From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta8.messagelabs.com (mail6.bemta8.messagelabs.com [216.82.243.55]) by kanga.kvack.org (Postfix) with ESMTP id EDF3D6B0069 for ; Fri, 18 Nov 2011 11:11:31 -0500 (EST) Date: Fri, 18 Nov 2011 17:11:28 +0100 From: Michal Hocko Subject: Re: [PATCH] hugetlb: detect race if fail to COW Message-ID: <20111118161128.GC23223@tiehlicka.suse.cz> References: <20111118150742.GA23223@tiehlicka.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Hillf Danton Cc: Andrea Arcangeli , Andrew Morton , Johannes Weiner , linux-mm@kvack.org, LKML On Fri 18-11-11 23:23:12, Hillf Danton wrote: > On Fri, Nov 18, 2011 at 11:07 PM, Michal Hocko wrote: > > On Fri 18-11-11 22:04:37, Hillf Danton wrote: > >> In the error path that we fail to allocate new huge page, before try again, we > >> have to check race since page_table_lock is re-acquired. > > > > I do not think we can race here because we are serialized by > > hugetlb_instantiation_mutex AFAIU. Without this lock, however, we could > > fall into avoidcopy and shortcut despite the fact that other thread has > > already did the job. > > > > The mutex usage is not obvious in hugetlb_cow so maybe we want to be > > explicit about it (either a comment or do the recheck). > > > > Then the following check is unnecessary, no? Hmm, thinking about it some more, I guess we have to recheck because we can still race with page migration. So we need you patch. Reviewed-by: Michal Hocko > > Thanks, > Hillf > > /* > * Retake the page_table_lock to check for racing updates > * before the page tables are altered > */ > spin_lock(&mm->page_table_lock); > ptep = huge_pte_offset(mm, address & huge_page_mask(h)); > if (likely(pte_same(huge_ptep_get(ptep), pte))) { > /* Break COW */ > > -- > 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 -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- 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