From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with ESMTP id 2EC7C8D003B for ; Thu, 21 Apr 2011 02:59:51 -0400 (EDT) Received: by iyh42 with SMTP id 42so1805262iyh.14 for ; Wed, 20 Apr 2011 23:59:47 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20110415101248.GB22688@suse.de> References: <20110415101248.GB22688@suse.de> Date: Thu, 21 Apr 2011 15:59:47 +0900 Message-ID: Subject: Re: [PATCH] mm: Check if PTE is already allocated during page fault From: Minchan Kim Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable 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 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 > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A02.6.38-vanilla 2.6= .38-checkptenone > creat-clo =C2=A0 =C2=A0 =C2=A0446.10 ( 0.00%) =C2=A0 424.47 (-5.10%) > page_test =C2=A0 =C2=A0 =C2=A0 38.10 ( 0.00%) =C2=A0 =C2=A042.04 ( 9.37%) > brk_test =C2=A0 =C2=A0 =C2=A0 =C2=A052.45 ( 0.00%) =C2=A0 =C2=A051.57 (-1= .71%) > exec_test =C2=A0 =C2=A0 =C2=A0382.00 ( 0.00%) =C2=A0 456.90 (16.39%) > fork_test =C2=A0 =C2=A0 =C2=A0 60.11 ( 0.00%) =C2=A0 =C2=A067.79 (11.34%) > MMTests Statistics: duration > Total Elapsed Time (seconds) =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0611.90 =C2=A0 =C2=A0612.22 > > (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 > -- > =C2=A0mm/memory.c | =C2=A0 =C2=A02 +- > =C2=A01 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, > =C2=A0 =C2=A0 =C2=A0 =C2=A0 * run pte_offset_map on the pmd, if an huge p= md could > =C2=A0 =C2=A0 =C2=A0 =C2=A0 * materialize from under us from a different = thread. > =C2=A0 =C2=A0 =C2=A0 =C2=A0 */ > - =C2=A0 =C2=A0 =C2=A0 if (unlikely(__pte_alloc(mm, vma, pmd, address))) > + =C2=A0 =C2=A0 =C2=A0 if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vm= a, pmd, address)) > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return VM_FAULT_OO= M; > =C2=A0 =C2=A0 =C2=A0 =C2=A0/* if an huge pmd materialized from under us j= ust retry later */ > =C2=A0 =C2=A0 =C2=A0 =C2=A0if (unlikely(pmd_trans_huge(*pmd))) > Reviewed-by: Minchan Kim 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. --=20 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org