From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031255Ab2CFU4W (ORCPT ); Tue, 6 Mar 2012 15:56:22 -0500 Received: from mail-iy0-f174.google.com ([209.85.210.174]:60807 "EHLO mail-iy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1031124Ab2CFU4T (ORCPT ); Tue, 6 Mar 2012 15:56:19 -0500 Authentication-Results: mr.google.com; spf=pass (google.com: domain of hughd@google.com designates 10.42.131.129 as permitted sender) smtp.mail=hughd@google.com; dkim=pass header.i=hughd@google.com Date: Tue, 6 Mar 2012 12:55:42 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Daisuke Nishimura cc: Naoya Horiguchi , linux-mm@kvack.org, Andrew Morton , Andrea Arcangeli , KAMEZAWA Hiroyuki , Hillf Danton , linux-kernel@vger.kernel.org Subject: Re: [PATCH] memcg: fix mapcount check in move charge code for anonymous page In-Reply-To: <20120305091719.c9f93f1a.nishimura@mxp.nes.nec.co.jp> Message-ID: References: <1330720508-21019-1-git-send-email-n-horiguchi@ah.jp.nec.com> <20120305091719.c9f93f1a.nishimura@mxp.nes.nec.co.jp> User-Agent: Alpine 2.00 (LSU 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 5 Mar 2012, Daisuke Nishimura wrote: > Hi, Horiguchi-san. > On Fri, 2 Mar 2012 15:35:08 -0500 > Naoya Horiguchi wrote: > > > Currently charge on shared anonyous pages is supposed not to moved > > in task migration. To implement this, we need to check that mapcount > 1, > > instread of > 2. So this patch fixes it. > > > > Signed-off-by: Naoya Horiguchi > > --- > > mm/memcontrol.c | 2 +- > > 1 files changed, 1 insertions(+), 1 deletions(-) > > > > diff --git linux-next-20120228.orig/mm/memcontrol.c linux-next-20120228/mm/memcontrol.c > > index b6d1bab..785f6d3 100644 > > --- linux-next-20120228.orig/mm/memcontrol.c > > +++ linux-next-20120228/mm/memcontrol.c > > @@ -5102,7 +5102,7 @@ static struct page *mc_handle_present_pte(struct vm_area_struct *vma, > > return NULL; > > if (PageAnon(page)) { > > /* we don't move shared anon */ > > - if (!move_anon() || page_mapcount(page) > 2) > > + if (!move_anon() || page_mapcount(page) > 1) > > return NULL; > > } else if (!move_file()) > > /* we ignore mapcount for file pages */ > > -- > > 1.7.7.6 > > > Sorry, it's my fault.. > Thank you for catching this. > > Reviewed-by: Daisuke Nishimura I'm perversely sorry to see this fix already wing its way into 3.3-rc, but never mind. I was puzzling over that same "> 2" test when thinking through the stats move locking, and again when swap accounting appeared to be broken through and through (now fixed by two-liner in page_cgroup.c). Why is there any test on page_mapcount(page) there at all? 2.6.34 comments it * TODO: We don't move charges of shared(used by multiple * processes) pages for now. as if it's an unwelcome restriction to be eliminated later. I don't understand why it was ever there, and would like to remove it (and update the Documentation file) - just to remove a little unnecessary complication, including mem_cgroup_count_swap_user(). The file case moves account, even when the page is not mapped into this address space, even when it's mapped into a thousand others. Why treat the anonymous so differently here? I'd have thought it quite likely (by no means certain, but quite likely) that when you move a task sharing an anon page from one cg to another, you'll move the other task(s) sharing it immediately after - strange that these shared pages should then get left behind. I was pleased by the "> 2" bug, there almost all the life of move_charge_at_immigrate, demonstrating that nobody was depending upon the documented behaviour. I've a few more cleanups in the swap accounting area, I guess I should just post this change along with them and we discuss then, unless you can enlighten me what it's about before I get there. Thanks, Hugh