linux-kernel - Re: [PATCH 0/3] cgroup: nmi safe css_rstat

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Message-ID: <rtgbcuvajr6oql5xfe5qp7cman2ucatnohux47upknwfoduc5q@63ywqn4tg3jr>
Date: Tue, 10 Jun 2025 12:53:11 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Tejun Heo <tj@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Vlastimil Babka <vbabka@...e.cz>, Alexei Starovoitov <ast@...nel.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Harry Yoo <harry.yoo@...cle.com>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>, bpf@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 0/3] cgroup: nmi safe css_rstat_updated

On Mon, Jun 09, 2025 at 03:56:08PM -0700, Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> BPF programs can run in nmi context and may trigger memcg charged memory
> allocation in such context. Recently linux added support to nmi safe
> page allocation along with memcg charging of such allocations. However
> the kmalloc/slab support and corresponding memcg charging is still
> lacking,
> 
> To provide nmi safe support for memcg charging for kmalloc/slab
> allocations, we need nmi safe memcg stats and for that we need nmi safe
> css_rstat_updated() which adds the given cgroup state whose stats are
> updated into the per-cpu per-ss update tree. This series took the aim to
> make css_rstat_updated() nmi safe.

memcg charging relies on page counters and per-cpu stocks.
css_rstat_updated() is "only" for statistics (which has admiteddly some
in-kernel consumers but those are already affected by batching and
flushing errors).

Have I missed some updates that make css_rstat_updated() calls critical
for memcg charging? I'd find it useful to explain this aspect more in
the cover letter.

Thanks,
Michal

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