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mm: don't hold css->refcnt during traversal

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To obtain the pointer to the next memcg position, mem_cgroup_iter()
currently holds css->refcnt during memcg traversal only to put css->refcnt
at the end of the routine.  This isn't necessary as an rcu_read_lock is
already held throughout the function.  The use of the RCU read lock with
css_next_descendant_pre() guarantees that sibling linkage is safe without
holding a ref on the passed-in @css.

Remove css->refcnt usage during traversal by leveraging RCU.

Link: https://lkml.kernel.org/r/20240905003058.1859929-3-kinseyho@google.com
Signed-off-by: Kinsey Ho <kinseyho@google.com>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Kinsey Ho 2024-09-05 00:30:51 +00:00 committed by Andrew Morton
parent 0e40cf2a8b
commit 4a2698b013
1 changed files with 1 additions and 17 deletions
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18
mm/memcontrol.c
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@ -1013,20 +1013,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
else if (reclaim->generation != iter->generation)
goto out_unlock;
while (1) {
pos = READ_ONCE(iter->position);
if (!pos || css_tryget(&pos->css))
break;
/*
* css reference reached zero, so iter->position will
* be cleared by ->css_released. However, we should not
* rely on this happening soon, because ->css_released
* is called from a work queue, and by busy-waiting we
* might block it. So we clear iter->position right
* away.
*/
(void)cmpxchg(&iter->position, pos, NULL);
}
pos = READ_ONCE(iter->position);
} else if (prev) {
pos = prev;
}
@ -1067,9 +1054,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
*/
(void)cmpxchg(&iter->position, pos, memcg);
if (pos)
css_put(&pos->css);
if (!memcg)
iter->generation++;
}