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

From: Michal Hocko
Date: Tue May 24 2016 - 04:47:46 EST

On Tue 24-05-16 11:43:19, Vladimir Davydov wrote:
> 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.

I thought that the readahead is __GFP_NORETRY so we do not trigger OOM

> 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.

Yeah, debugfs interface would make some sense.
Michal Hocko