On Thu, Apr 21, 2011 at 03:59:47PM +0900, Minchan Kim wrote: > Hi Mel, > > On Fri, Apr 15, 2011 at 7:12 PM, Mel Gorman <mgorman@xxxxxxx> 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@xxxxxxxxx> > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx> > > -- > > 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@xxxxxxxxx> > > Sorry for jumping in too late. I have a just nitpick. > Better late than never :) > 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. We already checked pmd_none() in handle_mm_fault() before calling into do_huge_pmd_anonymous_page(). We could race for the fault while attempting to allocate a huge page but it wouldn't be as severe a problem particularly as it is encountered after failing a 2M allocation. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>