linux-kernel - Re: [PATCH] mm: memcontrol: fix possible css ref leak on oom

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Message-ID: <20160524084319.GH7917@esperanza>
Date:	Tue, 24 May 2016 11:43:19 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Michal Hocko <mhocko@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: memcontrol: fix possible css ref leak on oom

On Mon, May 23, 2016 at 07:44:43PM +0200, Michal Hocko wrote:
> On Mon 23-05-16 19:02:10, Vladimir Davydov wrote:
> > mem_cgroup_oom may be invoked multiple times while a process is handling
> > a page fault, in which case current->memcg_in_oom will be overwritten
> > leaking the previously taken css reference.
> 
> Have you seen this happening? I was under impression that the page fault
> paths that have oom enabled will not retry allocations.

filemap_fault will, for readahead.

This is rather unlikely, just like the whole oom scenario, so I haven't
faced this leak in production yet, although it's pretty easy to
reproduce using a contrived test. However, even if this leak happened on
my host, I would probably not notice, because currently we have no clear
means of catching css leaks. I'm thinking about adding a file to debugfs
containing brief information about all memory cgroups, including dead
ones, so that we could at least see how many dead memory cgroups are
dangling out there.

>  
> > Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>
> 
> That being said I do not have anything against the patch. It is a good
> safety net I am just not sure this might happen right now and so the
> patch is not stable candidate.
> 
> After clarification
> Acked-by: Michal Hocko <mhocko@...e.com>

Thanks.