From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759305Ab2DJTwM (ORCPT ); Tue, 10 Apr 2012 15:52:12 -0400 Received: from mail-lb0-f174.google.com ([209.85.217.174]:36765 "EHLO mail-lb0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759281Ab2DJTwK (ORCPT ); Tue, 10 Apr 2012 15:52:10 -0400 Message-ID: <4F848F65.7000402@openvz.org> Date: Tue, 10 Apr 2012 23:52:05 +0400 From: Konstantin Khlebnikov User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.2) Gecko/20120217 Firefox/10.0.2 Iceape/2.7.2 MIME-Version: 1.0 To: Andrew Morton CC: Hugh Dickins , "linux-kernel@vger.kernel.org" , Oleg Nesterov , "linux-mm@kvack.org" , Markus Trippelsdorf , KAMEZAWA Hiroyuki Subject: Re: [PATCH v2] mm: correctly synchronize rss-counters at exit/exec References: <20120409200336.8368.63793.stgit@zurg> <20120410170732.18750.64274.stgit@zurg> <20120410114329.138fe242.akpm@linux-foundation.org> In-Reply-To: <20120410114329.138fe242.akpm@linux-foundation.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andrew Morton wrote: > On Tue, 10 Apr 2012 21:07:32 +0400 > Konstantin Khlebnikov wrote: > >> mm->rss_stat counters have per-task delta: task->rss_stat, before changing >> task->mm pointer kernel must flush this delta with help of sync_mm_rss(). >> >> do_exit() already calls sync_mm_rss() to flush rss-counters before commiting >> rss-statistics into task->signal->maxrss, taskstats, audit and other stuff. >> Unfortunately kernel do this before calling mm_relese(), which can call put_user() >> for processing task->clear_child_tid. So at this point we can trigger page-faults >> and task->rss_stat becomes non-zero again, as result mm->rss_stat becomes >> inconsistent and check_mm() will print something like this: >> >> | BUG: Bad rss-counter state mm:ffff88020813c380 idx:1 val:-1 >> | BUG: Bad rss-counter state mm:ffff88020813c380 idx:2 val:1 >> >> This patch moves sync_mm_rss() into mm_release(), and moves mm_release() out of >> do_exit() and calls it earlier. After mm_release() there should be no page-faults. >> >> ... >> >> --- a/kernel/fork.c >> +++ b/kernel/fork.c >> @@ -751,6 +751,14 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm) >> } >> tsk->clear_child_tid = NULL; >> } >> + >> + /* >> + * Final rss-counter synchronization. After this point must be >> + * no page-faults into this mm from current context, otherwise >> + * mm->rss_stat will be inconsistent. >> + */ >> + if (mm) >> + sync_mm_rss(mm); >> } >> > > Well that's scary. AFACIT `mm' can indeed be NULL here, when a kernel > thread calls do_exit(). No implementation of deactivate_mm() actually > uses its `mm' arg and I guess that kernel threads never set > tsk->clear_child_tid. Whee. But it works as designed =) > > > Do we think we should backport this into -stable kernels? How hard is > it to make that warning come out? > Warning was introduced in v3.3-3720-gc3f0327, so no releases with them. And I think older kernels can live fine with slightly racy rss counters.