From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with ESMTP id C20618D003B for ; Thu, 21 Apr 2011 10:26:52 -0400 (EDT) Received: by pvg4 with SMTP id 4so1242219pvg.14 for ; Thu, 21 Apr 2011 07:26:47 -0700 (PDT) Date: Thu, 21 Apr 2011 23:26:36 +0900 From: Minchan Kim Subject: Re: [PATCH] mm: Check if PTE is already allocated during page fault Message-ID: <20110421142636.GA1835@barrios-desktop> References: <20110415101248.GB22688@suse.de> <20110421110841.GA612@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20110421110841.GA612@suse.de> Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: akpm@linux-foundation.org, Andrea Arcangeli , raz ben yehuda , riel@redhat.com, kosaki.motohiro@jp.fujitsu.com, lkml , linux-mm@kvack.org, stable@kernel.org 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 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 > > > Signed-off-by: Mel Gorman > > > -- > > > 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 > > > > 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. 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. 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)) 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. Thanks, Mel. > > -- > Mel Gorman > SUSE Labs -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . 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