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 7EDFF5328AD; Wed, 23 Sep 2026 14:27:22 +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=1790173643; cv=none; b=njjCCnsyTNogQb1gZM23eJ6eL9zgfE3jjIyziTwlZiNWXh+OZRwSj4B9Yn8tLV3D9UEQqfgaiMB56NjUONt3CuTkjPYlhG+xMvQ2l656R0uBSi/hzxjUezDhS9JF4MasR+cQ18uudbsdK/WKnb8s0+18XU0/Dh06wKo5krWrXd4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790173643; c=relaxed/simple; bh=xroo30XIGd1/SKQfHN/BJrspdU4qxclGy/vkJvenVes=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=O+pVRTKKVa1meltvNwmJLd3k4aZVLgSIl6OJVxL/ZUXpPdnZxpdfxI9p/niz9SXkAbHrbOauacxEW3vSAVVMO3pUKi97QAFWm33SHXmTTeTRmnB49YlSlqZoJit5UaDGTLkhTiJJ1+J++NGLuXeg9WR4yqtNqp+cnAVSpkbxgRI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=QRGtZUun; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="QRGtZUun" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D283E1F000FF; Wed, 23 Sep 2026 14:27:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1790173642; bh=5s0a36Qrv63XfO50xbG+YSAC7XonqMCjk8OjFBWCQAw=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=QRGtZUun3ZvpNsF+EFOZpkgNPUR5kgAmJXLmp1Jmzbnphtp9dmRTHC0mQlvX+9IJl NNjodU5KjKfPiaDa2cCVCbzfU7Afi86EWCLteS0EaF9cpRyMo1lwkx/JgqtOBJq6YL soKG9j6U7kIcz2MAF83YXMURYR9WufgDCwjJzLD8= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Shakeel Butt , Farhad Alemi , Muchun Song , Johannes Weiner , Michal Hocko , Roman Gushchin , Andrew Morton Subject: [PATCH 7.2 311/438] memcg: avoid charging the root memcg from obj_cgroup_charge_pages() Date: Wed, 23 Sep 2026 16:05:32 +0200 Message-ID: <20260923140652.839610268@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260923140644.756254324@linuxfoundation.org> References: <20260923140644.756254324@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Shakeel Butt commit 6e673d0879ef78c395cfe0d3ba316690a60055d8 upstream. obj_cgroup_charge_pages() resolves the objcg to its memcg and calls try_charge_memcg(), which does not short circuit the root memcg. That memcg can be the root memcg: obj_cgroup_is_root() reflects the memcg the objcg was created for and is never updated, while memcg_reparent_objcgs() does redirect objcg->memcg to the parent on rmdir. An objcg of a dying child of root therefore passes every obj_cgroup_is_root() filter but resolves to the root memcg. Folios keep the objcg they were charged with, so this is easy to reach through zswap: allocate anon memory in a cgroup, move the task out, remove the cgroup, then write to the root cgroup's memory.reclaim. The reclaimed folios are charged through the reparented objcg and end up in refill_stock() with the root memcg: WARNING: mm/memcontrol.c:2198 at refill_stock+0x644/0x940 refill_stock+0x644/0x940 try_charge_memcg+0x12d6/0x1570 __obj_cgroup_charge+0x35/0xf0 obj_cgroup_charge+0x1de/0x210 obj_cgroup_charge_zswap+0x83/0x270 zswap_store+0x1620/0x2000 swap_writeout+0x94c/0x14c0 shrink_folio_list+0x3388/0x52b0 [...] try_to_free_mem_cgroup_pages+0x30d/0x830 user_proactive_reclaim+0x504/0x840 memory_reclaim+0x1f/0x30 Beyond the warning, the charge is asymmetric: obj_cgroup_uncharge_pages() skips refill_stock() for the root memcg, so the root's page counter grows and is never uncharged. It is not user visible, since memory.current is not exposed on the root, but it is a leak. Use try_charge(), which returns early for the root memcg, restoring the symmetry with obj_cgroup_uncharge_pages(). The above sequence was scripted into a standalone reproducer (zswap on, swap on a virtio disk, 512MB of anon memory faulted in inside a child of the root cgroup, the task then migrated to the root cgroup, the child removed, followed by "echo 600M swappiness=max > memory.reclaim" on the root) and run in a CONFIG_DEBUG_VM=y VM. It reproduces the splat on the first zswap store of a reparented folio, with the same call chain as the report. With this patch applied the splat is gone while the zswap store count over the run is unchanged, so the same path is still exercised. cgroup selftests test_zswap, test_kmem and test_memcontrol show no new failures. Link: https://lore.kernel.org/20260829023251.474083-1-shakeel.butt@linux.dev Fixes: 20d6c1725228 ("memcg: avoid refill_stock for root memcg") Signed-off-by: Shakeel Butt Reported-by: Farhad Alemi Closes: https://lore.kernel.org/all/CA+0ovCgWzUMK+nNbbtH7eV65Ca=fDN4Ozu7iASgryjvv8Tk8zQ@mail.gmail.com/ Reviewed-by: Muchun Song Reviewed-by: Johannes Weiner Cc: Michal Hocko Cc: Roman Gushchin Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- mm/memcontrol.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3107,7 +3107,7 @@ static int obj_cgroup_charge_pages(struc memcg = get_mem_cgroup_from_objcg(objcg); - ret = try_charge_memcg(memcg, gfp, nr_pages); + ret = try_charge(memcg, gfp, nr_pages); if (ret) goto out;