linux-kernel - Re: [QUESTION] What memcg lifetime is required by list_lru

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Message-ID: <siexzzafqmgtya5jas4v5pjpartt2h4l2amimjhaaztqidapht@2rexlquzrdsx>
Date: Tue, 3 Dec 2024 11:44:00 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Dave Chinner <david@...morbit.com>, Alice Ryhl <aliceryhl@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Nhat Pham <nphamcs@...il.com>, 
	Qi Zheng <zhengqi.arch@...edance.com>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Linux Memory Management List <linux-mm@...ck.org>, 
	Michal Hocko <mhocko@...nel.org>, Shakeel Butt <shakeel.butt@...ux.dev>, cgroups@...r.kernel.org, 
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [QUESTION] What memcg lifetime is required by list_lru_add?

On Thu, Nov 28, 2024 at 09:05:34AM GMT, Dave Chinner <david@...morbit.com> wrote:
> It's enforced by the -complex- state machine used to tear down
> control groups.

True.

> tl;dr: If the memcg gets torn down, it will reparent the objects on
> the LRU to it's parent memcg during the teardown process.
> 
> This reparenting happens in the cgroup ->css_offline() method, which
> only happens after the cgroup reference count goes to zero and is
> waited on via:

What's waited for is seeing "killing" of the _initial_ reference, the
refcount may be still non-zero. I.e. ->css_offline() happens with some
referencs around (e.g. from struct page^W folio) and only
->css_released() is called after refs drop to zero (and ->css_free()
even after RCU period given there were any RCU readers who didn't
css_get()).

> See the comment above css_free_rwork_fn() for more details about the
> teardown process:
> 
> /*
>  * css destruction is four-stage process.
>  *
>  * 1. Destruction starts.  Killing of the percpu_ref is initiated.
>  *    Implemented in kill_css().
>  *
>  * 2. When the percpu_ref is confirmed to be visible as killed on all CPUs
>  *    and thus css_tryget_online() is guaranteed to fail, the css can be
>  *    offlined by invoking offline_css().  After offlining, the base ref is
>  *    put.  Implemented in css_killed_work_fn().
>  *
>  * 3. When the percpu_ref reaches zero, the only possible remaining
>  *    accessors are inside RCU read sections.  css_release() schedules the
>  *    RCU callback.
>  *
>  * 4. After the grace period, the css can be freed.  Implemented in
>  *    css_free_rwork_fn().

This is a useful comment.

HTH,
Michal

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