From mboxrd@z Thu Jan 1 00:00:00 1970 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 7984D38553A for ; Wed, 1 Apr 2026 07:30:19 +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=1775028619; cv=none; b=TT3irbLMUSigIOTbTqsaEC5HAXqmM42zzSHU0iLOvNNha5ggg85tJhq949QZfmgc3iKPgjc6IyaI+rWIlHdlBLxvijjG8+xq/QkE+X5ubzb6ThwyJHHC81MbmAzcJm6MwyG5Rb/lzTFvJA3ynn8TbMrfG6UChaWmbbeoqeNuTTQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775028619; c=relaxed/simple; bh=f1Ngdbz7uczIY4RWfNQQvAw4tyC9TKeVNLanQ2yWR14=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=N8bIfYQLJoKtSbGRjFcT9vguU4I/xZN/UhBgwl4pQumSE+zpy6PnuCmAwvC9g+8iWMGBVZ5jEkQBGWQrbMCtffwdUA+SaKSPa4HNlkGV1wVDat1+YH/otodLmBF1LzTEIA2cnBdA6WrVqd84/bj5/Il0SujJ0d09PShUV+a+aaQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=qTjlT+Z3; 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="qTjlT+Z3" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 43C40C4CEF7; Wed, 1 Apr 2026 07:30:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775028619; bh=f1Ngdbz7uczIY4RWfNQQvAw4tyC9TKeVNLanQ2yWR14=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=qTjlT+Z33eRpwuN3qRicMSrxRdg+ybY/sOd850b1NNr7iV9YylLG1TIu7htcvWEPe PHS2kK7t164uPMtgxaSJWoytBMROaoHFIA+zD1FuW6WI7vo56Y5V1f08v1GoTb7D1P zVkWh+YNPmGx4lj2Azna2IfuiHOEPOs0JKJQIdQGfBOTqMU9wypeTe3piIiPUz3lCo Oq3/Uv0hI5C6AoiMYKGLqQms82bvPDnkQ6Sk9sFVcq4gti7llUxibJrn3xn/uJqsKM 7z/6L+/LYV3TZz4Z9+7KKN6pvLtlYlu1BxruR81iDsTUsm/l5y6HJxiPLXNdgzCBGU Qw4tB6QbsjKBg== Message-ID: Date: Wed, 1 Apr 2026 09:30:14 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/page_io: fix PSWPIN undercount for large folios in sio_read_complete() To: David CARLIER , Barry Song <21cnbao@gmail.com> Cc: Kairui Song , Chris Li , Andrew Morton , Kemeng Shi , Nhat Pham , Baoquan He , Youngjun Park , NeilBrown , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20260330071229.14614-1-devnexen@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 4/1/26 09:10, David CARLIER wrote: > On Tue, 31 Mar 2026 at 23:33, Barry Song <21cnbao@gmail.com> wrote: >> >> On Mon, Mar 30, 2026 at 3:12 PM David Carlier wrote: >>> >>> sio_read_complete() uses sio->pages to account global PSWPIN vm events, >>> but sio->pages tracks the number of bvec entries (folios), not base >>> pages. For large folios this undercounts compared to the per-memcg path >>> which correctly uses folio_nr_pages(), and compared to the bdev read >>> paths which also use folio_nr_pages(). >>> >>> Use sio->len >> PAGE_SHIFT instead, which gives the correct base page >>> count since sio->len is accumulated via folio_size(folio). >>> >>> Fixes: a1a0dfd56f97 ("mm: handle THP in swap_*page_fs()") >>> Signed-off-by: David Carlier >> >> The patch seems theoretically correct, but I’m wondering >> where we can swap in mTHP for filesystem-based swap？ >> >> In both do_swap_page() and shmem_swapin_folio(), we check >> data_race(si->flags & SWP_SYNCHRONOUS_IO) before allocating >> large folios. Am I missing something? > > ▎ The patch seems theoretically correct, but I'm wondering > ▎ where we can swap in mTHP for filesystem-based swap? > > ▎ In both do_swap_page() and shmem_swapin_folio(), we check > ▎ data_race(si->flags & SWP_SYNCHRONOUS_IO) before allocating > ▎ large folios. Am I missing something? > > You're right, I missed that. SWP_FS_OPS is only set by NFS and > SMB which have no bdev, so SWP_SYNCHRONOUS_IO can never be set > alongside it. Large folios can't currently reach this path since > both do_swap_page() and shmem_swapin_folio() gate mTHP allocation > on SWP_SYNCHRONOUS_IO. > > That said, sio_read_complete() already calls count_mthp_stat() > and the per-memcg accounting uses folio_nr_pages(), so the code > seems written with large folios in mind even if the path is > currently unreachable. Using sio->pages (bvec entry count) for > a base-page count is still semantically wrong, but I understand > the practical impact is nil today. > > Happy to either drop this or keep it as a correctness cleanup, > whatever you and Andrew prefer. Best to rework the patch description (clarify that it's a cleanup) and drop the Fixes: I guess. -- Cheers, David