From nobody Sat Sep 26 13:09:04 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 74D722C0F8C for ; Tue, 1 Sep 2026 06:49:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788245365; cv=none; b=EHMFAV2bkaRj+makAbfTOFsDiYtLzr6JvNdwdN6meqwXgeZUOc3VIxnw1B01g7uTTvkrGLgXtZEDt23uix2tt5gDDeZF8HgdqBfSnK3oEQlOjIGe7oCwYIAo8SnWKpgr0KRyyGZBBW7Jia261XiFSg6bCEb5x28cDosDeIk4khI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788245365; c=relaxed/simple; bh=jQ0/pgiELTyKSv5BNA3IsrLBQ9kpwm2mvn3Md2YSgV8=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:To:Cc; b=f4BDjgRr31KDDY8BmZG1ktBgyueUzkdzbGruHfzMT2qnG836ldvAy0kYZSJzc0GeOaYl96+aPaSgD+G3N1umlSpvbouNwadlt22DVbiCzdJ02Zex8aD0LbJhn2sy0Beq5uS9gQoA/EXS/ZRq4jTAKhqGdE63e9qx0iudxN44j4Y= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=OAGkSyH/; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="OAGkSyH/" Received: by smtp.kernel.org (Postfix) with ESMTPS id F080FC2BCF5; Tue, 1 Sep 2026 06:49:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1788245365; bh=jQ0/pgiELTyKSv5BNA3IsrLBQ9kpwm2mvn3Md2YSgV8=; h=From:Date:Subject:To:Cc:Reply-To:From; b=OAGkSyH/cHcRpa/rxlmPqLpBtqva002/BA2z/BEBxHKu3mkQO2g+ram0jL0aYLC3R yeH+q5BnvPF3r0lJFKqxWtgQSn5DBnOLcIYvjBGxTi3Ldluyt4NDGPLoj5q0VV8k3D 4/03WIChjiro5qru6g/IQvmAs3a8Yaw3FEmdCeTvvf3ArZQoP6jZjYBfh66RNlATbX Pxia50VPsp6YGcXEIyvCcuqlVjJ/vdOzY7OkcBt0i/TbM8SaXF4+qNk0oBYpocMXVt Dcx/EFqbFihcNn1xLPxgSVyNXwLkF7PT4pQt+AHifNviDzFAZqIFZBiWvCwoD8qgi+ X6joIf2GGBwyQ== Received: from aws-us-west-2-korg-lkml-1.web.codeaurora.org (localhost.localdomain [127.0.0.1]) by smtp.lore.kernel.org (Postfix) with ESMTP id C9F87C61DD3; Tue, 1 Sep 2026 06:49:24 +0000 (UTC) From: Bingfang Guo via B4 Relay Date: Tue, 01 Sep 2026 14:49:11 +0800 Subject: [PATCH v2] mm/memcg: clear folio memcg after changing per memcg stats Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260901-memcg-swapcache-stats-fix-v2-1-9caad330459b@tencent.com> X-B4-Tracking: v=1; b=H4sIAGZ1lmoC/32NwQqDMBBEf0X23C0mgqQ99T+Kh2RdNQejZENsE f+9qfRc5vQG5s0OwtGzwL3aIXL24pdQQF8qoMmGkdH3hUHXuq2NNjjzTCPKZleyNDFKsklw8C9 Ujnloemrs4KDs18ilPt3PrvDkJS3xfV5l9W1/1kb9sWaFJVQbdWPXO9M+EgfikK60zNAdx/EBV /EG3cQAAAA= To: Andrew Morton , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Barry Song , Youngjun Park , Qi Zheng , Shakeel Butt , Axel Rasmussen , Yuanchu Xie , Wei Xu , Johannes Weiner , David Hildenbrand , Michal Hocko , Lorenzo Stoakes , Bingfang Guo Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Bingfang Guo X-Mailer: b4 0.13.0 X-Developer-Signature: v=1; a=ed25519-sha256; t=1788245363; l=7992; i=bingfangguo@tencent.com; s=20260812; h=from:subject:message-id; bh=4FztJIrokZHGN2SlXXzmWENVmLBTnEVTBI5yiE1+1VQ=; b=DiK5n7qAly2OitjlWEKUNrJhfm6FISgJap/2n5ed6BgAJEOSzjG6/k8IEIZJjx/oAnldr5O4V x50SbV7azrsAjXMbu4PFAlC5dzGpjQKLAuHd1zd5krNpG+4lwBUsy2h X-Developer-Key: i=bingfangguo@tencent.com; a=ed25519; pk=q7u9S7d5e7VijWrYF9O+vv9/XvqWduVGtwGtuCObAHE= X-Endpoint-Received: by B4 Relay for bingfangguo@tencent.com/20260812 with auth_id=942 X-Original-From: Bingfang Guo Reply-To: bingfangguo@tencent.com From: Bingfang Guo I notice extremely high swapcached count in the per memcg level memory.stat when running tests with cgroupv1 setup by swapping pages in and out. It seems that the counter never gets decreased so the value is rather useless and confusing to users reading it. So I think fixing it so that the value can reflect the actual swapcache usage correctly could be helpful. __memcg1_swapout() transfers