From: Shakeel Butt Date: Mon, 1 Jun 2026 16:15:01 +0000 (-0700) Subject: mm/list_lru: drain before clearing xarray entry on reparent X-Git-Tag: v7.1~25^2~2 X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=98733f3f0becb1ae0701d021c1748e974e5fa55c;p=thirdparty%2Fkernel%2Fstable.git mm/list_lru: drain before clearing xarray entry on reparent memcg_reparent_list_lrus() clears the dying memcg's xarray entry with xas_store(&xas, NULL) before reparenting its per-node lists into the parent. This opens a window where a concurrent list_lru_del() arriving for the dying memcg sees xa_load() == NULL, walks to the parent in lock_list_lru_of_memcg(), takes the parent's per-node lock, and calls list_del_init() on an item still physically linked on the dying memcg's list. If another in-flight thread holds the dying memcg's per-node lock at the same moment (another list_lru_del, or a list_lru_walk_one running an isolate callback), both threads modify ->next/->prev pointers on the same physical list under different locks. Adjacent items can corrupt each other's links. Fix it by reversing the order: reparent each per-node list and mark the child's list lru dead and then clear the xarray entry. Any concurrent list_lru op that finds the still-set xarray entry either takes the dying memcg's per-node lock (synchronizing with the drain) or sees LONG_MIN and walks to the parent, where the items now live. Link: https://lore.kernel.org/20260601161501.1444829-1-shakeel.butt@linux.dev Fixes: fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup scope") Signed-off-by: Shakeel Butt Reported-by: Chris Mason Reviewed-by: Kairui Song Acked-by: Muchun Song Cc: Dave Chinner Cc: Johannes Weiner Cc: Roman Gushchin Cc: Signed-off-by: Andrew Morton --- diff --git a/mm/list_lru.c b/mm/list_lru.c index dd29bcf8eb5f..9bf7f524796b 100644 --- a/mm/list_lru.c +++ b/mm/list_lru.c @@ -473,26 +473,29 @@ void memcg_reparent_list_lrus(struct mem_cgroup *memcg, struct mem_cgroup *paren mutex_lock(&list_lrus_mutex); list_for_each_entry(lru, &memcg_list_lrus, list) { struct list_lru_memcg *mlru; - XA_STATE(xas, &lru->xa, memcg->kmemcg_id); /* - * Lock the Xarray to ensure no on going list_lru_memcg - * allocation and further allocation will see css_is_dying(). + * css_is_dying() check in memcg_list_lru_alloc() avoids + * allocating a new mlru since CSS_DYING is already set for this + * memcg a rcu grace period ago. */ - xas_lock_irq(&xas); - mlru = xas_store(&xas, NULL); - xas_unlock_irq(&xas); + mlru = xa_load(&lru->xa, memcg->kmemcg_id); if (!mlru) continue; /* - * With Xarray value set to NULL, holding the lru lock below - * prevents list_lru_{add,del,isolate} from touching the lru, - * safe to reparent. + * Reparent each per-node list and mark the child dead + * (LONG_MIN) before clearing xarray entry otherwise a + * concurrent list_lru_del() may corrupt the list if it arrives + * after xarray clear but before reparenting as + * lock_list_lru_of_memcg will acquire parent's lock while the + * item is still on child's list. */ for_each_node(i) memcg_reparent_list_lru_one(lru, i, &mlru->node[i], parent); + xa_erase_irq(&lru->xa, memcg->kmemcg_id); + /* * Here all list_lrus corresponding to the cgroup are guaranteed * to remain empty, we can safely free this lru, any further