Re: [PATCH] memcg: bypass the reclaim and oom killer for dying tasks once oom_reaper is done

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 28 Jul 2026 19:46:12 -0700 Shakeel Butt <shakeel.butt@linux.dev> wrote:

> At Meta, we are seeing instances where an OOM killed job is stuck in the
> exit path for several hours. In one particular case, the job was stuck
> for more than 8 hours and I had to manually remove the memory.max limits
> to allow the process to exit.

Not seeing the problem here - that sounds like normal kernel behavior ;)

> Reproduced with 20k threads, each parking a robust futex head on
> its own zswapped page, OOM-group-killed while a sibling holds mmap_lock
> for write so the reaper gives up and sets MMF_OOM_SKIP. Tested on
> next-20260728 and baseline show ~90 seconds exit time while with the
> patch the exit time reduced to ~3 seconds.

Very positive.

But it doesn't sound like you're fully confident that the problem is
fully solved?

> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2653,6 +2653,19 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	if (!gfpflags_allow_blocking(gfp_mask))
>  		goto nomem;
>  
> +	/*
> +	 * OOM victim still needs to charge memory to exit. OOM reaper should
> +	 * help but it might fail on mmap_lock contention. If the victim is a
> +	 * large thread group then all exiting threads might compete on oom_lock
> +	 * just to learn that there is nothing really killable anymore. Bail
> +	 * out early and fail the charge to expedite their exit. They are
> +	 * considered fully reclaimed by the oom reaper and they shouldn't
> +	 * contribute further charges.
> +	 */
> +	if (tsk_is_oom_victim(current) &&
> +	    mm_flags_test(MMF_OOM_SKIP, current->signal->oom_mm))
> +		goto nomem;
> +
>  	__memcg_memory_event(mem_over_limit, MEMCG_MAX, allow_spinning);
>  	raised_max_event = true;

Should we backport this?