The patch titled
Subject: mm: increment gen # before restarting traversal
has been added to the -mm mm-unstable branch. Its filename is
mm-increment-gen-before-restarting-traversal.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-increment-gen-before-restarting-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: increment gen # before restarting traversal
Date: Thu, 5 Sep 2024 00:30:52 +0000
The generation number in struct mem_cgroup_reclaim_iter should be
incremented on every round-trip. Currently, it is possible for a
concurrent reclaimer to jump in at the end of the hierarchy, causing a
traversal restart (resetting the iteration position) without incrementing
the generation number.
By resetting the position without incrementing the generation, it's
possible for another ongoing mem_cgroup_iter() thread to walk the tree
twice.
Move the traversal restart such that the generation number is
incremented before the restart.
Link: https://lkml.kernel.org/r/20240905003058.1859929-4-kinseyho@xxxxxxxxxx
Signed-off-by: Kinsey Ho <kinseyho@xxxxxxxxxx>
Reviewed-by: T.J. Mercier <tjmercier@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>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 22 ++++++++++++----------
1 file changed, 12 insertions(+), 10 deletions(-)
--- a/mm/memcontrol.c~mm-increment-gen-before-restarting-traversal
+++ a/mm/memcontrol.c
@@ -999,7 +999,7 @@ struct mem_cgroup *mem_cgroup_iter(struc
root = root_mem_cgroup;
rcu_read_lock();
-
+restart:
if (reclaim) {
struct mem_cgroup_per_node *mz;
@@ -1026,14 +1026,6 @@ struct mem_cgroup *mem_cgroup_iter(struc
for (;;) {
css = css_next_descendant_pre(css, &root->css);
if (!css) {
- /*
- * Reclaimers share the hierarchy walk, and a
- * new one might jump in right at the end of
- * the hierarchy - make sure they see at least
- * one group and restart from the beginning.
- */
- if (!prev)
- continue;
break;
}
@@ -1056,8 +1048,18 @@ struct mem_cgroup *mem_cgroup_iter(struc
*/
(void)cmpxchg(&iter->position, pos, memcg);
- if (!memcg)
+ if (!memcg) {
iter->generation++;
+
+ /*
+ * Reclaimers share the hierarchy walk, and a
+ * new one might jump in right at the end of
+ * the hierarchy - make sure they see at least
+ * one group and restart from the beginning.
+ */
+ if (!prev)
+ goto restart;
+ }
}
out_unlock:
_
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
