memcg, oom: do not bypass oom killer for dying tasks

memcg, oom: do not bypass oom killer for dying tasks

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.

Link: https://lore.kernel.org/all/20241212183012.GB1026@cmpxchg.org/T/#u
Link: https://lkml.kernel.org/r/20250402090117.130245-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>