From: Mel Gorman Date: Wed, 12 Oct 2011 19:06:51 +0000 (+0200) Subject: mm: Check if PTE is already allocated during page fault X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=4fd017708c4a067da51a2b5cf8aedddf4e840b1f;p=linux-beck.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 Signed-off-by: Andrea Arcangeli Cc: Linus Torvalds Cc: Andrew Morton Cc: Peter Zijlstra Cc: Rik van Riel [ Picked this up from the AutoNUMA tree to help it upstream and to allow apples-to-apples performance comparisons. ] Signed-off-by: Ingo Molnar --- diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 40f17c34b415..35c66a269bcc 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -710,7 +710,8 @@ out: * 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)) && + unlikely(__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))) diff --git a/mm/memory.c b/mm/memory.c index 221fc9ffcab1..7cf762857baa 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3560,7 +3560,8 @@ retry: * run pte_offset_map on the pmd, if an huge pmd could * materialize from under us from a different thread. */ - if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vma, pmd, address)) + if (unlikely(pmd_none(*pmd)) && + unlikely(__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)))