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From: Roman Gushchin <roman.gushchin@linux.dev>
To: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,  Kumar Kartikeya
 Dwivedi <memxor@gmail.com>,  linux-mm <linux-mm@kvack.org>,  bpf
 <bpf@vger.kernel.org>,  Suren Baghdasaryan <surenb@google.com>,  Johannes
 Weiner <hannes@cmpxchg.org>,  Michal Hocko <mhocko@suse.com>,  David
 Rientjes <rientjes@google.com>,  Matt Bobrowski
 <mattbobrowski@google.com>,  Song Liu <song@kernel.org>,  Alexei
 Starovoitov <ast@kernel.org>,  Andrew Morton <akpm@linux-foundation.org>,
  LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v1 01/14] mm: introduce bpf struct ops for OOM handling
In-Reply-To: <a76ad1e9-07d5-4ba1-83e4-22fe36a32df0@linux.dev> (Martin KaFai
	Lau's message of "Tue, 2 Sep 2025 15:30:04 -0700")
References: <20250818170136.209169-1-roman.gushchin@linux.dev>
	<20250818170136.209169-2-roman.gushchin@linux.dev>
	<CAP01T76AUkN_v425s5DjCyOg_xxFGQ=P1jGBDv6XkbL5wwetHA@mail.gmail.com>
	<87ms7tldwo.fsf@linux.dev>
	<1f2711b1-d809-4063-804b-7b2a3c8d933e@linux.dev>
	<87wm6rwd4d.fsf@linux.dev>
	<ef890e96-5c2a-4023-bcb2-7ffd799155be@linux.dev>
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	<87iki0n4lm.fsf@linux.dev>
	<a76ad1e9-07d5-4ba1-83e4-22fe36a32df0@linux.dev>
Date: Fri, 03 Oct 2025 19:00:38 -0700
Message-ID: <877bxb77eh.fsf@linux.dev>
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Martin KaFai Lau <martin.lau@linux.dev> writes:

> On 9/2/25 10:31 AM, Roman Gushchin wrote:
>> Btw, what's the right way to attach struct ops to a cgroup, if there is
>> one? Add a cgroup_id field to the struct and use it in the .reg()
>
> Adding a cgroup id/fd field to the struct bpf_oom_ops will be hard to
> attach the same bpf_oom_ops to multiple cgroups.
>
>> callback? Or there is something better?
>
> There is a link_create.target_fd in the "union bpf_attr". The
> cgroup_bpf_link_attach() is using it as cgroup fd. May be it can be
> used here also. This will limit it to link attach only. Meaning the
> SEC(".struct_ops.link") is supported but not the older
> SEC(".struct_ops"). I think this should be fine.

I thought a bit more about it (sorry for the delay):
if we want to be able to attach a single struct ops to multiple cgroups
(and potentially other objects, e.g. sockets), we can't really
use the existing struct ops's bpf_link.

So I guess we need to add a new .attach() function beside .reg()
which will take the existing link and struct bpf_attr as arguments and
return a new bpf_link. And in libbpf we need a corresponding new
bpf_link__attach_cgroup().

Does it sound right?