Re: [PATCH bpf-next v3 07/17] mm: introduce BPF OOM struct ops

[Roman Gushchin]

Michal Hocko <mhocko@xxxxxxxx> writes:

> On Mon 26-01-26 18:44:10, Roman Gushchin wrote:
>> Introduce a bpf struct ops for implementing custom OOM handling
>> policies.
>> 
>> It's possible to load one bpf_oom_ops for the system and one
>> bpf_oom_ops for every memory cgroup. In case of a memcg OOM, the
>> cgroup tree is traversed from the OOM'ing memcg up to the root and
>> corresponding BPF OOM handlers are executed until some memory is
>> freed. If no memory is freed, the kernel OOM killer is invoked.
>> 
>> The struct ops provides the bpf_handle_out_of_memory() callback,
>> which expected to return 1 if it was able to free some memory and 0
>> otherwise. If 1 is returned, the kernel also checks the bpf_memory_freed
>> field of the oom_control structure, which is expected to be set by
>> kfuncs suitable for releasing memory (which will be introduced later
>> in the patch series). If both are set, OOM is considered handled,
>> otherwise the next OOM handler in the chain is executed: e.g. BPF OOM
>> attached to the parent cgroup or the kernel OOM killer.
>
> I still find this dual reporting a bit confusing. I can see your
> intention in having a pre-defined "releasers" of the memory to trust BPF
> handlers more but they do have access to oc->bpf_memory_freed so they
> can manipulate it. Therefore an additional level of protection is rather
> weak.

No, they can't. They have only a read-only access.

> It is also not really clear to me how this works while there is OOM
> victim on the way out. (i.e. tsk_is_oom_victim() -> abort case). This
> will result in no killing therefore no bpf_memory_freed, right? Handler
> itself should consider its work done. How exactly is this handled.

It's a good question, I see your point...
Basically we want to give a handler an option to exit with "I promise,
some memory will be freed soon" without doing anything destructive.
But keeping it save at the same time.

I don't have a perfect answer out of my head, maybe some sort of a
rate-limiter/counter might work? E.g. a handler can promise this N times
before the kernel kicks in? Any ideas?

> Also is there any way to handle the oom by increasing the memcg limit?
> I do not see a callback for that.

There is no kfunc yet, but it's a good idea (which we accidentally
discussed few days ago). I'll implement it.

Thank you!