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Date: Wed, 20 Mar 2024 18:38:31 +0100
From: Michal Hocko <mhocko@suse.com>
To: Waiman Long <longman@redhat.com>
Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Shakeel Butt <shakeel.butt@linux.dev>,
	Muchun Song <muchun.song@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>, cgroups@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel@openvz.org
Subject: Re: [PATCH] mm/memcontrol: stop resize loop if limit was changed
 again
Message-ID: <ZfsfFyo4fRKX4h9Z@tiehlicka>
References: <20240320100556.463266-1-ptikhomirov@virtuozzo.com>
 <be05a470-bb31-47ef-b786-557c347de429@redhat.com>
 <ZfsZElNXNf6bOKSt@tiehlicka>
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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