path: root/mm/memory.c
diff options
authorMel Gorman <>2011-10-12 21:06:51 +0200
committerMel Gorman <>2012-12-11 14:28:34 +0000
commit4fd017708c4a067da51a2b5cf8aedddf4e840b1f (patch)
treec2b3c9f42a957a0d54fd95411fb3c5626a7efc14 /mm/memory.c
parent8d1acce4537c4e2f5889ed9ba9b8eddb80d99820 (diff)
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 <>
Diffstat (limited to 'mm/memory.c')
1 files changed, 2 insertions, 1 deletions
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)))