+ memcg-oom-remove-explicit-wakeup-in-mem_cgroup_oom_synchronize.patch added to mm-unstable branch

+ memcg-oom-remove-explicit-wakeup-in-mem_cgroup_oom_synchronize.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: memcg, oom: remove explicit wakeup in mem_cgroup_oom_synchronize()
has been added to the -mm mm-unstable branch.  Its filename is
     memcg-oom-remove-explicit-wakeup-in-mem_cgroup_oom_synchronize.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/memcg-oom-remove-explicit-wakeup-in-mem_cgroup_oom_synchronize.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Haifeng Xu <haifeng.xu@shopee.com>
Subject: memcg, oom: remove explicit wakeup in mem_cgroup_oom_synchronize()
Date: Wed, 19 Apr 2023 03:07:39 +0000

Before commit 29ef680ae7c2 ("memcg, oom: move out_of_memory back to the
charge path"), all memcg oom killers were delayed to page fault path.  And
the explicit wakeup is used in this case:

thread A:
        ...
        if (locked) {           // complete oom-kill, hold the lock
                mem_cgroup_oom_unlock(memcg);
                ...
        }
        ...

thread B:
        ...

        if (locked && !memcg->oom_kill_disable) {
                ...
        } else {
                schedule();     // can't acquire the lock
                ...
        }
        ...

The reason is that thread A kicks off the OOM-killer, which leads to
wakeups from the uncharges of the exiting task.  But thread B is not
guaranteed to see them if it enters the OOM path after the OOM kills but
before thread A releases the lock.

Now only oom_kill_disable case is handled from the #PF path.  In that case
it is userspace to trigger the wake up not the #PF path itself.  All
potential paths to free some charges are responsible to call
memcg_oom_recover() , so the explicit wakeup is not needed in the
mem_cgroup_oom_synchronize() path which doesn't release any memory itself.

Link: https://lkml.kernel.org/r/20230419030739.115845-2-haifeng.xu@shopee.com
Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memcontrol.c |    9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

--- a/mm/memcontrol.c~memcg-oom-remove-explicit-wakeup-in-mem_cgroup_oom_synchronize
+++ a/mm/memcontrol.c
@@ -2028,15 +2028,8 @@ bool mem_cgroup_oom_synchronize(bool han
 	mem_cgroup_unmark_under_oom(memcg);
 	finish_wait(&memcg_oom_waitq, &owait.wait);
 
-	if (locked) {
+	if (locked)
 		mem_cgroup_oom_unlock(memcg);
-		/*
-		 * There is no guarantee that an OOM-lock contender
-		 * sees the wakeups triggered by the OOM kill
-		 * uncharges.  Wake any sleepers explicitly.
-		 */
-		memcg_oom_recover(memcg);
-	}
 cleanup:
 	current->memcg_in_oom = NULL;
 	css_put(&memcg->css);
_

Patches currently in -mm which might be from haifeng.xu@shopee.com are

cpuset-clean-up-cpuset_node_allowed.patch
memcg-oom-remove-unnecessary-check-in-mem_cgroup_oom_synchronize.patch
memcg-oom-remove-explicit-wakeup-in-mem_cgroup_oom_synchronize.patch