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SubjectRe: [PATCH] mm: Check if PTE is already allocated during page fault
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@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.
> > >
> >
> > 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
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