From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759292Ab1D0Nu7 (ORCPT ); Wed, 27 Apr 2011 09:50:59 -0400 Received: from zene.cmpxchg.org ([85.214.230.12]:46513 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759258Ab1D0Nu6 (ORCPT ); Wed, 27 Apr 2011 09:50:58 -0400 Date: Wed, 27 Apr 2011 15:50:40 +0200 From: Johannes Weiner 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 Subject: Re: [PATCH] mm: Check if PTE is already allocated during page fault Message-ID: <20110427135040.GA12437@cmpxchg.org> References: <20110415101248.GB22688@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110415101248.GB22688@suse.de> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 15, 2011 at 11:12:48AM +0100, 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 Acked-by: Johannes Weiner