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    Subject[PATCH] mm: Check if PTE is already allocated during page fault
    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.

    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 patch)

    Reported-by: Raz Ben Yehuda <raziebe@gmail.com>
    Signed-off-by: Mel Gorman <mgorman@suse.de>
    --
    mm/memory.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/mm/memory.c b/mm/memory.c
    index 5823698..1659574 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -3322,7 +3322,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
    * 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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