From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with ESMTP id 9A64A6B006C for ; Mon, 21 Nov 2011 09:16:31 -0500 (EST) Date: Mon, 21 Nov 2011 15:16:25 +0100 From: Michal Hocko Subject: Re: [PATCH] hugetlb: detect race if fail to COW Message-ID: <20111121141625.GB16957@tiehlicka.suse.cz> References: <20111118150742.GA23223@tiehlicka.suse.cz> <20111118161128.GC23223@tiehlicka.suse.cz> <20111121122303.GA13594@tiehlicka.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111121122303.GA13594@tiehlicka.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Hillf Danton Cc: Andrea Arcangeli , Andrew Morton , Johannes Weiner , linux-mm@kvack.org, LKML On Mon 21-11-11 13:23:03, Michal Hocko wrote: > On Fri 18-11-11 17:11:28, Michal Hocko wrote: > > 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. > > OK, so looked at it again and we cannot race with page migration because > the page is locked (by unmap_and_move_*page) migration and we have the > old page locked here as well (hugetlb_fault). > > Or am I missing something? And the updated patch: ---