On Thu, Apr 21, 2011 at 11:26:36PM +0900, Minchan Kim wrote: > On Thu, Apr 21, 2011 at 12:08:41PM +0100, Mel Gorman wrote: > > 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. > > Right you are. Fail ot 2M allocation would affect as throttle. > Thanks. > > As I failed let you add the check, I have to reveal my mind. :) > Actually, what I want is consistency of the code. This is a stronger arguement than as a performance fix. I was concerned that if such a check was added that it would confuse someone in a years time trying to figure out why the pmd_none check was really necessary. > The code have been same in two places but you find the problem in page_test of aim9, > you changed one of them slightly. I think in future someone will > have a question about that and he will start grep git log but it will take > a long time as the log is buried other code piled up. > Fair point. > I hope adding the comment in this case. > > /* > * PTEs may be allocated unnecessarily and the page table lock taken. > * The useless PTE does get cleaned up but it's a performance hit in > * some micro-benchmark. Let's check pmd_none before __pte_alloc to > * reduce the overhead. > */ > - if (unlikely(__pte_alloc(mm, vma, pmd, address))) > + if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vma, pmd, address)) > I think a better justification is /* * Even though handle_mm_fault has already checked pmd_none, we * have failed a huge allocation at this point during which a * valid PTE could have been inserted. Double check a PTE alloc * is still necessary to avoid additional overhead */ > If you mind it as someone who have a question can find the log at last > although he need some time, I wouldn't care of the nitpick any more. :) > It's up to you. > If you want to create a new patch with either your comment or mine (whichever you prefer) I'll add my ack. I'm about to drop offline for a few days but if it's still there Tuesday, I'll put together an appropriate patch and submit. I'd keep it separate from the other patch because it's a performance fix (which I'd like to see in -stable) where as this is more of a cleanup IMO. Thanks -- 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>