From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with ESMTP id C85628D0001 for ; Thu, 4 Nov 2010 13:32:55 -0400 (EDT) Received: from hpaq14.eem.corp.google.com (hpaq14.eem.corp.google.com [172.25.149.14]) by smtp-out.google.com with ESMTP id oA4HWNTT026670 for ; Thu, 4 Nov 2010 10:32:23 -0700 Received: from pwj7 (pwj7.prod.google.com [10.241.219.71]) by hpaq14.eem.corp.google.com with ESMTP id oA4HWJLe003985 for ; Thu, 4 Nov 2010 10:32:22 -0700 Received: by pwj7 with SMTP id 7so285213pwj.19 for ; Thu, 04 Nov 2010 10:32:19 -0700 (PDT) Date: Thu, 4 Nov 2010 10:31:58 -0700 (PDT) From: Hugh Dickins Subject: Re: [BUGFIX][PATCH] fix wrong VM_BUG_ON() in try_charge()'s mm->owner check In-Reply-To: Message-ID: References: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Hiroyuki Kamezawa Cc: linux-mm@kvack.org, KAMEZAWA Hiroyuki , balbir@linux.vnet.ibm.com, nishimura@mxp.nes.nec.co.jp, Andrew Morton , linux-kernel@vger.kernel.org, kosaki.motohiro@jp.fujitsu.com List-ID: On Wed, 3 Nov 2010, Hiroyuki Kamezawa wrote: > I'm sorry for attached file, I have to use unusual mailer this time. > This is a fix for wrong VM_BUG_ON() for mm/memcontol.c Thanks, Kame, that's good: I've inlined it below with Balbir's Review, my Ack, and a Cc: stable@kernel.org. Hugh [PATCH] memcg: fix wrong VM_BUG_ON() in try_charge()'s mm->owner check At __mem_cgroup_try_charge(), VM_BUG_ON(!mm->owner) is checked. But as commented in mem_cgroup_from_task(), mm->owner can be NULL in some racy case. This check of VM_BUG_ON() is bad. A possible story to hit this is at swapoff()->try_to_unuse(). It passes mm_struct to mem_cgroup_try_charge_swapin() while mm->owner is NULL. If we can't get proper mem_cgroup from swap_cgroup information, mm->owner is used as charge target and we see NULL. Cc: Daisuke Nishimura Cc: KOSAKI Motohiro Reported-by: Hugh Dickins Signed-off-by: KAMEZAWA Hiroyuki Reviewed-by: Balbir Singh Acked-by: Hugh Dickins Cc: stable@kernel.org --- mm/memcontrol.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-) Index: linux-2.6.36/mm/memcontrol.c =================================================================== --- linux-2.6.36.orig/mm/memcontrol.c +++ linux-2.6.36/mm/memcontrol.c @@ -1729,19 +1729,18 @@ again: rcu_read_lock(); p = rcu_dereference(mm->owner); - VM_BUG_ON(!p); /* - * because we don't have task_lock(), "p" can exit while - * we're here. In that case, "mem" can point to root - * cgroup but never be NULL. (and task_struct itself is freed - * by RCU, cgroup itself is RCU safe.) Then, we have small - * risk here to get wrong cgroup. But such kind of mis-account - * by race always happens because we don't have cgroup_mutex(). - * It's overkill and we allow that small race, here. + * Because we don't have task_lock(), "p" can exit. + * In that case, "mem" can point to root or p can be NULL with + * race with swapoff. Then, we have small risk of mis-accouning. + * But such kind of mis-account by race always happens because + * we don't have cgroup_mutex(). It's overkill and we allo that + * small race, here. + * (*) swapoff at el will charge against mm-struct not against + * task-struct. So, mm->owner can be NULL. */ mem = mem_cgroup_from_task(p); - VM_BUG_ON(!mem); - if (mem_cgroup_is_root(mem)) { + if (!mem || mem_cgroup_is_root(mem)) { rcu_read_unlock(); goto done; } -- 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 policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: email@kvack.org