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>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Rik van Riel <riel@redhat.com>
[ 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@kernel.org>
2 files changed