linux-kernel - Re: [PATCH v6 1/2] mm/vmscan: Skip memcg with !usage in shrink_node

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Message-ID: <20250414164721.GA741145@cmpxchg.org>
Date: Mon, 14 Apr 2025 12:47:21 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Michal Koutný <mkoutny@...e.com>
Cc: Waiman Long <llong@...hat.com>, Michal Hocko <mhocko@...nel.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>,
	Tejun Heo <tj@...nel.org>, Shuah Khan <shuah@...nel.org>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v6 1/2] mm/vmscan: Skip memcg with !usage in
 shrink_node_memcgs()

On Mon, Apr 14, 2025 at 03:55:39PM +0200, Michal Koutný wrote:
> On Mon, Apr 14, 2025 at 09:15:57AM -0400, Waiman Long <llong@...hat.com> wrote:
> > I did see some low event in the no usage case because of the ">=" comparison
> > used in mem_cgroup_below_min().
> 
> Do you refer to A/B/E or A/B/F from the test?
> It's OK to see some events if there was non-zero usage initially.
> 
> Nevertheless, which situation this patch changes that is not handled by
> mem_cgroup_below_min() already?

It's not a functional change to the protection semantics or the
reclaim behavior.

The problem is if we go into low_reclaim and encounter an empty group,
we'll issue "low-protected group is being reclaimed" events, which is
kind of absurd (nothing will be reclaimed) and thus confusing to users
(I didn't even configure any protection!)

I suggested, instead of redefining the protection definitions for that
special case, to bypass all the checks and the scan count calculations
when we already know the group is empty and none of this applies.

https://lore.kernel.org/linux-mm/20250404181308.GA300138@cmpxchg.org/