From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5AA1D48EC88 for ; Thu, 10 Sep 2026 15:47:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789055270; cv=none; b=lVekq9YrGAffNvHLAPEVURMsAY9TZwvaO9r/6+h92RWor7cWlR8fDXBjXqAoY9G0HYVQUhPPrDKKDw1F1tOPI3fhtZOOvzxPZHRr0EL+wofewyCz84GwMQdP1t+a1wQFPzcYsi7fzSc/pRn5hslnRAxH/2s7YGprZB6Vaae6/WQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789055270; c=relaxed/simple; bh=cE6ZH5i+do18EzZe8BgZNylCKZsFAk/fHM6qVE4gpkI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=II2oLCCfVrC1hffoXrIgzOq2PjYLdeZd8d91chr6oCXibrW8HDGRTE9Dvs7Fq6k08QB+mPfnSBO+JEMewNf+4r0NFset9+Wqh2ZzJcc4ZG/PgQyGtYhASqFSbEWJu5/8YQETvw8rDLyrPi7t/MyEgZKiwIoumB331sT9unZ0O/4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=a1HhBAR6; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="a1HhBAR6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D3C211F000FF; Thu, 10 Sep 2026 15:47:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789055268; bh=evomDcAdWKURPQFilS/ODkODT9xt93Jpj2agY7hg1Ro=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=a1HhBAR67HgYN7Denn0z/7Fl8HzMzdHirNsa51YjVG9S0tZs/WxROCf8uOlOS5He+ 0skVMfYF7ri7s1Wbtw7XbCRbfohfgeNxpisle27tmqdh26wadxzG2EQwCoYt5xJuqe ia5WvNKw4tBRe/Au/MBOFhymgxhxRq7qPbrFjVS6d8fhnLSzO7YtDozzEaAcNom7aj 6dEl0fajNYsClJ/7ZBGKAPjEYH/QTCjv7uIYIgBOazSl/hSJL0wEnK6juF2gZkZV1b h98AE1R5DIQi0CPBzgmVq9+tug/4vGSB9vHhxzs0DRDONajlWdGI/BCDanhDaJUXLr CNRfrXddX0FeA== From: Sasha Levin To: stable@vger.kernel.org Cc: Shakeel Butt , Michal Hocko , Johannes Weiner , Roman Gushchin , Muchun Song , Andrew Morton , Sasha Levin Subject: [PATCH 7.2.y] memcg: move LRU size accounting on reparenting instead of copying it Date: Thu, 10 Sep 2026 11:47:41 -0400 Message-ID: <20260910154741.1039372-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026090956-reorder-flypaper-cf1a@gregkh> References: <2026090956-reorder-flypaper-cf1a@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Shakeel Butt [ Upstream commit 0e0ac326c511d514817cc7b6d7741afd59098ce2 ] 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 Signed-off-by: Sasha Levin --- mm/swap.c | 9 +++++++++ mm/vmscan.c | 5 +++++ 2 files changed, 14 insertions(+) diff --git a/mm/swap.c b/mm/swap.c index 588f50d8f1a8c..1a87e56bdcc52 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -1153,7 +1153,16 @@ 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 ee3efc6213f49..8781dbcdb99a6 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -4554,7 +4554,12 @@ 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