Re: [Regression] 3.19-rc3 : memcg: Hang in mount memcg

[Tejun Heo <tj@kernel.org>]

On Thu, Jan 22, 2015 at 10:19:43AM -0500, Johannes Weiner wrote:
> From 3d7ae5aeb16ce6118d8bff17194e791339a1f06c Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@cmpxchg.org>
> Date: Thu, 22 Jan 2015 08:16:31 -0500
> Subject: [patch] kernel: cgroup: prevent mount hang due to memory controller
>  lifetime
> 
> Since b2052564e66d ("mm: memcontrol: continue cache reclaim from
> offlined groups"), re-mounting the memory controller after using it is
> very likely to hang.
> 
> The cgroup core assumes that any remaining references after deleting a
> cgroup are temporary in nature, and synchroneously waits for them, but
> the above-mentioned commit has left-over page cache pin its css until
> it is reclaimed naturally.  That being said, swap entries and charged
> kernel memory have been doing the same indefinite pinning forever, the
> bug is just more likely to trigger with left-over page cache.
> 
> Reparenting kernel memory is highly impractical, which leaves changing
> the cgroup assumptions to reflect this: once a controller has been
> mounted and used, it has internal state that is independent from mount
> and cgroup lifetime.  It can be unmounted and remounted, but it can't
> be reconfigured during subsequent mounts.
> 
> Don't offline the controller root as long as there are any children,
> dead or alive.  A remount will no longer wait for these old references
> to drain, it will simply mount the persistent controller state again.
> 
> Reported-by: "Suzuki K. Poulose" <Suzuki.Poulose@arm.com>
> Reported-by: Will Deacon <will.deacon@arm.com>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Applied to cgroup/for-3.19-fixes.

Thanks.

-- 
tejun

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