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SubjectRe: [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

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