Commit-ID: b0a798a54f8a80c34887ef17a81843ec60669b3d
Gitweb: http://git.kernel.org/tip/b0a798a54f8a80c34887ef17a81843ec60669b3d
Author: Mel Gorman <mgorman@xxxxxxx>
AuthorDate: Wed, 12 Oct 2011 21:06:51 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 23 Oct 2012 11:41:27 +0200
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 <raziebe@xxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
[ Picked this up from the AutoNUMA tree to help
it upstream and to allow apples-to-apples
performance comparisons. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
mm/huge_memory.c | 3 ++-
mm/memory.c | 3 ++-
2 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 40f17c3..35c66a2 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 fb135ba..a241ce3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3562,7 +3562,8 @@ 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)))
--
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