linux-kernel - Re: [PATCH bpf-next v2 2/7] mm: introduce BPF kfuncs to deal with memcg pointers

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Message-ID: <z77od372jts3d73n77v3o7dk4ro7hizuj2zn235vmbltwjnwfy@zkamyu7gf5jj>
Date: Fri, 19 Dec 2025 21:20:48 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: bpf@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	JP Kobryn <inwardvessel@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Michal Hocko <mhocko@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH bpf-next v2 2/7] mm: introduce BPF kfuncs to deal with
 memcg pointers

On Fri, Dec 19, 2025 at 08:12:45PM -0800, Roman Gushchin wrote:
> To effectively operate with memory cgroups in BPF there is a need
> to convert css pointers to memcg pointers. A simple container_of
> cast which is used in the kernel code can't be used in BPF because
> from the verifier's point of view that's a out-of-bounds memory access.
> 
> Introduce helper get/put kfuncs which can be used to get
> a refcounted memcg pointer from the css pointer:
>   - bpf_get_mem_cgroup,
>   - bpf_put_mem_cgroup.
> 
> bpf_get_mem_cgroup() can take both memcg's css and the corresponding
> cgroup's "self" css. It allows it to be used with the existing cgroup
> iterator which iterates over cgroup tree, not memcg tree.
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>

Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>