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mm: increment gen # before restarting traversal

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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@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 00:30:52 +00:00 committed by Andrew Morton
parent 4a2698b013
commit 3d150e31a1
1 changed files with 12 additions and 10 deletions
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22
mm/memcontrol.c
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@ -997,7 +997,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
root = root_mem_cgroup; root = root_mem_cgroup;
rcu_read_lock(); rcu_read_lock();
restart:
if (reclaim) { if (reclaim) {
struct mem_cgroup_per_node *mz; struct mem_cgroup_per_node *mz;
@ -1024,14 +1024,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
for (;;) { for (;;) {
css = css_next_descendant_pre(css, &root->css); css = css_next_descendant_pre(css, &root->css);
if (!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; break;
} }
@ -1054,8 +1046,18 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
*/ */
(void)cmpxchg(&iter->position, pos, memcg); (void)cmpxchg(&iter->position, pos, memcg);
if (!memcg) if (!memcg) {
iter->generation++; 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: out_unlock: