linux-kernel - Re: [PATCH v1 01/14] mm: introduce bpf struct ops for OOM handling

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Message-ID: <CAADnVQ+LGbXXHHTbBB9b-RjAXO4B6=3Z=G0=7ToZVuH61OONWA@mail.gmail.com>
Date: Tue, 26 Aug 2025 12:52:26 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>, Kumar Kartikeya Dwivedi <memxor@...il.com>, 
	linux-mm <linux-mm@...ck.org>, bpf <bpf@...r.kernel.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...e.com>, 
	David Rientjes <rientjes@...gle.com>, Matt Bobrowski <mattbobrowski@...gle.com>, 
	Song Liu <song@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 01/14] mm: introduce bpf struct ops for OOM handling

On Tue, Aug 26, 2025 at 11:01 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 8/25/25 10:00 AM, Roman Gushchin wrote:
> > Martin KaFai Lau <martin.lau@...ux.dev> writes:
> >
> >> On 8/20/25 5:24 PM, Roman Gushchin wrote:
> >>>> How is it decided who gets to run before the other? Is it based on
> >>>> order of attachment (which can be non-deterministic)?
> >>> Yeah, now it's the order of attachment.
> >>>
> >>>> There was a lot of discussion on something similar for tc progs, and
> >>>> we went with specific flags that capture partial ordering constraints
> >>>> (instead of priorities that may collide).
> >>>> https://lore.kernel.org/all/20230719140858.13224-2-daniel@iogearbox.net
> >>>> It would be nice if we can find a way of making this consistent.
> >>
> >> +1
> >>
> >> The cgroup bpf prog has recently added the mprog api support also. If
> >> the simple order of attachment is not enough and needs to have
> >> specific ordering, we should make the bpf struct_ops support the same
> >> mprog api instead of asking each subsystem creating its own.
> >>
> >> fyi, another need for struct_ops ordering is to upgrade the
> >> BPF_PROG_TYPE_SOCK_OPS api to struct_ops for easier extension in the
> >> future. Slide 13 in
> >> https://drive.google.com/file/d/1wjKZth6T0llLJ_ONPAL_6Q_jbxbAjByp/view
> >
> > Does it mean it's better now to keep it simple in the context of oom
> > patches with the plan to later reuse the generic struct_ops
> > infrastructure?
> >
> > Honestly, I believe that the simple order of attachment should be
> > good enough for quite a while, so I'd not over-complicate this,
> > unless it's not fixable later.
>
> I think the simple attachment ordering is fine. Presumably the current link list
> in patch 1 can be replaced by the mprog in the future. Other experts can chime
> in if I have missed things.

I don't think the proposed approach of:
list_for_each_entry_srcu(bpf_oom, &bpf_oom_handlers, node, false) {
is extensible without breaking things.
Sooner or later people will want bpf-oom handlers to be per
container, so we have to think upfront how to do it.
I would start with one bpf-oom prog per memcg and extend with mprog later.
Effectively placing 'struct bpf_oom_ops *' into oc->memcg,
and having one global bpf_oom_ops when oc->memcg == NULL.
I'm sure other designs are possible, but lets make sure container scope
is designed from the beginning.
mprog-like multi prog behavior per container can be added later.