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 >        Â2.6.38-vanilla 2.6.38-checkptenone > creat-clo   Â446.10 ( 0.00%)  424.47 (-5.10%) > page_test    38.10 ( 0.00%)  Â42.04 ( 9.37%) > brk_test    Â52.45 ( 0.00%)  Â51.57 (-1.71%) > exec_test   Â382.00 ( 0.00%)  456.90 (16.39%) > fork_test    60.11 ( 0.00%)  Â67.79 (11.34%) > MMTests Statistics: duration > Total Elapsed Time (seconds)        Â611.90  Â612.22 > > (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> > -- > Âmm/memory.c |  Â2 +- > Â1 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)) >        Âreturn VM_FAULT_OOM; >    Â/* if an huge pmd materialized from under us just retry later */ >    Âif (unlikely(pmd_trans_huge(*pmd))) > Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx> 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, 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