From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C6B82C4320A for ; Mon, 2 Aug 2021 09:00:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AB00C610D0 for ; Mon, 2 Aug 2021 09:00:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232987AbhHBJAb (ORCPT ); Mon, 2 Aug 2021 05:00:31 -0400 Received: from mail.kernel.org ([198.145.29.99]:54936 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232983AbhHBJA0 (ORCPT ); Mon, 2 Aug 2021 05:00:26 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 6832860EBD; Mon, 2 Aug 2021 09:00:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1627894817; bh=NpGYxMxPtr0QSqg+p3P23qf5aNI0HOKU5NMbrglC06Y=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=RdoQGF598VsHZscJC5HMR3we486QnR63NlWZ5ruA3kXqcyZu1Xq5eGtri+xlTiPFZ z+Pzx3D4O+YR5+qVkKcRl9Lp7NtBn4jBuNi2KOmY5qq3fzArDmV432u2NUXjwtpPDB vX/7/MQbJnNmRXLtumUOdf5EHFZHfKUr96PqD1Yn5PclZQvXQYNketFumi4dcgsI0k X7RImEgtiF1tVMlS0ImsrsZ37cnFyq/Aoh533Cjzq/3VH2BABrTQAe4ZoQeEke4/rW I6YZb1I5/DL/NEarggj7M4msoiSivQpXYNOVBih4zMEloTJF2bhwC2SSRvDJA9srnB q8sIRtOirdNMw== Subject: Re: [PATCH] f2fs: remove broken support for allocating DIO writes To: Eric Biggers , Jaegeuk Kim , Theodore Ts'o Cc: linux-f2fs-devel@lists.sourceforge.net, linux-fsdevel@vger.kernel.org, stable@vger.kernel.org References: <20210728015154.171507-1-ebiggers@kernel.org> From: Chao Yu Message-ID: Date: Mon, 2 Aug 2021 17:00:15 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 2021/8/2 12:39, Eric Biggers wrote: > On Fri, Jul 30, 2021 at 10:46:16PM -0400, Theodore Ts'o wrote: >> On Fri, Jul 30, 2021 at 12:17:26PM -0700, Eric Biggers wrote: >>>> Currently, non-overwrite DIO writes are fundamentally unsafe on f2fs as >>>> they require preallocating blocks, but f2fs doesn't support unwritten >>>> blocks and therefore has to preallocate the blocks as regular blocks. >>>> f2fs has no way to reliably roll back such preallocations, so as a >>>> result, f2fs will leak uninitialized blocks to users if a DIO write >>>> doesn't fully complete. >> >> There's another way of solving this problem which doesn't require >> supporting unwritten blocks. What a file system *could* do is to >> allocate the blocks, but *not* update the on-disk data structures --- >> so the allocation happens in memory only, so you know that the >> physical blocks won't get used for another files, and then issue the >> data block writes. On the block I/O completion, trigger a workqueue >> function which updates the on-disk metadata to assign physical blocks >> to the inode. >> >> That way if you crash before the data I/O has a chance to complete, >> the on-disk logical block -> physical block map hasn't been updated >> yet, and so you don't need to worry about leaking uninitialized blocks. Thanks for your suggestion, I think it makes sense. >> >> Cheers, >> >> - Ted > > Jaegeuk and Chao, any idea how feasible it would be for f2fs to do this? Firstly, let's notice that below metadata will be touched during DIO preallocation flow: - log header - sit bitmap/count - free seg/sec bitmap/count - dirty seg/sec bitmap/count And there is one case we need to concern about is: checkpoint() can be triggered randomly in between dio_preallocate() and dio_end_io(), we should not persist any DIO preallocation related metadata during checkpoint(), otherwise, sudden power-cut after the checkpoint will corrupt filesytem. So it needs to well separate two kinds of metadata update: a) belong to dio preallocation b) the left one After that, it will simply checkpoint() flow to just flush metadata b), for other flow, like GC, data/node allocation, it needs to query/update metadata after we combine metadata a) and b). In addition, there is an existing in-memory log header framework in f2fs, based on this fwk, it's very easy for us to add a new in-memory log header for DIO preallocation. So it seems feasible for me until now... Jaegeuk, any other concerns about the implementation details? Thanks, > > - Eric >