linux-kernel - Re: [PATCH] mm/memcontrol: respect zswap.writeback setting from parent cg too

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Message-ID: <zsj4ilkso7p43qexiumk42bkzuqt5bxi3u5pys5arfpjodqszd@4jomnqwf4vim>
Date: Wed, 21 Aug 2024 18:14:52 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Nhat Pham <nphamcs@...il.com>, Mike Yuan <me@...dnzj.com>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Muchun Song <muchun.song@...ux.dev>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Michal Hocko <mhocko@...nel.org>, Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] mm/memcontrol: respect zswap.writeback setting from
 parent cg too

On Wed, Aug 14, 2024 at 01:22:01PM GMT, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> Anyway, both use cases make sense to me, disabling writeback
> system-wide or in an entire subtree, and disabling writeback on the
> root and then selectively enabling it. I am slightly inclined to the
> first one (what this patch does).
> 
> Considering the hierarchical cgroup knobs work, we usually use the
> most restrictive limit among the ancestors. I guess it ultimately
> depends on how we define "most restrictive". Disabling writeback is
> restrictive in the sense that you don't have access to free some zswap
> space to reclaim more memory. OTOH, disabling writeback also means
> that your zswapped memory won't go to disk under memory pressure, so
> in that sense it would be restrictive to force writeback :)
> 
> Usually, the "default" is the non-restrictive thing, and then you can
> set restrictions that apply to all children (e.g. no limits are set by
> default). Since writeback is enabled by default, it seems like the
> restriction would be disabling writeback. Hence, it would make sense
> to inherit zswap disabling (i.e. only writeback if all ancestors allow
> it, like this patch does).
> 
> What we do today dismisses inheritance completely, so it seems to me
> like it should be changed anyway.

I subscribe to inheritance (at cgroup creation) not proving well (in
general). Here's the case of expecting hierarchical semantic of the
attribute.

With this change -- is there any point in keeping the inheritance
around? (Simply default to enabled.)

Thanks,
Michal

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