linux-kernel - [PATCH mm-unstable v3 2/5] mm: don't hold css->refcnt during traversal

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Message-ID: <20240827230753.2073580-3-kinseyho@google.com>
Date: Tue, 27 Aug 2024 23:07:39 +0000
From: Kinsey Ho <kinseyho@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	Yosry Ahmed <yosryahmed@...gle.com>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>, mkoutny@...e.com, 
	Kinsey Ho <kinseyho@...gle.com>
Subject: [PATCH mm-unstable v3 2/5] 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.

Signed-off-by: Kinsey Ho <kinseyho@...gle.com>
---
 mm/memcontrol.c | 18 +-----------------
 1 file changed, 1 insertion(+), 17 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 35431035e782..67b1994377b7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -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++;
 	}
-- 
2.46.0.295.g3b9ea8a38a-goog