the memsw charge of a folio to its swap entry and clears folio->memcg_data as part of that. In the vmscan swapout path it runs before __swap_cache_del_folio(), which then decrements the swapcache stats through lruvec_stat_mod_folio(). Since folio->memcg_data has already been cleared, folio_memcg() returns NULL and the NR_SWAPCACHE decrement only updates the node-level counter instead of the memcg's lruvec, leaking the per-memcg swapcache count. Move the __memcg1_swapout() call into __swap_cache_del_folio(), after the NR_FILE_PAGES and NR_SWAPCACHE updates but before __swap_cache_do_del_folio() removes the folio from the swap cache. This keeps the stats attributed to the folio's memcg while still recording the swap cgroup with a valid folio->swap. Add a swapout parameter so the plain swap_cache_del_folio() path is left unchanged. Fixes: b197d41462c20 ("mm/memcg, swap: store cgroup id in cluster table dir= ectly") Signed-off-by: Bingfang Guo --- The problem is reproducible using the following script and program: ``` #!/bin/bash set -e CG=3D/sys/fs/cgroup/memory/swapcache-leak-test SIZE=3D$((256 * 1024 * 1024)) # 256 MiB of anon memory [ "$(id -u)" -eq 0 ] || { echo "must run as root"; exit 1; } grep -q . /proc/swaps <<<"$(tail -n +2 /proc/swaps)" || { echo "no swap act= ive; run: swapon "; exit 1; } cleanup() { rmdir "$CG" 2>/dev/null || true; } trap cleanup EXIT cc -O2 swapout.c -o swapout mkdir -p "$CG" echo "+memory" > /sys/fs/cgroup/cgroup.subtree_control 2>/dev/null || true echo "=3D=3D before reclaim =3D=3D" grep -E '^(swapcached|anon) ' "$CG/memory.stat" # Put ourselves in the cgroup, allocate & touch anon memory, then wait to b= e reclaimed. ( echo $BASHPID > "$CG/cgroup.procs" # Allocate and dirty SIZE bytes of anonymous memory. ./swapout ) & WORKER=3D$! sleep 2 echo "=3D=3D after reclaim (swap cache should drain to ~0) =3D=3D" grep -E '^(swapcached|anon) ' "$CG/memory.stat" SWAPCACHED=3D$(awk '/^swapcached /{print $2}' "$CG/memory.stat") echo if [ "$SWAPCACHED" -gt $((1024 * 1024)) ]; then echo "LEAK DETECTED: swapcached =3D $SWAPCACHED bytes (expected ~0)= [BUGGY kernel]" RC=3D1 else echo "OK: swapcached =3D $SWAPCACHED bytes [FIXED kernel]" RC=3D0 fi kill "$WORKER" 2>/dev/null || true wait "$WORKER" 2>/dev/null || true exit $RC ``` swapout.c: ``` #include #include #include #include #include int main(int argc, char **argv) { size_t mib =3D (argc > 1) ? strtoul(argv[1], NULL, 10) : 256; size_t size =3D mib * 1024UL * 1024UL; long page =3D sysconf(_SC_PAGESIZE); char *buf; size_t i; buf =3D mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); if (buf =3D=3D MAP_FAILED) { perror("mmap"); return 1; } /* Fault in and dirty every page so it becomes reclaimable anon. */ for (i =3D 0; i < size; i +=3D page) buf[i] =3D 1; printf("allocated and dirtied %zu MiB, paging out...\n", mib); /* Force the whole range out to swap. */ if (madvise(buf, size, MADV_PAGEOUT)) { perror("madvise(MADV_PAGEOUT)"); return 1; } /* Give reclaim a moment, then stay alive so the cgroup can be insp= ected. */ printf("paged out; sleeping so memory.stat can be read. pid=3D%d\n"= , getpid()); sleep(30); munmap(buf, size); return 0; } ``` Test result: before: ``` =3D=3D before reclaim =3D=3D swapcached 0 allocated and dirtied 256 MiB, paging out... paged out; sleeping so memory.stat can be read. pid=3D4778 =3D=3D after reclaim (swap cache should drain to ~0) =3D=3D swapcached 268435456 LEAK DETECTED: swapcached =3D 