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[109.81.81.112]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6a59e00108asm12355655a12.2.2026.08.24.04.47.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 Aug 2026 04:47:55 -0700 (PDT) Date: Mon, 24 Aug 2026 13:47:54 +0200 From: Michal Hocko To: Shakeel Butt Cc: Andrew Morton , Johannes Weiner , Muchun Song , Qi Zheng , Roman Gushchin , Meta kernel team , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] memcg: move LRU size accounting on reparenting instead of copying it Message-ID: References: <20260822024707.77192-1-shakeel.butt@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260822024707.77192-1-shakeel.butt@linux.dev> X-Rspam-User: X-Stat-Signature: 7xi39mg4mzwyaxe5juspxuxsght5yfm4 X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 935DC100005 X-HE-Tag: 1787572077-945172 X-HE-Meta: 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 XYIw9dwE zjrjGKiwJLqX7q3v7NFV1zHAmZ6vaGTKCobhj3gMTXaJS4nEMO1pTNyu0C8mcK+Xp3VPz2FdONlbCowDMT84sMHiwTvbe9RrWrGbrTIlxRc97cDdFDspcn4Z2x7bo7Z4SCdaEfCz5VBauA9J3wU7E1jx3aGpLZKabkx4UGy4vaj3RkNF4A2TPfszcs10M1eqx7WklaUX3vtzwZBrEqfrt0SM8bJp5B6QyJ3hwS7hnI68MUEdh+VLbfwDAYx0TrodshweoU8ZQFMr2o8galFzcBYGe575O6dwMuWfRXvnyWFY1JloD29DvPzqorRLzx2JrZsDEv2/dkcP/+PUodVExcaVnV2KQxwP+v/6ufDEViK1q3fhslkiZFTVYUuJxh4lnOIVR2h4nELhp/4PjewIxZjLUbDUt87QMV8GHIpus79evqJtYUDbI6G9cx2rURHflJNA2 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri 21-08-26 19:47:07, Shakeel Butt wrote: > 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(). > > Fixes: 07a6e9a2c199 ("mm: vmscan: prepare for reparenting traditional LRU folios") > Fixes: f304652609ea ("mm: vmscan: prepare for reparenting MGLRU folios") > Cc: # After: bf4ade7dbd76: memcg: keep folio's objcg same as its node > Signed-off-by: Shakeel Butt Acked-by: Michal Hocko Thanks! > --- > 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 fe7f0c52a18c..561eeec5628c 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); > } > } > } > -- > 2.53.0-Meta -- Michal Hocko SUSE Labs