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    Subject[41/55] mm: check if PTE is already allocated during page fault
    2.6.38-stable review patch.  If anyone has any objections, please let us know.


    From: Mel Gorman <>

    commit cc03638df20acbec5d0d0d9e07234aadde9e698d upstream.

    With transparent hugepage support, handle_mm_fault() has to be careful
    that a normal PMD has been established before handling a PTE fault. To
    achieve this, it used __pte_alloc() directly instead of pte_alloc_map as
    pte_alloc_map is unsafe to run against a huge PMD. pte_offset_map() is
    called once it is known the PMD is safe.

    pte_alloc_map() is smart enough to check if a PTE is already present
    before calling __pte_alloc but this check was lost. As a consequence,
    PTEs may be allocated unnecessarily and the page table lock taken. Thi
    useless PTE does get cleaned up but it's a performance hit which is
    visible in page_test from aim9.

    This patch simply re-adds the check normally done by pte_alloc_map to
    check if the PTE needs to be allocated before taking the page table lock.
    The effect is noticable in page_test from aim9.

    2.6.38-vanilla 2.6.38-checkptenone
    creat-clo 446.10 ( 0.00%) 424.47 (-5.10%)
    page_test 38.10 ( 0.00%) 42.04 ( 9.37%)
    brk_test 52.45 ( 0.00%) 51.57 (-1.71%)
    exec_test 382.00 ( 0.00%) 456.90 (16.39%)
    fork_test 60.11 ( 0.00%) 67.79 (11.34%)
    MMTests Statistics: duration
    Total Elapsed Time (seconds) 611.90 612.22

    (While this affects 2.6.38, it is a performance rather than a functional
    bug and normally outside the rules -stable. While the big performance
    differences are to a microbench, the difference in fork and exec
    performance may be significant enough that -stable wants to consider the

    Reported-by: Raz Ben Yehuda <>
    Signed-off-by: Mel Gorman <>
    Reviewed-by: Rik van Riel <>
    Reviewed-by: Andrea Arcangeli <>
    Reviewed-by: Minchan Kim <>
    Acked-by: Johannes Weiner <>
    Signed-off-by: Andrew Morton <>
    Signed-off-by: Linus Torvalds <>
    Signed-off-by: Greg Kroah-Hartman <>

    mm/memory.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -3332,7 +3332,7 @@ int handle_mm_fault(struct mm_struct *mm
    * run pte_offset_map on the pmd, if an huge pmd could
    * materialize from under us from a different thread.
    - if (unlikely(__pte_alloc(mm, vma, pmd, address)))
    + if (unlikely(pmd_none(*pmd)) && __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)))

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      Last update: 2011-04-29 21:05    [W:0.021 / U:0.088 seconds]
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