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    SubjectRe: [PATCH] mm: Check if PTE is already allocated during page fault
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    Hi Mel,

    On Fri, Apr 15, 2011 at 7:12 PM, Mel Gorman <mgorman@suse.de> wrote:
    > 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)))
    >

    Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

    Sorry for jumping in too late. I have a just nitpick.

    We have another place, do_huge_pmd_anonymous_page.
    Although it isn't workload of page_test, is it valuable to expand your
    patch to cover it?
    If there is workload there are many thread and share one shared anon
    vma in ALWAYS THP mode, same problem would happen.


    --
    Kind regards,
    Minchan Kim
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