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Subject[41/55] mm: check if PTE is already allocated during page fault
2.6.38-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Mel Gorman <mgorman@suse.de>

commit cc03638df20acbec5d0d0d9e07234aadde9e698d upstream.

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>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
mm/memory.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3332,7 +3332,7 @@ int handle_mm_fault(struct mm_struct *mm
* 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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