From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758667Ab0E0LSQ (ORCPT ); Thu, 27 May 2010 07:18:16 -0400 Received: from casper.infradead.org ([85.118.1.10]:46138 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758641Ab0E0LSM convert rfc822-to-8bit (ORCPT ); Thu, 27 May 2010 07:18:12 -0400 Subject: Re: [PATCH 00/11] V4: rwsem changes + down_read_critical() proposal From: Peter Zijlstra To: Michel Lespinasse Cc: Linus Torvalds , David Howells , Ingo Molnar , Thomas Gleixner , LKML , Andrew Morton , Mike Waychison , Suleiman Souhlal , Ying Han In-Reply-To: <20100527105948.GA20625@google.com> References: <1274733081-4623-1-git-send-email-walken@google.com> <1274777239.5882.581.camel@twins> <1274779675.5882.684.camel@twins> <20100527105948.GA20625@google.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Thu, 27 May 2010 13:18:00 +0200 Message-ID: <1274959080.27810.4438.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2010-05-27 at 03:59 -0700, Michel Lespinasse wrote: > On Tue, May 25, 2010 at 11:27:55AM +0200, Peter Zijlstra wrote: > > On Tue, 2010-05-25 at 02:12 -0700, Michel Lespinasse wrote: > > > Yes, we do have patches trying to release the mmap_sem when a page > > > fault for a file backed VMA blocks on accessing the corresponding > > > file. We have not given up on these, and we intend to try submitting > > > them again. However, these patches do *not* address the case of a page > > > fault blocking while trying to get a free page (i.e. when you get > > > under high memory pressure). > > > > But I guess they could, right? Simply make the allocation under mmap_sem > > be __GFP_HARDWALL|__GFP_HIGHMEM|__GFP_MOVABLE__GFP_NOWARN or > > (GFP_HUGHUSER_MOVABLE & ~(__GFP_WAIT|__GFP_IO|__GFP_FS))|__GFP_NOWARN > > > > and drop the mmap_sem when that fails. > > It's not clear to me if this can lead to a clean uncontroversial solution. > Doing this for file backed VMAs does not sound any harder in principle, > but we could not get it past linus's NACK last time. I think it's worth > exploring again, but I don't expect it to be so easy :) Right, I was planning to have a look again, but my todo list is getting out of hand again. > > > > I really don't like people tinkering with the lock implementations like > > > > this. Nor do I like the naming, stats are in no way _critical_. > > > > > > Critical here refers to the fact that you're not allowed to block > > > while holding the unfairly acquired rwsem. > > > > We usually call that atomic, your 0/n patch didn't explain any of that. > > Would replacing the 'critical' name with 'atomic' address your concern > though, or would you remain fundamentally opposed to anything that involves > an unfair acquire path ? Yeah, I don't think its a good idea to add unfairness to the lock, such things tend to backfire and generate starvation cases. For instance, a tight loop around your /proc/$pid/maps mod will starve $pid from doing anything requiring mmap_sem for writing. > What about patches 1-7 which don't deal with the critical/atomic API; > can we agree to get these in before we we figure out what to do with > the the last 4 ? patch 6 looks like it will break fairness, either that or the changelog got me totally confused (already had to read it twice).