From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753750Ab2HBMhE (ORCPT ); Thu, 2 Aug 2012 08:37:04 -0400 Received: from cantor2.suse.de ([195.135.220.15]:35291 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753053Ab2HBMhC (ORCPT ); Thu, 2 Aug 2012 08:37:02 -0400 Date: Thu, 2 Aug 2012 14:36:58 +0200 From: Michal Hocko To: Mel Gorman Cc: Larry Woodman , Rik van Riel , Hugh Dickins , Linux-MM , David Gibson , Ken Chen , Cong Wang , LKML Subject: Re: [PATCH -alternative] mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables V2 (resend) Message-ID: <20120802123658.GA5194@dhcp22.suse.cz> References: <5016DC5F.7030604@redhat.com> <20120731124650.GO612@suse.de> <50181AA1.0@redhat.com> <20120731200650.GB19524@tiehlicka.suse.cz> <50189857.4000501@redhat.com> <20120801082036.GC4436@tiehlicka.suse.cz> <20120801123209.GK4436@tiehlicka.suse.cz> <501945F9.2030402@redhat.com> <20120802071934.GA7557@dhcp22.suse.cz> <20120802073757.GC29814@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120802073757.GC29814@suse.de> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 02-08-12 08:37:57, Mel Gorman wrote: > On Thu, Aug 02, 2012 at 09:19:34AM +0200, Michal Hocko wrote: [...] > > On the other hand, mine is more coupled with the sharing code so it > > makes the code easier to follow and also makes the sharing more > > effective because racing processes see pmd populated when checking for > > shareable mappings. > > > > It could do with a small comment above huge_pmd_share() explaining that > calling pmd_alloc() under the i_mmap_mutex is necessary to prevent two > parallel faults missing a sharing opportunity with each other but it's > not mandatory. Sure, that's a good idea. What about the following: diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c index 40b2500..51839d1 100644 --- a/arch/x86/mm/hugetlbpage.c +++ b/arch/x86/mm/hugetlbpage.c @@ -56,7 +56,13 @@ static int vma_shareable(struct vm_area_struct *vma, unsigned long addr) } /* - * search for a shareable pmd page for hugetlb. + * search for a shareable pmd page for hugetlb. In any case calls + * pmd_alloc and returns the corresponding pte. While this not necessary + * for the !shared pmd case because we can allocate the pmd later as + * well it makes the code much cleaner. pmd allocation is essential for + * the shared case though because pud has to be populated inside the + * same i_mmap_mutex section otherwise racing tasks could either miss + * the sharing (see huge_pte_offset) or selected a bad pmd for sharing. */ static pte_t* huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud) > > > So I am more inclined to mine but I don't want to push it because both > > are good and make sense. What other people think? > > > > I vote yours > > Reviewed-by: Mel Gorman Thanks! -- Michal Hocko SUSE Labs