linux-kernel - Re: [PATCH] zswap: improve memory.zswap.writeback inheritance

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Message-ID: <CAKEwX=MmCKrOkvDO5Yc_M8EB+k5U9AZ3boEiu4u2HUb7p0z+Kw@mail.gmail.com>
Date: Thu, 26 Sep 2024 16:12:58 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Ivan Shapovalov <intelfx@...elfx.name>
Cc: linux-kernel@...r.kernel.org, Mike Yuan <me@...dnzj.com>, Tejun Heo <tj@...nel.org>, 
	Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Koutný <mkoutny@...e.com>, 
	Jonathan Corbet <corbet@....net>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, 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>, 
	Chris Li <chrisl@...nel.org>, cgroups@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH] zswap: improve memory.zswap.writeback inheritance

On Thu, Sep 26, 2024 at 3:55 PM Ivan Shapovalov <intelfx@...elfx.name> wrote:
>
> Improve the inheritance behavior of the `memory.zswap.writeback` cgroup
> attribute introduced during the 6.11 cycle. Specifically, in 6.11 we
> walk the parent cgroups until we find a _disabled_ writeback, which does
> not allow the user to selectively enable zswap writeback while having it
> disabled by default.

Is there an actual need for this? This is a theoretical use case I
thought of (and raised), but I don't think anybody actually wants
this...?

Besides, most people who want this can just:

1. Enable zswap writeback on root cgroup (and all non-leaf cgroups).

2. Disable zswap writeback on leaf cgroups on creation by default.

3. Selectively enable zswap writeback for the leaf cgroups.

All of this is quite doable in userspace. It's not even _that_ racy -
just do this before adding tasks etc. to the cgroup?