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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=HSxzzXAQ; 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 C85F9606A330 for ; Thu, 12 Nov 2026 08:54:14 +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=HSxzzXAQ Received: by smtp.kernel.org (Postfix) with ESMTPSA id DED451F00A05; Thu, 12 Nov 2026 08:54:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794473654; bh=OT4UYzJ55zbPPzE3sSYyUxZKuwkGev+001urY+Jtyv0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=HSxzzXAQAXajBmw3uzuM1xXEfDxqd9aJG7PLJu051MyVrGQ1NaJm3jkY2BfPcSs61 KQVrEw+f/uZxjoPuRgJso2EKttZxcRCo50io39UiuYuf4Il8I1iDf4lFu29Je5hkke wMxFFXLGjAE0evcUlapK2syy4lY2aGUBLtmwfUcQP5TEPNcCO9sC/3Oe+1isVNwt6l Oe6xf2kLohBkQTpw7HlPYlVGuPoUTVTpLqvCRH7aIGvPwbYKHiCVA5kXV0VH7M8nio zhe7wrGIvhQ6/RoYNJw700AstBFvZkICaw2fkm+pVWHqezWETqQZc1oun5vXPru4x1 ZsuSAGWiWPm2Q== Message-ID: Date: Thu, 12 Nov 2026 09:54:03 +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: Song Hu Cc: akpm@linux-foundation.org, apopple@nvidia.com, byungchul@sk.com, gourry@gourry.net, joshua.hahnjy@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, matthew.brost@intel.com, ridong.chen@linux.dev, shivankg@amd.com, willy@infradead.org, ying.huang@linux.alibaba.com, ziy@nvidia.com References: <67275d3d-cc60-406c-a088-298b948d7577@kernel.org> <20261112024855.424402-1-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 11/12/26 03:48, Song Hu wrote: >> On 11/10/26 04:09, Zi Yan wrote: >>> >>> It would be better to simplify the above like Joshua suggested. >>> >>> >>> 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 > > Sure. Here's an updated commit message incorporating Joshua's > suggestion to describe the accounting model first and then explain why > migration gets it wrong. > > mm/migrate: don't move memcg NR_FILE_PAGES for swapcache > > NR_FILE_PAGES is accounted on both the node and memcg planes for page > cache folios. Swapcache folios are the exception: they keep > NR_FILE_PAGES only on the node plane, while their memcg accounting is > tracked as NR_SWAPCACHE. > > The cross-zone path in __folio_migrate_mapping() currently migrates > NR_FILE_PAGES through mod_lruvec_state(), which updates both planes. > This is correct for normal page cache, but wrong for swapcache folios > because there is no memcg NR_FILE_PAGES state to move. As a result, > migration incorrectly increments the destination memcg counter and > decrements the source one, leaving the per-memcg accounting incorrect > until the memcg is destroyed. > > 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. > > Fix it by updating the node accounting directly for swapcache folios > while leaving the nonexistent memcg NR_FILE_PAGES state untouched. > Non-swapcache folios continue to use mod_lruvec_state(), preserving > the existing behaviour. This mirrors the existing swapcache handling > for NR_SHMEM in the same block. 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"). > > Reviewed-by: Joshua Hahn > Acked-by: Zi Yan > Signed-off-by: Song Hu > > Thanks to Joshua and Zi for the helpful reviews. Updated, thanks! -- Cheers, David