Commit cc03638df20acbec5d0d0d9e07234aadde9e698d
Committed by
Linus Torvalds
1 parent
1409f141ac
Exists in
master
and in
4 other branches
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@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> Cc: <stable@kernel.org> [2.6.38.x] Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Showing 1 changed file with 1 additions and 1 deletions Side-by-side Diff
mm/memory.c
... | ... | @@ -3396,7 +3396,7 @@ |
3396 | 3396 | * run pte_offset_map on the pmd, if an huge pmd could |
3397 | 3397 | * materialize from under us from a different thread. |
3398 | 3398 | */ |
3399 | - if (unlikely(__pte_alloc(mm, vma, pmd, address))) | |
3399 | + if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vma, pmd, address)) | |
3400 | 3400 | return VM_FAULT_OOM; |
3401 | 3401 | /* if an huge pmd materialized from under us just retry later */ |
3402 | 3402 | if (unlikely(pmd_trans_huge(*pmd))) |