The patch titled
Subject: mm: don't hold css->refcnt during traversal
has been added to the -mm mm-unstable branch. Its filename is
mm-dont-hold-css-refcnt-during-traversal.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-dont-hold-css-refcnt-during-traversal.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: Kinsey Ho <kinseyho@xxxxxxxxxx>
Subject: mm: don't hold css->refcnt during traversal
Date: Tue, 27 Aug 2024 23:07:39 +0000
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/20240827230753.2073580-3-kinseyho@xxxxxxxxxx
Signed-off-by: Kinsey Ho <kinseyho@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Michal Koutný <mkoutny@xxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Cc: Zefan Li <lizefan.x@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 18 +-----------------
1 file changed, 1 insertion(+), 17 deletions(-)
--- a/mm/memcontrol.c~mm-dont-hold-css-refcnt-during-traversal
+++ a/mm/memcontrol.c
@@ -1013,20 +1013,7 @@ struct mem_cgroup *mem_cgroup_iter(struc
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(struc
*/
(void)cmpxchg(&iter->position, pos, memcg);
- if (pos)
- css_put(&pos->css);
-
if (!memcg)
iter->generation++;
}
_
Patches currently in -mm which might be from kinseyho@xxxxxxxxxx are
cgroup-clarify-css-sibling-linkage-is-protected-by-cgroup_mutex-or-rcu.patch
mm-dont-hold-css-refcnt-during-traversal.patch
mm-increment-gen-before-restarting-traversal.patch
mm-restart-if-multiple-traversals-raced.patch
mm-clean-up-mem_cgroup_iter.patch
