From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754266Ab3KALoO (ORCPT ); Fri, 1 Nov 2013 07:44:14 -0400 Received: from mga09.intel.com ([134.134.136.24]:14804 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750787Ab3KALoN (ORCPT ); Fri, 1 Nov 2013 07:44:13 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.93,535,1378882800"; d="scan'208";a="428228177" Date: Fri, 1 Nov 2013 19:44:29 +0800 From: Yuanhan Liu To: Peter Zijlstra Cc: Michel Lespinasse , linux-kernel@vger.kernel.org, Ingo Molnar , Linus Torvalds , Andrew Morton , Rik van Riel Subject: Re: [PATCH 1/4] mm/rmap: per anon_vma lock Message-ID: <20131101114429.GD30123@yliu-dev.sh.intel.com> References: <1383292467-28922-1-git-send-email-yuanhan.liu@linux.intel.com> <1383292467-28922-2-git-send-email-yuanhan.liu@linux.intel.com> <20131101084329.GB19466@laptop.lan> <20131101100707.GB30123@yliu-dev.sh.intel.com> <20131101101514.GD19466@laptop.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131101101514.GD19466@laptop.lan> 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 Fri, Nov 01, 2013 at 11:15:14AM +0100, Peter Zijlstra wrote: > On Fri, Nov 01, 2013 at 06:07:07PM +0800, Yuanhan Liu wrote: > > > I also want to point out that lately we've seen several changes sent > > > out that relax locking with no accompanying explanation of why the > > > relaxed locking would be safe. Please don't do that - having a lot of > > > performance data is worthless if you can't explain why the new locking > > > is safe. > > > > Agreed. > > > > > And I'm not asking to prove a negative ('lack of any possible > > > races') there, but at least in this case one could dig out why the > > > root anon vma locking was introduced and if they think that this > > > reason doesn't apply anymore, explain why... > > > > It was introduced by commit 2b575eb6(And, BTW, I'm sorry that this commit log > > about bb4aa39676f is wrong) > > > > commit 2b575eb64f7a9c701fb4bfdb12388ac547f6c2b6 > > Author: Peter Zijlstra > > Date: Tue May 24 17:12:11 2011 -0700 > > > > mm: convert anon_vma->lock to a mutex > > > > Straightforward conversion of anon_vma->lock to a mutex. > > > > As you can see, Peter didn't tell why before. Honestly speaking, that > > was my originaly concern as well. I tried to find some possible races; > > I guess I may miss something. > > Bullshit; I didn't change the locking. I only changed the lock primitive > from a spinlock to a mutex. The anon_vma->root->lock is completely > unrelated to this change. Oops, sorry for that. Just made a *horrible* mistake: it was commit 012f18004da33ba672e3c60838cc4898126174d3. commit 012f18004da33ba672e3c60838cc4898126174d3 Author: Rik van Riel Date: Mon Aug 9 17:18:40 2010 -0700 mm: always lock the root (oldest) anon_vma Always (and only) lock the root (oldest) anon_vma whenever we do something in an anon_vma. The recently introduced anon_vma scalability is due to the rmap code scanning only the VMAs that need to be scanned. Many common operations still took the anon_vma lock on the root anon_vma, so always taking that lock is not expected to introduce any scalability issues. However, always taking the same lock does mean we only need to take one lock, which means rmap_walk on pages from any anon_vma in the vma is excluded from occurring during an munmap, expand_stack or other operation that needs to exclude rmap_walk and similar functions. Also add the proper locking to vma_adjust. Signed-off-by: Rik van Riel Tested-by: Larry Woodman Acked-by: Larry Woodman Reviewed-by: Minchan Kim Reviewed-by: KAMEZAWA Hiroyuki Acked-by: Mel Gorman Acked-by: Linus Torvalds Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds