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

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

On Mon, May 23, 2016 at 07:44:43PM +0200, Michal Hocko wrote:

quoted

On Mon 23-05-16 19:02:10, Vladimir Davydov wrote:

quoted

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
killer.

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
SUSE Labs

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