linux-kernel - Re: [PATCH] mm/memcontrol: stop resize loop if limit was changed again

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Date: Wed, 20 Mar 2024 18:38:31 +0100
From: Michal Hocko <mhocko@...e.com>
To: Waiman Long <longman@...hat.com>
Cc: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vladimir Davydov <vdavydov.dev@...il.com>, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel@...nvz.org
Subject: Re: [PATCH] mm/memcontrol: stop resize loop if limit was changed
 again

On Wed 20-03-24 18:12:50, Michal Hocko wrote:
> On Wed 20-03-24 13:09:07, Waiman Long wrote:
> > 
> > On 3/20/24 06:03, Pavel Tikhomirov wrote:
> > > In memory_max_write() we first set memcg->memory.max and only then
> > > try to enforce it in loop. What if while we are in loop someone else
> > > have changed memcg->memory.max but we are still trying to enforce
> > > the old value? I believe this can lead to nasty consequence like getting
> > > an oom on perfectly fine cgroup within it's limits or excess reclaim.
> > 
> > Concurrent write to the same cgroup control file is not possible as the
> > underlying kernfs_open_file structure has a mutex that serialize access to
> > the file.
> 
> This is good to know and I was not aware of that. Thanks!

Btw. even if the interface itself is serialized then uncoordinated
userspace is still timing dependent so fundamentally racy.
-- 
Michal Hocko
SUSE Labs