Commit 4a2698b0 authored Sep 05, 2024 by Kinsey Ho Committed by Andrew Morton Sep 09, 2024

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

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>

parent 0e40cf2a

mm/memcontrol.c

+1 −17

Original line number	Diff line number	Diff line
		@@ -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);
		}
		} 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++;
		}