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

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Date: Wed, 20 Mar 2024 15:38:40 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
Cc: Michal Hocko <mhocko@...e.com>, Johannes Weiner <hannes@...xchg.org>,
	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, Mar 20, 2024 at 06:55:05PM +0800, Pavel Tikhomirov wrote:
> 
> 
> On 20/03/2024 18:28, Michal Hocko wrote:
> > On Wed 20-03-24 18:03:30, 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.
> > 
> > I would argue that uncoordinated hard limit configuration can cause
> > problems on their own.
> 
> Sorry, didn't know that.
> 
> > Beside how is this any different from changing
> > the high limit while we are inside the reclaim loop?
> 
> I believe reclaim loop rereads limits on each iteration, e.g. in
> reclaim_high(), so it should always be enforcing the right limit.
> 
> > 
> > > We also have exactly the same thing in memory_high_write().
> > > 
> > > So let's stop enforcing old limits if we already have a new ones.
> > 
> > I do see any reasons why this would be harmful I just do not see why
> > this is a real thing or why the new behavior is any better for racing
> > updaters as those are not deterministic anyway. If you have any actual
> > usecase then more details would really help to justify this change.
> > 
> > The existing behavior makes some sense as it enforces the given limit
> > deterministically.
> 
> I don't have any actual problem, usecase or reproduce at hand, I only see a
> potential problem:

If the problem is only potential and also not very severe (it's not a crash
or memory corruption or something like this), I'd say let's keep things as
they are.

Thanks!