Messages in this thread Patch in this message | | | Date | Fri, 15 Apr 2011 11:12:48 +0100 | From | Mel Gorman <> | Subject | [PATCH] mm: Check if PTE is already allocated during page fault |
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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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