linux-kernel - Re: [PATCH] mm: skip lru_note_cost() when scanning only file or anon

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Message-ID: <87o6tmu4j5.fsf@linux.dev>
Date: Mon, 14 Jul 2025 09:21:34 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,  Shakeel Butt
 <shakeel.butt@...ux.dev>,  Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
  Michal Hocko <mhocko@...nel.org>,  David Hildenbrand <david@...hat.com>,
  linux-mm@...ck.org,  linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: skip lru_note_cost() when scanning only file or anon

Johannes Weiner <hannes@...xchg.org> writes:

> On Fri, Jul 11, 2025 at 10:55:48AM -0700, Roman Gushchin wrote:
>> Johannes Weiner <hannes@...xchg.org> writes:
>> > The caveat with this patch is that, aside from the static noswap
>> > scenario, modes can switch back and forth abruptly or even overlap.
>> >
>> > So if you leave a pressure scenario and go back to cache trimming, you
>> > will no longer age the cost information anymore. The next spike could
>> > be starting out with potentially quite stale information.
>> >
>> > Or say proactive reclaim recently already targeted anon, and there
>> > were rotations and pageouts; that would be useful data for a reactive
>> > reclaimer doing work at around the same time, or shortly thereafter.
>> 
>> Agree, but at the same time it's possible to come up with the scenario
>> when it's not good.
>>   A
>>  / \
>> B  C  memory.max=X
>>   / \
>>  D   E
>> 
>> Let's say we have a cgroup structure like this, we apply a lot
>> of proactive anon pressure on E, then the pressure from on D from
>> C's limit will be biased towards file without a good reason.
>
> No, this is on purpose. D and E are not independent. They're in the
> same memory domain, C. So if you want to reclaim C, and a subset of
> its anon has already been pressured to resistance, then a larger part
> of the reclaim candidates in C will need to come from file.

So, basically you can create a tiny memcg without swap in a large
system, create a ton of memory pressure there and bias the global
memory reclaim? That's strange.