@ -2762,7 +2763,14 @@ void __split_huge_page_pmd(struct vm_area_struct *vma, unsigned long address,
    split_huge_page(page);

    put_page(page);
    - BUG_ON(pmd_trans_huge(*pmd));
    +
    + /*
    + * We don't always have down_write of mmap_sem here: a racing
    + * do_huge_pmd_wp_page() might have copied-on-write to another
    + * huge page before our split_huge_page() got the anon_vma lock.
    + */
    + if (unlikely(pmd_trans_huge(*pmd)))
    + goto again;
    }

    void split_huge_page_pmd_mm(struct mm_struct *mm, unsigned long address,
    --
    1.8.1.2


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