From 0e0ac326c511d514817cc7b6d7741afd59098ce2 Mon Sep 17 00:00:00 2001 From: Shakeel Butt Date: Fri, 21 Aug 2026 19:47:07 -0700 Subject: [PATCH] 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 bf4ade7dbd76 ("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: 07a6e9a2c199 ("mm: vmscan: prepare for reparenting traditional LRU folios") Fixes: f304652609ea ("mm: vmscan: prepare for reparenting MGLRU folios") Signed-off-by: Shakeel Butt Acked-by: Michal Hocko Cc: Johannes Weiner Cc: Roman Gushchin Cc: Muchun Song Cc: # After: bf4ade7dbd76: memcg: keep folio's objcg same as its node Signed-off-by: Andrew Morton --- mm/folio.c | 9 +++++++++ mm/vmscan.c | 5 +++++ 2 files changed, 14 insertions(+) diff --git a/mm/folio.c b/mm/folio.c index 59c477120b9a..c02dcea9c03c 100644 --- a/mm/folio.c +++ b/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); } } diff --git a/mm/vmscan.c b/mm/vmscan.c index b569eeca590d..73a81b4a3e16 100644 --- a/mm/vmscan.c +++ b/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); } } }