From: Mel Gorman Date: Wed, 27 Apr 2011 22:26:56 +0000 (-0700) Subject: mm: check if PTE is already allocated during page fault X-Git-Tag: firefly_0821_release~7613^2~1527 X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=cc03638df20acbec5d0d0d9e07234aadde9e698d;p=firefly-linux-kernel-4.4.55.git 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 Signed-off-by: Mel Gorman Reviewed-by: Rik van Riel Reviewed-by: Andrea Arcangeli Reviewed-by: Minchan Kim Acked-by: Johannes Weiner Cc: [2.6.38.x] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/mm/memory.c b/mm/memory.c index ce22a250926f..607098d47e74 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3396,7 +3396,7 @@ 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)))