From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with ESMTP id EFB226B0012 for ; Sat, 11 Jun 2011 14:04:29 -0400 (EDT) Date: Sat, 11 Jun 2011 20:04:15 +0200 From: Johannes Weiner Subject: Re: [PATCH] [BUGFIX] update mm->owner even if no next owner. Message-ID: <20110611180415.GB31154@cmpxchg.org> References: <20110610091355.2ce38798.kamezawa.hiroyu@jp.fujitsu.com> <20110610113311.409bb423.kamezawa.hiroyu@jp.fujitsu.com> <20110610121949.622e4629.kamezawa.hiroyu@jp.fujitsu.com> <20110610125551.385ea7ed.kamezawa.hiroyu@jp.fujitsu.com> <20110610133021.2eaaf0da.kamezawa.hiroyu@jp.fujitsu.com> <20110611163943.GA3238@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110611163943.GA3238@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Arcangeli Cc: Hugh Dickins , KAMEZAWA Hiroyuki , Hiroyuki Kamezawa , Ying Han , Dave Jones , Linux Kernel , "linux-mm@kvack.org" , Oleg Nesterov , Andrew Morton On Sat, Jun 11, 2011 at 06:39:43PM +0200, Andrea Arcangeli wrote: > On Sat, Jun 11, 2011 at 09:04:14AM -0700, Hugh Dickins wrote: > > I had another go at reproducing it, 2 hours that time, then a try with > > 692e0b35427a reverted: it ran overnight for 9 hours when I stopped it. > > > > Andrea, please would you ask Linus to revert that commit before -rc3? > > Or is there something else you'd like us to try instead? I admit that > > I've not actually taken the time to think through exactly how it goes > > wrong, but it does look dangerous. > > Here I was asked if the mem_cgroup_newpage_charge need the mmap_sem at > all. And if not why not to release the mmap_sem early. > > https://lkml.org/lkml/2011/3/14/276 > > So I didn't see why mmap_sem was needed, I also asked confirmation and > who answered agreed it was safe without mmap_sem even if it's the only > place doing that. Maybe that assumption was wrong and we need > mmap_sem after all if this commit is causing problems. > > Or did you find something wrong in the actual patch? > > Do I understand right that the bug just that we must run > alloc_hugepage_vma+mem_cgroup_newpage_charge within the same critical > section protected by the mmap_sem read mode? Do we know why? The problem is that mm->owner points to a stale task structure if the last possible owner is exiting. The mmap_sem just prevented the task from actually exiting through write-acquiring the mmap_sem in khugepaged_exit(). I think enforcing lifetime of an object through locks is not the nicest thing to do, so I stand by what I wrote in the mail you linked to above :) and agree with Kame that mm->owner should just not point to a stale task struct. The memcg code can handle it going NULL. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org