memcg: move LRU size accounting on reparenting instead of copying it

When a memory cgroup is offlined its LRU folios are reparented to the
parent.  lruvec_reparent_lru() splices the child's lists into the
parent's and credits the parent with the child's per-zone
lru_zone_size[], but never clears the child's copy, so the size is
copied rather than moved.  lru_gen_reparent_memcg() does the same for
MGLRU.

The parent is left correct, credited with exactly the folios it took
over.  The stale value sits on the child and nothing will correct it:
folio->memcg_data now resolves to the parent, so every later
update_lru_size() for those folios goes there.

Dying cgroups are not freed immediately and mem_cgroup_iter() still
walks them, so shrink_lruvec() keeps being called on them.
get_scan_count() reads the phantom counter through lruvec_lru_size() and
the scan loop then grinds through nr[] in SWAP_CLUSTER_MAX steps against
an empty list, for as long as the dead cgroup lives.  Under MGLRU the
MGLRU scanner runs instead, but count_shadow_nodes() sums all of
NR_LRU_LISTS through lruvec_lru_size() and over-budgets the shadow node
limit just the same.

On one 251 GiB host a sweep of every mz->lru_zone_size[] found 380
counters describing folios on no list at all: 124777314 pages, 476 GiB,
1.89x the machine's RAM, across 57 cgroups.  All were on memcgs with
CSS_DYING set and CSS_ONLINE clear, and parent/child pairs reported
byte-identical sizes.

LRU_UNEVICTABLE needs its size moved too.  Its list is deliberately not
spliced because lruvec_init() poisons the head - the unevictable LRU is
imaginary and folios are never threaded on it - but the size is kept by
lruvec_add_folio()/lruvec_del_folio() and those folios account to the
parent from here on.

This depends on commit bf4ade7dbd ("memcg: keep folio's objcg same as
its node") and must not be backported ahead of it.  Without that
invariant a folio's objcg can belong to another node, so a folio already
spliced onto the parent's list can still resolve to the child's lruvec
until the objcg's node is reparented in a later iteration of
memcg_reparent_objcgs(); clearing the child's counter early then lets
lruvec_del_folio() underflow it and trip the WARN_ONCE()/VM_BUG_ON() in
mem_cgroup_update_lru_size().

Link: https://lore.kernel.org/20260822024707.77192-1-shakeel.butt@linux.dev
Fixes: 07a6e9a2c1 ("mm: vmscan: prepare for reparenting traditional LRU folios")
Fixes: f304652609 ("mm: vmscan: prepare for reparenting MGLRU folios")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: <stable@vger.kernel.org> # After: bf4ade7dbd76: memcg: keep folio's objcg same as its node
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/folio.c

@@ -1130,7 +1130,16 @@ static void lruvec_reparent_lru(struct lruvec *child_lruvec,
 	for_each_managed_zone_pgdat(zone, NODE_DATA(nid), zid, MAX_NR_ZONES - 1) {
 		unsigned long size = mem_cgroup_get_zone_lru_size(child_lruvec, lru, zid);
 
+		if (!size)
+			continue;
+
+		/*
+		 * The folios are accounted to the parent from now on, so the
+		 * size has to be moved, not just copied. Leaving it behind
+		 * makes the dying child describe folios it no longer owns.
+		 */
 		mem_cgroup_update_lru_size(parent_lruvec, lru, zid, size);
+		mem_cgroup_update_lru_size(child_lruvec, lru, zid, -(long)size);
 	}
 }
 

mm/vmscan.c

@@ -4635,7 +4635,12 @@ void lru_gen_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent,
 		for_each_managed_zone_pgdat(zone, NODE_DATA(nid), zid, MAX_NR_ZONES - 1) {
 			unsigned long size = mem_cgroup_get_zone_lru_size(child_lruvec, lru, zid);
 
+			if (!size)
+				continue;
+
+			/* Move the accounting, do not duplicate it. */
 			mem_cgroup_update_lru_size(parent_lruvec, lru, zid, size);
+			mem_cgroup_update_lru_size(child_lruvec, lru, zid, -(long)size);
 		}
 	}
 }