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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=blfPTJf3; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 07103606A31C for ; Tue, 10 Nov 2026 12:30:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=blfPTJf3 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 55A811F0089F; Tue, 10 Nov 2026 12:29:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794313800; bh=BgSE/1fL0Gmuvdjo71x0BWWR6H8PrWT9eXmVlUS9oqw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=blfPTJf388MwCSPYtBP64cewc1b2uqBwZadbzZJb8OnU5AzPSUz7Ze9pmAulkvUGV KGs5AWiBuT8LkB86B+jsPYEfMqUOFv7Cm0Rnc0eDReHhdwvB3+Q8bZoVywnKEg+/Aa U4gXPKYx1tzcqu2FEcGmvHOyko63avnRy4Dn0xih/SgUdNl16nUTxI0bMjwe4tq+MG jCwEX12VoK+bAQ/5nwsJSX2QtmXuDIfu7AeNgn+SyOw7O4YusX+UHbctmDpxdjNy1J kKKtXmcYu6YGusfHauB8/pihppdE919Y59BVQkxIkGoZQaJ5G6zDG5+hwrYgONrJKE 6dCdyraaHfiFQ== Message-ID: <67275d3d-cc60-406c-a088-298b948d7577@kernel.org> Date: Tue, 10 Nov 2026 13:29:54 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/migrate: don't move memcg NR_FILE_PAGES for swapcache To: Zi Yan , Song Hu , akpm@linux-foundation.org Cc: matthew.brost@intel.com, joshua.hahnjy@gmail.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, willy@infradead.org, shivankg@amd.com, ridong.chen@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261104044313.2099309-1-husong@kylinos.cn> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/10/26 04:09, Zi Yan wrote: > On Tue Nov 3, 2026 at 11:43 PM EST, Song Hu wrote: >> NR_FILE_PAGES lives on both the node and the memcg plane for page >> cache folios: filemap_add_folio() and filemap_unaccount_folio() charge >> it to the folio's node and memcg, while __swap_cache_add_folio() and >> __swap_cache_del_folio() maintain the node plane of NR_FILE_PAGES and >> the memcg plane of NR_SWAPCACHE. A folio in the swap cache therefore >> carries no per-memcg NR_FILE_PAGES count: shmem folios drop their >> memcg file accounting when swapped out, and anon folios never had any. >> >> The cross-zone arm of __folio_migrate_mapping() moves NR_FILE_PAGES on >> both planes unconditionally. For swapcache folios the memcg plane is >> empty, so migrating one misaccounts it: the destination accrues >> nr_pages that were never accounted there while the source counter >> underflows. The wrong counts persist until the memcg is destroyed, >> but the aggregate in memory.stat stays correct, so the drift is easy >> to miss. NUMA balancing, migratepages, compaction and memory hotplug >> can all migrate such folios. > > It would be better to simplify the above like Joshua suggested. > >> >> Reproducer, on a 2-node QEMU guest with cgroup v2: >> >> 1. create a cgroup with memory.max = 50M and swap enabled; >> 2. memset 128M of anonymous memory in it, so reclaim swaps most of >> it out; >> 3. fork to raise the swap entry refcount, then fault the memory >> back in, so the resident folios are swapcache ones; >> 4. migrate_pages(pid, node0 -> node1). >> >> "file" row of memory.numa_stat (bytes) after step 4: >> >> without this patch with this patch >> N0 N1 N0 N1 >> 0 (*) 51691520 0 0 >> >> (*) the source counter has gone negative; negative values read as 0. >> >> Keep moving the node plane via mod_node_page_state() and skip only >> the empty memcg plane, mirroring the swapcache exclusion the NR_SHMEM >> handling in the same block already applies. It accompanies >> commit 4eef27af2932 ("mm/migrate: move NR_FILE_THPS on cross-node migration") >> and commit c5eef2e03618 ("mm/migrate: move NR_KERNEL_FILE_PAGES on cross-node migration"). >> >> Fixes: 0d1c20722ab3 ("mm: memcontrol: switch to native NR_FILE_PAGES and NR_SHMEM counters") >> Signed-off-by: Song Hu >> --- >> mm/migrate.c | 16 ++++++++++++++-- >> 1 file changed, 14 insertions(+), 2 deletions(-) >> >> diff --git a/mm/migrate.c b/mm/migrate.c >> index 28a8d7734735..bc60f6587d1b 100644 >> --- a/mm/migrate.c >> +++ b/mm/migrate.c >> @@ -690,8 +690,20 @@ static int __folio_migrate_mapping(struct address_space *mapping, >> old_lruvec = mem_cgroup_lruvec(memcg, oldzone->zone_pgdat); >> new_lruvec = mem_cgroup_lruvec(memcg, newzone->zone_pgdat); >> >> - mod_lruvec_state(old_lruvec, NR_FILE_PAGES, -nr); >> - mod_lruvec_state(new_lruvec, NR_FILE_PAGES, nr); >> + /* >> + * Swapcache folios carry NR_FILE_PAGES on the node plane >> + * only (their memcg footprint is NR_SWAPCACHE, see >> + * swap_state.c), so move only the node plane for them. >> + */ >> + if (folio_test_swapcache(folio)) { >> + mod_node_page_state(oldzone->zone_pgdat, NR_FILE_PAGES, >> + -nr); >> + mod_node_page_state(newzone->zone_pgdat, NR_FILE_PAGES, >> + nr); >> + } else { >> + mod_lruvec_state(old_lruvec, NR_FILE_PAGES, -nr); >> + mod_lruvec_state(new_lruvec, NR_FILE_PAGES, nr); >> + } > > Like Joshua said in another email, it will be more readable if > NR_SWAPCACHE updates can be moved into the then branch above. Can one of you write me the updated patch description? Thanks1 -- Cheers, David