From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 5494838E129 for ; Tue, 7 Jul 2026 09:44:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783417460; cv=none; b=ofP1PSB7sWu4A6AbpQ8EeD2fYfiSTh3pma/1cMpQjXnoyjcI4+MUHTpu3+xZo775FCP8G+oGKLY+CZ087vskSxKBeSC8jwmYEA/8fkjULUHxLc5Jmofrw8DWMSZvbGu1SQmvJvW2cIu51egdCcd6UWjVajUCkUqmV68rdUUCJJE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783417460; c=relaxed/simple; bh=BjEPAqSemin8teKoO+Us8bqOlcwhR4u+ZvPIH7xW1/4=; h=Message-ID:Date:MIME-Version:Cc:Subject:To:References:From: In-Reply-To:Content-Type; b=Iff2hGR4l8cX7z1FJIFVNtqc8W2lqHrNjsjXBRGk0iOijix5j0lIYgd0FddkeraSjfcaRJ6+CAv1qRWTljcR+u74qO7RWlG5P4rTuwA1Nqiqs5M9h43/E5IQyBYnV/7TlhpI7ZCKf0zLe03HKLXcNrdQWtur2VmGtzSVuDLXT/Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YPgbdaEV; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="YPgbdaEV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E83271F000E9; Tue, 7 Jul 2026 09:44:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783417459; bh=PB57BxWlwllznDrR6zlFL6wLD5avrGWOWu0LQzeEDQg=; h=Date:Cc:Subject:To:References:From:In-Reply-To; b=YPgbdaEVGcrRx3fmEtXe6Yekl8vs0E1Fme+ReyfPRTwWnBqwJO85a4NOi6h7pO/FU 4RsMJ/uDyCMHILfE57hapOZmZWGCireSHgCaY1q7HjPOJTy5p7/Ae2KAL3ycGF9EMh Kfaopuha4IARQdhwvjN0qdIv5KvDzAA+4UKU/bZ3ir5XwTQxqbfknX0Phel0+ancIy OtJub6w0OX7VyY7E7y39dZ19cLfm3wCp/vV1J6Z0Hlcd+BRLRZjm5xcoUKFrderXYR q7Tsw4djKH4JbAXl2DVCv587uTocfJ/sTN2Ehh3os0T3Uo5q7LmFLTzyWvx+kRAiSo +7xLehvLokzgA== Message-ID: Date: Tue, 7 Jul 2026 17:44:15 +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 Cc: chao@kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] f2fs: quota: use memalloc_nofs_{save,restore} instead of FGP_NOFS To: Matthew Wilcox , jaegeuk@kernel.org References: <20260706095943.2560208-1-chao@kernel.org> <20260706095943.2560208-2-chao@kernel.org> Content-Language: en-US From: Chao Yu In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/6/26 22:18, Matthew Wilcox wrote: > On Mon, Jul 06, 2026 at 05:59:43PM +0800, Chao Yu wrote: >> FGP_NOFS could be removed later, let's use memalloc_nofs_{save,restore} >> instead, which is recommended to be used to avoid potential deadlock >> when memory allocation in f2fs_quota_write() will call into filesystem >> interface again, e.g. .writepages, evict_inode, shrinker due to >> complicated lock race condition. > > I think we need to be clear on why we need the memalloc_nofs_save() > call here. What problem would it cause if we did call into the > filesystem to reclaim memory? Hmm, after taking another look at the code, I suspect below deadlock may occur w/o NOFS flag? User Sync (Thread 1) - f2fs_do_quota_sync - f2fs_lock_op : down_read(cp_rwsem) [HELD] (1st Read Lock) - f2fs_quota_sync_file - dquot_writeback_dquots - ... - f2fs_quota_write - f2fs_write_begin - f2fs_filemap_get_folio - __filemap_get_folio - ... (GFP_FS allocation triggers reclaim) - try_to_free_pages() - shrink_node() - shrink_lruvec() - shrink_page_list() - pageout() <-- Reclaim decides to write back a dirty F2FS data page - f2fs_writepages() - f2fs_write_cache_pages() - f2fs_write_single_data_page(allow_balance = true) - f2fs_balance_fs(need = true) - f2fs_gc() - f2fs_write_checkpoint() - block_operations() - f2fs_lock_all() : down_write(cp_rwsem) [BLOCKED] (Self-deadlock) To Jaegeuk, please help to double check this. :) > > I suspect this is the wrong place to insert this call and it should be > near the lock that causes the problem. I've attempted a rewrite of the > memalloc_nofs_save documentation; let me know what you think: > > /** > * memalloc_nofs_save - Prevent recursion into the filesystem. > * > * All memory allocations between calling this function and calling > * memalloc_nofs_restore() will be prevented from calling into filesystems > * to reclaim memory. Clean page cache memory can still be reclaimed, > * but (for example) inodes will not be. > * > * The primary reason to do this is that the caller has taken a lock > * which would be needed by FS reclaim. While we could theoretically > * call into a different filesystem in this case, it can be a deep call > * stack so it is better to avoid all filesystems. > * > * Filesystems often choose to incorporate a call to this function as part > * of starting a journal transaction. While not a lock in the normal > * sense, it has much the same effect as nested journal transactions > * are either prohibited or expensive. > * > * Also call this function if you need to allocate memory while holding > * a file folio locked. High order allocations (such as those requested > * by slab) can trigger compaction which will attempt to lock the folio. > * > * Context: This function is safe to be used from any context. > * Return: The saved flags to be passed to memalloc_nofs_restore. > */ It makes sense to me, thanks for updating. Thanks,