Re: [PATCH v2 06/23] mm: introduce BPF struct ops for OOM handling

[Matt Bobrowski <mattbobrowski@google.com>]

On Mon, Jan 12, 2026 at 09:20:13AM -0800, Roman Gushchin wrote:
> Matt Bobrowski <mattbobrowski@google.com> writes:
> 
> > On Mon, Oct 27, 2025 at 04:17:09PM -0700, Roman Gushchin wrote:
> >> Introduce a bpf struct ops for implementing custom OOM handling
> >> policies.
> >>
> >> ...
> >>
> >> +#ifdef CONFIG_MEMCG
> >> +	/* Find the nearest bpf_oom_ops traversing the cgroup tree upwards */
> >> +	for (memcg = oc->memcg; memcg; memcg = parent_mem_cgroup(memcg)) {
> >> +		bpf_oom_ops = READ_ONCE(memcg->bpf_oom);
> >> +		if (!bpf_oom_ops)
> >> +			continue;
> >> +
> >> +		/* Call BPF OOM handler */
> >> +		ret = bpf_ops_handle_oom(bpf_oom_ops, memcg, oc);
> >> +		if (ret && oc->bpf_memory_freed)
> >> +			goto exit;
> >
> > I have a question about the semantics of oc->bpf_memory_freed.
> >
> > Currently, it seems this flag is used to indicate that a BPF OOM
> > program has made forward progress by freeing some memory (i.e.,
> > bpf_oom_kill_process()), but if it's not set, it falls back to the
> > default in-kernel OOM killer.
> >
> > However, what if forward progress in some contexts means not freeing
> > memory? For example, in some bespoke container environments, the
> > policy might be to catch the OOM event and handle it gracefully by
> > raising the memory.limit_in_bytes on the affected memcg. In this kind
> > of resizing scenario, no memory would be freed, but the OOM event
> > would effectively be resolved.
> 
> I'd say we need to introduce a special kfunc which increases the limit
> and sets bpf_memory_freed. I think it's important to maintain safety
> guarantee, so that a faulty bpf program is not leading to the system
> being deadlocked on memory.

Yeah, I was thinking something along the same lines. We can always add
this kind of new BPF kfunc in at a later point, so need to directly
address this use case right now.