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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=Naie0bLo; 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 99781606A31F for ; Mon, 26 Oct 2026 11:19:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Naie0bLo Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6723F1F01453; Mon, 26 Oct 2026 11:19:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793013567; bh=ym2rUG4N9q5Vuz2Zf+g/cQPodhMY2XS2XpRVAbz6x2o=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Naie0bLom6KB1rvZiawwhNtlk55RBj7K9WkrJ62mpDzmd9p3ZjTeOhiy9/BYmyqqo SnNRmD9w2Kz+0bSLQRlg8Tui85D5ZH0652mrSWRzXhTB5K+jIu9ma16QMmKAtXShM8 DPHVOy6h8rmnPLxA+itKXfRJjJHDY/UsAnutAKmXMCh/pi+/jNoUCmRv2nSkUKchq5 iXCcTfsHnUyFmxEe5kxR3+PmC4oYX1reUzFTklfn/oI4jh476Y8y6Z0tz31GJGBHpW nDNDalO8lQY5sWzN7TmhP8cMk8olsxCglZgxn5aDh4gDUCv7xAUOoqxZ9Y1YxDVIeV uaJorTDd45ugg== Message-ID: Date: Mon, 26 Oct 2026 11:59:46 +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: move NR_FILE_THPS on cross-node migration To: Ridong Chen , Andrew Morton Cc: Zi Yan , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , "Matthew Wilcox (Oracle)" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ridong Chen References: <20261025110209.4105994-1-ridong.chen@linux.dev> 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 10/25/26 12:02, Ridong Chen wrote: > From: Ridong Chen > > When a page cache folio is migrated to another node, > __folio_migrate_mapping() moves the per-node and per-memcg counters > that are keyed off the folio's node: NR_FILE_PAGES, NR_SHMEM, > NR_SHMEM_THPS, NR_SWAPCACHE and NR_FILE_DIRTY. NR_FILE_THPS is not > moved, although __filemap_add_folio() charges it to the source node and > filemap_unaccount_folio() takes it off whatever node the folio lives on > when it is removed. > > Commit 0b52c420350e ("mm: fix shmem THP counters on migration") added > the NR_SHMEM_THPS case but not its file-backed counterpart. Now that > ext4, xfs, btrfs and erofs can all have PMD-sized page cache folios, > migrating them via NUMA balancing, mbind(), migratepages, demotion or > memory offlining leaks FileHugePages on the source node and underflows > it on the destination, in both /sys/devices/system/node/nodeN/meminfo > and memory.numa_stat. The global value is unaffected, which is why this > went unnoticed. > > Reproducer, on a 2-node QEMU guest with THP set to "always": > > 1. read a 256M ext4 file through a MADV_HUGEPAGE mapping while bound > to node 0, so its page cache is 128 PMD-sized folios on node 0; > 2. move the mapping to node 1 with mbind(MPOL_MF_MOVE); > 3. unmap the file and drop its page cache. > > nr_file_hugepages in /sys/devices/system/node/nodeN/vmstat after each > step (in units of PMD-sized folios): > > without this patch with this patch > node0 node1 node0 node1 > 1. cached 128 0 128 0 > 2. migrated 128 0 0 128 > 3. dropped 128 0 (*) 0 0 > > (*) the counter has underflowed to -128; negative values read as 0. > > Without the patch the 128 folios stay accounted to node 0 forever, and > every further round adds another 128. The same happens to the per-memcg > per-node counter: after step 3, memory.stat of the cgroup shows > "file_thp 0", while memory.numa_stat still shows > "file_thp N0=268435456 N1=0". With the patch both are 0. > > Move NR_FILE_THPS together with NR_FILE_PAGES, using the same condition > filemap_unaccount_folio() uses to take it off, so that THPs in the swap > cache are left alone. > > Fixes: d68eccad3706 ("mm/filemap: Allow large folios to be added to the page cache") > Signed-off-by: Ridong Chen > --- > Note: prepared with help from an LLM-based assistant (Kiro, claude-opus-5.5). > > mm/migrate.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/migrate.c b/mm/migrate.c > index 7bdcdb57652f..d55f576a2579 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -700,6 +700,10 @@ static int __folio_migrate_mapping(struct address_space *mapping, > mod_lruvec_state(old_lruvec, NR_SHMEM_THPS, -nr); > mod_lruvec_state(new_lruvec, NR_SHMEM_THPS, nr); > } > + } else if (!folio_test_swapbacked(folio) && > + folio_test_pmd_mappable(folio)) { > + mod_lruvec_state(old_lruvec, NR_FILE_THPS, -nr); > + mod_lruvec_state(new_lruvec, NR_FILE_THPS, nr); > } Looking at filemap_unaccount_folio(), I was (once again) surprise about the different handling. Key is that we might be migration aonymous folios in the swapcache. Acked-by: David Hildenbrand (Arm) -- Cheers, David