From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 16 May 2007 19:38:16 +0200 From: Nick Piggin Subject: Re: [rfc] optimise unlock_page Message-ID: <20070516173816.GB5883@wotan.suse.de> References: <1178659827.14928.85.camel@localhost.localdomain> <20070508224124.GD20174@wotan.suse.de> <20070508225012.GF20174@wotan.suse.de> <20070510033736.GA19196@wotan.suse.de> <20070511085424.GA15352@wotan.suse.de> <20070513033210.GA3667@wotan.suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org Return-Path: To: Hugh Dickins Cc: Benjamin Herrenschmidt , linux-arch@vger.kernel.org, Andrew Morton , Linux Kernel Mailing List , Linux Memory Management List List-ID: On Wed, May 16, 2007 at 06:21:09PM +0100, Hugh Dickins wrote: > On Sun, 13 May 2007, Nick Piggin wrote: > > On Fri, May 11, 2007 at 02:15:03PM +0100, Hugh Dickins wrote: > > > > > But again I wonder just what the gain has been, once your double > > > unmap_mapping_range is factored in. When I suggested before that > > > perhaps the double (well, treble including the one in truncate.c) > > > unmap_mapping_range might solve the problem you set out to solve > > > (I've lost sight of that!) without pagelock when faulting, you said: > > > > > > > Well aside from being terribly ugly, it means we can still drop > > > > the dirty bit where we'd otherwise rather not, so I don't think > > > > we can do that. > > > > > > but that didn't give me enough information to agree or disagree. > > > > Oh, well invalidate wants to be able to skip dirty pages or have the > > filesystem do something special with them first. Once you have taken > > the page out of the pagecache but still mapped shared, then blowing > > it away doesn't actually solve the data loss problem... only makes > > the window of VM inconsistency smaller. > > Right, I think I see what you mean now, thanks: userspace > must not for a moment be allowed to write to orphaned pages. Yep. > Whereas it's not an issue for the privately COWed pages you added > the second unmap_mapping_range for: because it's only truncation > that has to worry about them, so they're heading for SIGBUS anyway. > > Yes, and the page_mapped tests in mm/truncate.c are just racy > heuristics without the page lock you now put into faulting. Yes. -- 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/ . Don't email: email@kvack.org