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 CB619C432BE for ; Tue, 3 Aug 2021 01:19:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A8CCA60F9E for ; Tue, 3 Aug 2021 01:19:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233118AbhHCBTY (ORCPT ); Mon, 2 Aug 2021 21:19:24 -0400 Received: from mail.kernel.org ([198.145.29.99]:44892 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232891AbhHCBTX (ORCPT ); Mon, 2 Aug 2021 21:19:23 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id D6B6160EFF; Tue, 3 Aug 2021 01:19:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1627953553; bh=MHctK3vcMIjG05hTw3nq1KHBUvlKG/a6cUvMRnmiMgo=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=otLihmXIjm3Jzy/9vU4q9nxn0/nRX7PKb+ZudFug5dZkAcF5YJ1hEkacyu+pCUwyl kM6F9L9UtbV+xwka6uICieOxPxzHcnb5pdvLS4W9cg7qlxkid1m0wM53FChBbyDc0+ d49OmCuBKHGpculT+u4t9wZTV/OTQeC+NfTOnvvBoSYGTG/M4TTwbMHK7F5FYA2z3Z +V6D/flUop3Dr8KWHGC75G3rsKtO+r04100yeVIRSTUKeCevJJnJIJ4D4JMbIxByyR E36MPG7BgCJ0bHaT5d0GcPBd+rUjRa4ZrxvTu+FGH45MZHVhlxmDdZTnAK/eMRlQMy 4ueBpUe894USg== Subject: Re: [PATCH] f2fs: remove broken support for allocating DIO writes To: Jaegeuk Kim Cc: Eric Biggers , Theodore Ts'o , 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: Tue, 3 Aug 2021 09:19:11 +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 2021/8/3 2:23, Jaegeuk Kim wrote: > On 08/02, Chao Yu wrote: >> 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? > > Hmm, I'm still trying to deal with this as a corner case where the writes > haven't completed due to an error. How about keeping the preallocated block > offsets and releasing them if we get an error? Do we need to handle EIO right? What about the case that CP + SPO following DIO preallocation? User will encounter uninitialized block after recovery. Thanks, > >> >> Thanks, >> >>> >>> - Eric >>> 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=-5.3 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 054C8C4338F for ; Tue, 3 Aug 2021 01:19:27 +0000 (UTC) Received: from lists.sourceforge.net (lists.sourceforge.net [216.105.38.7]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id A788260C3E; Tue, 3 Aug 2021 01:19:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org A788260C3E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; 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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1627953553; bh=MHctK3vcMIjG05hTw3nq1KHBUvlKG/a6cUvMRnmiMgo=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=otLihmXIjm3Jzy/9vU4q9nxn0/nRX7PKb+ZudFug5dZkAcF5YJ1hEkacyu+pCUwyl kM6F9L9UtbV+xwka6uICieOxPxzHcnb5pdvLS4W9cg7qlxkid1m0wM53FChBbyDc0+ d49OmCuBKHGpculT+u4t9wZTV/OTQeC+NfTOnvvBoSYGTG/M4TTwbMHK7F5FYA2z3Z +V6D/flUop3Dr8KWHGC75G3rsKtO+r04100yeVIRSTUKeCevJJnJIJ4D4JMbIxByyR E36MPG7BgCJ0bHaT5d0GcPBd+rUjRa4ZrxvTu+FGH45MZHVhlxmDdZTnAK/eMRlQMy 4ueBpUe894USg== To: Jaegeuk Kim References: <20210728015154.171507-1-ebiggers@kernel.org> From: Chao Yu Message-ID: Date: Tue, 3 Aug 2021 09:19:11 +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-Language: en-US X-Headers-End: 1mAj5X-00Ah0L-Sv Subject: Re: [f2fs-dev] [PATCH] f2fs: remove broken support for allocating DIO writes X-BeenThere: linux-f2fs-devel@lists.sourceforge.net X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eric Biggers , linux-fsdevel@vger.kernel.org, Theodore Ts'o , stable@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: linux-f2fs-devel-bounces@lists.sourceforge.net On 2021/8/3 2:23, Jaegeuk Kim wrote: > On 08/02, Chao Yu wrote: >> 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? > > Hmm, I'm still trying to deal with this as a corner case where the writes > haven't completed due to an error. How about keeping the preallocated block > offsets and releasing them if we get an error? Do we need to handle EIO right? What about the case that CP + SPO following DIO preallocation? User will encounter uninitialized block after recovery. Thanks, > >> >> Thanks, >> >>> >>> - Eric >>> _______________________________________________ Linux-f2fs-devel mailing list Linux-f2fs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel