linux-kernel - Re: [PATCH] fs, mm: account filp and names caches to kmemcg

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Date:   Fri, 13 Oct 2017 08:51:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Greg Thelen <gthelen@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, linux-fsdevel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs, mm: account filp and names caches to kmemcg

On Thu 12-10-17 16:57:22, Greg Thelen wrote:
[...]
> Overcharging kmem with deferred reconciliation sounds good to me.
> 
> A few comments (not reasons to avoid this):
> 
> 1) If a task is moved between memcg it seems possible to overcharge
>    multiple oom memcg for different kmem/user allocations.
>    mem_cgroup_oom_synchronize() would see at most one oom memcg in
>    current->memcg_in_oom.  Thus it'd only reconcile a single memcg.  But
>    that seems pretty rare and the next charge to any of the other memcg
>    would reconcile them.

This is a general problem for the cgroup v2 memcg oom handling. 

> 2) if a kernel thread charges kmem on behalf of a client mm then there
>    is no good place to call mem_cgroup_oom_synchronize(), short of
>    launching a work item in mem_cgroup_oom().  I don't we have anything
>    like that yet.  So nothing to worry about.

If we do invoke the OOM killer from the charge path, it shouldn't be a
problem.

> 3) it's debatable if mem_cgroup_oom_synchronize() should first attempt
>    reclaim before killing.  But that's a whole 'nother thread.

Again, this shouldn't be an issue if we invoke the oom killer from the
charge path.

> 4) overcharging with deferred reconciliation could also be used for user
>    pages.  But I haven't looked at the code long enough to know if this
>    would be a net win.

It would solve g-u-p issues failing with ENOMEM unexpectedly just
because of memcg charge failure.
-- 
Michal Hocko
SUSE Labs