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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=gscrHbez; 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) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4BB25606A31A for ; Wed, 28 Oct 2026 08:36:29 +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=gscrHbez Received: by smtp.kernel.org (Postfix) with ESMTPSA id B53CB1F01825; Wed, 28 Oct 2026 08:36:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793176588; bh=5SoDZH5y48If1elxYBxox/ESd5HONTLAzfzat25FiII=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=gscrHbezavwEXPMK6kRtD1Li4ys3C9HLDDcREx2x3C4ZU+hKrPPb5KQNBUEbdLlQY ik7pQY48RPsjcXmvEgt4f+rB/CJC2AhN0emliAY8OEThJ8U+DhCWrmYwCva3eDua9L nTih5IXDxDPGSjm7+mS7lb5Ho6Juj3DaFaQKnqQP1nA3O9YLfBRLAPChxvSOP+FN8A FLx31mhQFZsIrw43VTDsVQ88/sugKAsqdT2CYk5TMXIuaPB607YtcnDUSHfA5XkZyB kf5WlT2HMzWa9ZMBYA19s+VfZ1A6JWgDbQVwQx/5/PLbla8SPSwRNr5rOgqzi/FG64 6Tc8/f/0pzpjA== Message-ID: Date: Wed, 28 Oct 2026 09:36:22 +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-new] mm/migrate: move NR_KERNEL_FILE_PAGES on cross-node migration To: Ridong Chen , Andrew Morton Cc: Zi Yan , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Boris Burkov , Shakeel Butt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Shivank Garg , Ridong Chen References: <20261027043214.2248986-1-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 10/27/26 05:32, Ridong Chen wrote: > From: Ridong Chen > > NR_KERNEL_FILE_PAGES counts page cache folios of mappings flagged > AS_KERNEL_FILE (currently the btrfs btree inode). It is a per-node > counter: filemap_add_folio() adds to folio_pgdat() of the new folio and > filemap_unaccount_folio() subtracts from folio_pgdat() of the folio > being removed. > > __folio_migrate_mapping() does not move it when such a folio is > migrated to another node, so the source node keeps the count forever > and the destination node underflows once the folio is freed. btree > folios are migratable through btree_migrate_folio(), so demotion, > DAMON-based migration (DAMOS_MIGRATE_{HOT,COLD}) or memory offlining > all hit this. > > Reproducer, on a QEMU guest with two memory tiers (node0 is DRAM with > all CPUs, node1 is a CPU-less slower node described via HMAT) and > /sys/kernel/mm/numa/demotion_enabled set: > > 1. mount a btrfs and create 20000 empty files, so that btree folios > are cached on node0; > 2. echo 2G > /sys/devices/system/node/node0/reclaim, which demotes > them to node1; > 3. umount and drop caches. > > nr_kernel_file_pages in /sys/devices/system/node/nodeN/vmstat after > each step: > > without this patch with this patch > node0 node1 node0 node1 > 1. cached 1868 0 1868 0 > 2. demoted 1868 0 36 1832 > 3. dropped 1832 0 (*) 0 0 > > (*) the counter has underflowed to -1832; negative values read as 0. > > The global value in /proc/vmstat is 0 after step 3 in both cases. > > Move it together with the other node-keyed page cache counters. It is > a node-only counter, so use the node accessor, matching how it is > accounted elsewhere. Interrupts are disabled here, as for the zone > counters below. > > This was pointed out by Shivank while reviewing the NR_FILE_THPS fix > for the same function. > > Fixes: e3a9ac4e866e ("mm: add vmstat for kernel_file pages") > Suggested-by: Shivank Garg > Assisted-by: LLM > Signed-off-by: Ridong Chen > --- > mm/migrate.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/mm/migrate.c b/mm/migrate.c > index 99e3026e3126..f829c5017c96 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -705,6 +705,12 @@ static int __folio_migrate_mapping(struct address_space *mapping, > mod_lruvec_state(old_lruvec, NR_FILE_THPS, -nr); > mod_lruvec_state(new_lruvec, NR_FILE_THPS, nr); > } > + if (test_bit(AS_KERNEL_FILE, &mapping->flags)) { > + __mod_node_page_state(oldzone->zone_pgdat, > + NR_KERNEL_FILE_PAGES, -nr); > + __mod_node_page_state(newzone->zone_pgdat, > + NR_KERNEL_FILE_PAGES, nr); > + } > #ifdef CONFIG_SWAP > if (folio_test_swapcache(folio)) { > mod_lruvec_state(old_lruvec, NR_SWAPCACHE, -nr); Today I learned about NR_KERNEL_FILE_PAGES :D Acked-by: David Hildenbrand (Arm) -- Cheers, David