Re: [PATCH] memcg, oom: do not bypass oom killer for dying tasks

[Johannes Weiner]

On Wed, Apr 02, 2025 at 11:01:17AM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> 7775face2079 ("memcg: killed threads should not invoke memcg OOM killer") has added
> a bypass of the oom killer path for dying threads because a very
> specific workload (described in the changelog) could hit "no killable
> tasks" path. This itself is not fatal condition but it could be annoying
> if this was a common case.
> 
> On the other hand the bypass has some issues on its own. Without
> triggering oom killer we won't be able to trigger async oom reclaim
> (oom_reaper) which can operate on killed tasks as well as long as they
> still have their mm available. This could be the case during futex
> cleanup when the memory as pointed out by Johannes in [1]. The said case
> is still not fully understood but let's drop this bypass that was mostly
> driven by an artificial workload and allow dying tasks to go into oom
> path. This will make the code easier to reason about and also help
> corner cases where oom_reaper could help to release memory.
> 
> [1] https://lore.kernel.org/all/20241212183012.GB1026@xxxxxxxxxxx/T/#u
> 
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks, yeah, the investigation stalled out over the new years break
and then... distractions.

I think we'll eventually still need the second part of [2], to force
charge from dying OOM victims, but let's go with this for now.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

[2] https://lore.kernel.org/all/20241212183012.GB1026@xxxxxxxxxxx/

> ---
>  mm/memcontrol.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7b3503d12aaf..9c30c442e3b0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1627,7 +1627,7 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	 * A few threads which were not waiting at mutex_lock_killable() can
>  	 * fail to bail out. Therefore, check again after holding oom_lock.
>  	 */
> -	ret = task_is_dying() || out_of_memory(&oc);
> +	ret = out_of_memory(&oc);
>  
>  unlock:
>  	mutex_unlock(&oom_lock);
> -- 
> 2.49.0