linux-kernel - Re: [PATCH rfc 10/12] mm: introduce bpf_out_of

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Message-ID: <aBFFNyGjDAekx58J@google.com>
Date: Tue, 29 Apr 2025 21:31:35 +0000
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Michal Hocko <mhocko@...e.com>
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Suren Baghdasaryan <surenb@...gle.com>,
	David Rientjes <rientjes@...gle.com>, Josh Don <joshdon@...gle.com>,
	Chuyi Zhou <zhouchuyi@...edance.com>, cgroups@...r.kernel.org,
	linux-mm@...ck.org, bpf@...r.kernel.org
Subject: Re: [PATCH rfc 10/12] mm: introduce bpf_out_of_memory() bpf kfunc

On Tue, Apr 29, 2025 at 01:46:07PM +0200, Michal Hocko wrote:
> On Mon 28-04-25 03:36:15, Roman Gushchin wrote:
> > Introduce bpf_out_of_memory() bpf kfunc, which allows to declare
> > an out of memory events and trigger the corresponding kernel OOM
> > handling mechanism.
> > 
> > It takes a trusted memcg pointer (or NULL for system-wide OOMs)
> > as an argument, as well as the page order.
> > 
> > Only one OOM can be declared and handled in the system at once,
> > so if the function is called in parallel to another OOM handling,
> > it bails out with -EBUSY.
> 
> This makes sense for the global OOM handler because concurrent handlers
> are cooperative. But is this really correct for memcg ooms which could
> happen for different hierarchies? Currently we do block on oom_lock in
> that case to make sure one oom doesn't starve others. Do we want the
> same behavior for custom OOM handlers?

It's a good point and I had similar thoughts when I was working on it.
But I think it's orthogonal to the customization of the oom handling.
Even for the existing oom killer it makes no sense to serialize memcg ooms
in independent memcg subtrees. But I'm worried about the dmesg reporting,
it can become really messy for 2+ concurrent OOMs.

Also, some memory can be shared, so one OOM can eliminate a need for another
OOM, even if they look independent.

So my conclusion here is to leave things as they are until we'll get signs
of real world problems with the (lack of) concurrency between ooms.

Thanks