Messages in this thread | | | Date | Thu, 21 Apr 2011 15:59:47 +0900 | Subject | Re: [PATCH] mm: Check if PTE is already allocated during page fault | From | Minchan Kim <> |
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Hi Mel,
On Fri, Apr 15, 2011 at 7:12 PM, Mel Gorman <mgorman@suse.de> wrote: > 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> > -- > mm/memory.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 5823698..1659574 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -3322,7 +3322,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma, > * 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))) >
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Sorry for jumping in too late. I have a just nitpick.
We have another place, do_huge_pmd_anonymous_page. Although it isn't workload of page_test, is it valuable to expand your patch to cover it? If there is workload there are many thread and share one shared anon vma in ALWAYS THP mode, same problem would happen.
-- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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