linux-kernel - Re: [PATCH v2 14/23] mm: allow specifying custom oom constraint for BPF triggers

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Message-ID: <aRQ8pu_57IeA_Jn_@tiehlicka>
Date: Wed, 12 Nov 2025 08:52:06 +0100
From: Michal Hocko <mhocko@...e.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
	Alexei Starovoitov <ast@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	JP Kobryn <inwardvessel@...il.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, bpf@...r.kernel.org,
	Martin KaFai Lau <martin.lau@...nel.org>,
	Song Liu <song@...nel.org>,
	Kumar Kartikeya Dwivedi <memxor@...il.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2 14/23] mm: allow specifying custom oom constraint for
 BPF triggers

On Tue 11-11-25 11:17:48, Roman Gushchin wrote:
> Michal Hocko <mhocko@...e.com> writes:
> 
> > On Mon 27-10-25 16:21:57, Roman Gushchin wrote:
> >> Currently there is a hard-coded list of possible oom constraints:
> >> NONE, CPUSET, MEMORY_POLICY & MEMCG. Add a new one: CONSTRAINT_BPF.
> >> Also, add an ability to specify a custom constraint name
> >> when calling bpf_out_of_memory(). If an empty string is passed
> >> as an argument, CONSTRAINT_BPF is displayed.
> >
> > Constrain is meant to define the scope of the oom handler but to me it
> > seems like you want to specify the oom handler and (ab)using scope for
> > that. In other words it still makes sense to distinguesh memcg, global,
> > mempolicy wide OOMs with global vs. bpf handler, right?
> 
> I use the word "constraint" as the "reason" why an OOM was declared (in
> other words which constraint was violated). And memcg vs global define
> the scope. Right now the only way to trigger a memcg oom is to exceed
> the memory.max limit. But with bpf oom there will others, e.g. exceed a
> certain PSI threshold. So you can have different constraints violated
> within the same scope.

Please use a different placeholder for that. Current constrains have a
well defined semantic. They are not claiming why the OOM happened but
what is the scope of the oom action (domain if you will). The specific
handler has a sufficient knowledge to explain why the OOM killing is
happening and on which domain/scope/constrain.

-- 
Michal Hocko
SUSE Labs