From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-98.freemail.mail.aliyun.com (out30-98.freemail.mail.aliyun.com [115.124.30.98]) (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 46AD638C for ; Tue, 24 Sep 2024 01:55:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.98 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727142949; cv=none; b=Vw+VjNd8Fjr4G3gSe1UGcYWWC/mQGGNK/GAu2+S4aVp2xdtgA58LiRPX8wKsjoQWC5q3FOYzfY6yH/nHIWtlB2XsycGmWYclsSYpmLrlWxQzXVzDVTYTf/ZXJchlCwCmpkoerlg+n4QkEfj86EsBobdsDkgDU6xTLD2VjvyZrrs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727142949; c=relaxed/simple; bh=eQGfIO9HSMg5Exet2LE6v9ak0XY9XAXLv94DDnuCHJQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YtAVytvK8pJDosXnScqHthFCT/J3T5bJMkKuYy3c8TJpLp4+2zCTFZndmFH42yoh1DXKdzcg7LDoH6TPzji9MNF4y5JqeWVWbWpueOVWAc5hTpM9sN4eSJzzowMTeeRQNZ3yQ6AaRC1mA7rlCau7KsKxUUbDVFBSgm/1gairYGY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=YnIjX8nw; arc=none smtp.client-ip=115.124.30.98 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="YnIjX8nw" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1727142944; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=QK7m63y5+dM5qtwR3OerysVuM/WrkXtW1Hv0gD6hl6k=; b=YnIjX8nwVNg27ERRcjFzVwyz60P0Eug6l5pH5CL9Ez/eb0YOWMz5HobVz8N0mmKR9rOys1ak059xAdOkkx0pkCHk4dVjm2SOD8bRGZaQ2L1KsZRvaeyfd9laIn+9/w3WRA67CphG5H0h2TqWEl739KvJ/t4/rZuBR3NCAnNo3mo= Received: from 30.74.144.117(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0WFe6d.J_1727142941) by smtp.aliyun-inc.com; Tue, 24 Sep 2024 09:55:42 +0800 Message-ID: <4d38c65d-760c-43a5-bb47-8e0235c13a51@linux.alibaba.com> Date: Tue, 24 Sep 2024 09:55:39 +0800 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: [RFC PATCH 0/2] remove SWAP_MAP_SHMEM To: Nhat Pham , akpm@linux-foundation.org Cc: hannes@cmpxchg.org, yosryahmed@google.com, hughd@google.com, shakeel.butt@linux.dev, ryan.roberts@arm.com, ying.huang@intel.com, chrisl@kernel.org, david@redhat.com, kasong@tencent.com, willy@infradead.org, viro@zeniv.linux.org.uk, baohua@kernel.org, chengming.zhou@linux.dev, linux-mm@kvack.org, kernel-team@meta.com, linux-kernel@vger.kernel.org References: <20240923231142.4155415-1-nphamcs@gmail.com> From: Baolin Wang In-Reply-To: <20240923231142.4155415-1-nphamcs@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2024/9/24 07:11, Nhat Pham wrote: > The SWAP_MAP_SHMEM state was originally introduced in the commit > aaa468653b4a ("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a > swap entry belongs to shmem during swapoff. > > However, swapoff has since been rewritten drastically in the commit > b56a2d8af914 ("mm: rid swapoff of quadratic complexity"). Now > having swap count == SWAP_MAP_SHMEM value is basically the same as having > swap count == 1, and swap_shmem_alloc() behaves analogously to > swap_duplicate() > > This RFC proposes the removal of this state and the associated helper to > simplify the state machine (both mentally and code-wise). We will also > have an extra state/special value that can be repurposed (for swap entries > that never gets re-duplicated). > > Another motivation (albeit a bit premature at the moment) is the new swap > abstraction I am currently working on, that would allow for swap/zswap > decoupling, swapoff optimization, etc. The fewer states and swap API > functions there are, the simpler the conversion will be. > > I am sending this series first as an RFC, just in case I missed something > or misunderstood this state, or if someone has a swap optimization in mind > for shmem that would require this special state. The idea makes sense to me. I did a quick test with shmem mTHP, and encountered the following warning which is triggered by 'VM_WARN_ON(usage == 1 && nr > 1)' in __swap_duplicate(). [ 81.064967] ------------[ cut here ]------------ [ 81.064968] WARNING: CPU: 4 PID: 6852 at mm/swapfile.c:3623 __swap_duplicate+0x1d0/0x2e0 [ 81.064994] pstate: 23400005 (nzCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--) [ 81.064995] pc : __swap_duplicate+0x1d0/0x2e0 [ 81.064997] lr : swap_duplicate_nr+0x30/0x70 [......] [ 81.065019] Call trace: [ 81.065019] __swap_duplicate+0x1d0/0x2e0 [ 81.065021] swap_duplicate_nr+0x30/0x70 [ 81.065022] shmem_writepage+0x24c/0x438 [ 81.065024] pageout+0x104/0x2e0 [ 81.065026] shrink_folio_list+0x7f0/0xe60 [ 81.065027] reclaim_folio_list+0x90/0x178 [ 81.065029] reclaim_pages+0x128/0x1a8 [ 81.065030] madvise_cold_or_pageout_pte_range+0x80c/0xd10 [ 81.065031] walk_pmd_range.isra.0+0x1b8/0x3a0 [ 81.065033] walk_pud_range+0x120/0x1b0 [ 81.065035] walk_pgd_range+0x150/0x1a8 [ 81.065036] __walk_page_range+0xa4/0xb8 [ 81.065038] walk_page_range+0x1c8/0x250 [ 81.065039] madvise_pageout+0xf4/0x280 [ 81.065041] madvise_vma_behavior+0x268/0x3f0 [ 81.065042] madvise_walk_vmas.constprop.0+0xb8/0x128 [ 81.065043] do_madvise.part.0+0xe8/0x2a0 [ 81.065044] __arm64_sys_madvise+0x64/0x78 [ 81.065046] invoke_syscall.constprop.0+0x54/0xe8 [ 81.065048] do_el0_svc+0xa4/0xc0 [ 81.065050] el0_svc+0x2c/0xb0 [ 81.065052] el0t_64_sync_handler+0xb8/0xc0 [ 81.065054] el0t_64_sync+0x14c/0x150 > Swap experts, let me know if I'm mistaken :) Otherwise if there is no > objection I will resend this patch series again for merging. > > Nhat Pham (2): > swapfile: add a batched variant for swap_duplicate() > swap: shmem: remove SWAP_MAP_SHMEM > > include/linux/swap.h | 16 ++++++++-------- > mm/shmem.c | 2 +- > mm/swapfile.c | 28 +++++++++------------------- > 3 files changed, 18 insertions(+), 28 deletions(-) > > > base-commit: acfabf7e197f7a5bedf4749dac1f39551417b049