--
Best regards,
Konstantin Khorenko,
Virtuozzo Linux Kernel Team
On 02/26/2021 12:12 PM, Kirill Tkhai wrote:
On 24.02.2021 21:55, Konstantin Khorenko wrote:
It's quite strange to have rcu section in mem_cgroup_iter(),
but do not use rcu_dereference/rcu_assign for pointers being defended.
We plan to access/assign '->last_visited' during iterator invalidation,
so we'll need the protection there anyway.
https://jira.sw.ru/browse/PSBM-123655
Signed-off-by: Konstantin Khorenko <[email protected]>
---
mm/memcontrol.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8040f09425bf..d0251d27de00 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -194,8 +194,18 @@ struct mem_cgroup_reclaim_iter {
/*
* last scanned hierarchy member. Valid only if last_dead_count
* matches memcg->dead_count of the hierarchy root group.
+ *
+ * Memory pointed by 'last_visited' is freed not earlier than
+ * one rcu period after we accessed it:
+ * cgroup_offline_fn()
+ * offline_css()
+ * list_del_rcu()
+ * dput()
+ * ...
+ * cgroup_diput()
+ * call_rcu(&cgrp->rcu_head, cgroup_free_rcu)
*/
- struct mem_cgroup *last_visited;
+ struct mem_cgroup __rcu *last_visited;
unsigned long last_dead_count;
/* scan generation, increased every round-trip */
@@ -1591,8 +1601,7 @@ mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
*/
*sequence = atomic_read(&root->dead_count);
if (iter->last_dead_count == *sequence) {
- smp_rmb();
- position = iter->last_visited;
+ position = rcu_dereference(iter->last_visited);
/*
* We cannot take a reference to root because we might race
@@ -1620,8 +1629,7 @@ static void mem_cgroup_iter_update(struct
mem_cgroup_reclaim_iter *iter,
* don't lose destruction events in between. We could have
* raced with the destruction of @new_position after all.
*/
- iter->last_visited = new_position;
- smp_wmb();
We can't remove barriers in this patch, this makes the patch wrong.
We should remove barriers somewhere in [9/9].
JFYI: i've moved this not to [9/9], but to
[PATCH rh7 v4 7/9] mm/mem_cgroup_iter: Don't bother checking 'dead_count'
anymore
+ rcu_assign_pointer(iter->last_visited, new_position);
iter->last_dead_count = sequence;
/* root reference counting symmetric to mem_cgroup_iter_load */
@@ -1681,7 +1689,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup
*root,
mz = mem_cgroup_zoneinfo(root, nid, zid);
iter = &mz->reclaim_iter[reclaim->priority];
if (prev && reclaim->generation != iter->generation) {
- iter->last_visited = NULL;
+ rcu_assign_pointer(iter->last_visited, NULL);
goto out_unlock;
}
.
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