linux-kernel - Re: [PATCH v2 1/2] memcg: introduce kfuncs for fetching memcg stats

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Message-ID: <CAHzjS_s3L7f=Rgux_Y3NQ7tz+Jmec5T8hLyQCxseLJ9-T-9xuQ@mail.gmail.com>
Date: Wed, 15 Oct 2025 16:12:06 -0700
From: Song Liu <song@...nel.org>
To: JP Kobryn <inwardvessel@...il.com>
Cc: shakeel.butt@...ux.dev, andrii@...nel.org, ast@...nel.org, 
	mkoutny@...e.com, yosryahmed@...gle.com, hannes@...xchg.org, tj@...nel.org, 
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, 
	cgroups@...r.kernel.org, linux-mm@...ck.org, bpf@...r.kernel.org, 
	kernel-team@...a.com
Subject: Re: [PATCH v2 1/2] memcg: introduce kfuncs for fetching memcg stats

On Wed, Oct 15, 2025 at 12:08 PM JP Kobryn <inwardvessel@...il.com> wrote:
>
[...]
> ---
>  mm/memcontrol.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 67 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 4deda33625f4..6547c27d4430 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -871,6 +871,73 @@ unsigned long memcg_events_local(struct mem_cgroup *memcg, int event)
>  }
>  #endif
>
> +static inline struct mem_cgroup *memcg_from_cgroup(struct cgroup *cgrp)
> +{
> +       return cgrp ? mem_cgroup_from_css(cgrp->subsys[memory_cgrp_id]) : NULL;
> +}
> +

We should add __bpf_kfunc_start_defs() here, and __bpf_kfunc_end_defs()
after all the kfuncs.

> +__bpf_kfunc static void memcg_flush_stats(struct cgroup *cgrp)

We mostly do not make kfunc static, but it seems to also work.

> +{
> +       struct mem_cgroup *memcg = memcg_from_cgroup(cgrp);
> +
> +       if (!memcg)
> +               return;

Maybe we can let memcg_flush_stats return int, and return -EINVAL
on memcg == NULL cases?

> +
> +       mem_cgroup_flush_stats(memcg);
> +}
> +
[...]