268435456 bytes (expected ~0) [BUGGY kernel] ``` after the patch: ``` =3D=3D before reclaim =3D=3D swapcached 0 allocated and dirtied 256 MiB, paging out... paged out; sleeping so memory.stat can be read. pid=3D2601 =3D=3D after reclaim (swap cache should drain to ~0) =3D=3D swapcached 0 OK: swapcached =3D 0 bytes [FIXED kernel] ``` --- Changes in v2: - Update the commit message to describe the problem in the beginnning. - Change function declaration for !CONFIG_SWAP as well. - Link to v1: https://lore.kernel.org/r/20260831-memcg-swapcache-stats-fix-= v1-1-1c0819ebdb86@tencent.com --- mm/swap.h | 6 ++++-- mm/swap_state.c | 10 +++++++--- mm/vmscan.c | 3 +-- 3 files changed, 12 insertions(+), 7 deletions(-) diff --git a/mm/swap.h b/mm/swap.h index 0b5d507739bcb..b3b54c28929a1 100644 --- a/mm/swap.h +++ b/mm/swap.h @@ -319,7 +319,8 @@ struct folio *swap_cache_alloc_folio(swp_entry_t target= _entry, gfp_t gfp_mask, void __swap_cache_add_folio(struct swap_cluster_info *ci, struct folio *folio, swp_entry_t entry); void __swap_cache_del_folio(struct swap_cluster_info *ci, - struct folio *folio, swp_entry_t entry, void *shadow); + struct folio *folio, swp_entry_t entry, void *shadow, + bool swapout); void __swap_cache_replace_folio(struct swap_cluster_info *ci, struct folio *old, struct folio *new); =20 @@ -452,7 +453,8 @@ static inline void swap_cache_del_folio(struct folio *f= olio) } =20 static inline void __swap_cache_del_folio(struct swap_cluster_info *ci, - struct folio *folio, swp_entry_t entry, void *shadow) + struct folio *folio, swp_entry_t entry, void *shadow, + bool swapout) { } =20 diff --git a/mm/swap_state.c b/mm/swap_state.c index 305877e1f4d7b..9e0684166b78a 100644 --- a/mm/swap_state.c +++ b/mm/swap_state.c @@ -314,13 +314,17 @@ static void __swap_cache_do_del_folio(struct swap_clu= ster_info *ci, * using the index of @entry, and lock the cluster that holds the entries. */ void __swap_cache_del_folio(struct swap_cluster_info *ci, struct folio *fo= lio, - swp_entry_t entry, void *shadow) + swp_entry_t entry, void *shadow, bool swapout) { unsigned long nr_pages =3D folio_nr_pages(folio); =20 - __swap_cache_do_del_folio(ci, folio, entry, shadow); node_stat_mod_folio(folio, NR_FILE_PAGES, -nr_pages); lruvec_stat_mod_folio(folio, NR_SWAPCACHE, -nr_pages); + + if (swapout) + __memcg1_swapout(folio, ci); + + __swap_cache_do_del_folio(ci, folio, entry, shadow); } =20 /** @@ -339,7 +343,7 @@ void swap_cache_del_folio(struct folio *folio) swp_entry_t entry =3D folio->swap; =20 ci =3D swap_cluster_lock(__swap_entry_to_info(entry), swp_offset(entry)); - __swap_cache_del_folio(ci, folio, entry, NULL); + __swap_cache_del_folio(ci, folio, entry, NULL, false); swap_cluster_unlock(ci); =20 folio_ref_sub(folio, folio_nr_pages(folio)); diff --git a/mm/vmscan.c b/mm/vmscan.c index b4c9b8f3dfe99..af7dfa905cb50 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -735,8 +735,7 @@ static int __remove_mapping(struct address_space *mappi= ng, struct folio *folio, =20 if (reclaimed && !mapping_exiting(mapping)) shadow =3D workingset_eviction(folio, target_memcg); - __memcg1_swapout(folio, ci); - __swap_cache_del_folio(ci, folio, swap, shadow); + __swap_cache_del_folio(ci, folio, swap, shadow, true); swap_cluster_unlock_irq(ci); } else { void (*free_folio)(struct folio *); --- base-commit: 88297631d4d42f6004cb39c0ba3da7d2d10a616f change-id: 20260828-memcg-swapcache-stats-fix-1beef3dc3afb Best regards, --=20 Bingfang Guo