linux-kernel - Re: [PATCH] mm: vmscan: skip the file folios in proactive reclaim if swappiness is MAX

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Message-ID: <20250312231656.GB1247787@cmpxchg.org>
Date: Wed, 12 Mar 2025 19:16:56 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Zhongkun He <hezhongkun.hzk@...edance.com>
Cc: akpm@...ux-foundation.org, mhocko@...e.com, muchun.song@...ux.dev,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: vmscan: skip the file folios in proactive reclaim if
 swappiness is MAX

On Wed, Mar 12, 2025 at 05:43:37PM +0800, Zhongkun He wrote:
> With this patch 'commit <68cd9050d871> ("mm: add swappiness= arg to
> memory.reclaim")', we can submit an additional swappiness=<val> argument
> to memory.reclaim. It is very useful because we can dynamically adjust
> the reclamation ratio based on the anonymous folios and file folios of
> each cgroup. For example,when swappiness is set to 0, we only reclaim
> from file pages.
> 
> However,we have also encountered a new issue: when swappiness is set to
> the MAX_SWAPPINESS, it may still only reclaim file folios. This is due
> to the knob of cache_trim_mode, which depends solely on the ratio of
> inactive folios, regardless of whether there are a large number of cold
> folios in anonymous folio list.
> 
> So, we hope to add a new control logic where proactive memory reclaim only
> reclaims from anonymous folios when swappiness is set to MAX_SWAPPINESS.
> For example, something like this:
> 
> echo "2M swappiness=200" > /sys/fs/cgroup/memory.reclaim
> 
> will perform reclaim on the rootcg with a swappiness setting of 200 (max
> swappiness) regardless of the file folios. Users have a more comprehensive
> view of the application's memory distribution because there are many
> metrics available.

I'm not opposed but can you be a bit more specific on your usecase?

Presumably this is with zram/zswap, where the IO tradeoff that
cache_trim_mode is making doesn't hold - file refaults will cause IO,
whereas anon decompression will not.

> With this patch, the swappiness argument of memory.reclaim has a more
> precise semantics: 0 means reclaiming only from file pages, while 200
> means reclaiming just from anonymous pages.
> 
> Signed-off-by: Zhongkun He <hezhongkun.hzk@...edance.com>

Makes sense to me. With the doc update Andrew had asked for,

Acked-by: Johannes Weiner <hannes@...xchg.org>