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=-9.7 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 5F901C4320A for ; Fri, 20 Aug 2021 09:35:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3E40560F44 for ; Fri, 20 Aug 2021 09:35:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235001AbhHTJgE (ORCPT ); Fri, 20 Aug 2021 05:36:04 -0400 Received: from mail.kernel.org ([198.145.29.99]:44838 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233976AbhHTJgA (ORCPT ); Fri, 20 Aug 2021 05:36:00 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 04AD860ED5; Fri, 20 Aug 2021 09:35:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1629452123; bh=wBrcxx/2SIRw6Sko2YlVtKQdiFRwlnbLVzaL4jAOvTk=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=IhN5QQiy3x0pZAEnBpZC7W889BoW5A3lAnCz9DjjoES0mtXygKSAAsIjy5d/i6hJ+ 2D3vbN4KMfvngGNN21EewUgz4SPbQdb5fOftk0mnXQtUEhNpNUVsqwAvEaiBjpKmSo +Dqc3t7ws3uDegim5tfkTetoDnzgDCn4ac+HFQpQvvGDP9jO3FfRdvnhCIJNQyw63S 4keI8FM8vdDmnRoRlfqxfUziZzqrCfDToYA457Ny9rGSvlB5YJyjB6K6Kpl6xJ9EUW HGdiovOUne2hdKM0ds8qKbABc01w1CDKgJavmb8Fxz5dloxkhKZs4T9jtEjdkXmwpk jg27wj2ZFa/Mw== Subject: Re: [PATCH] f2fs: remove broken support for allocating DIO writes To: Eric Biggers , Jaegeuk Kim Cc: 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: Fri, 20 Aug 2021 17:35:21 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 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 2021/8/17 10:03, Eric Biggers wrote: > On Mon, Aug 02, 2021 at 06:34:48PM -0700, Jaegeuk Kim wrote: >> On 08/03, Chao Yu wrote: >>> 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. >> >> I think buffered writes as a workaround can expose the last unwritten block as >> well, if SPO happens right after block allocation. We may need to compromise >> at certain level? >> > > Freeing preallocated blocks on error would be better than nothing, although note > that the preallocated blocks may have filled an arbitrary sequence of holes -- > so simply truncating past EOF would *not* be sufficient. > > But really filesystems need to be designed to never expose uninitialized data, > even if I/O errors or a sudden power failure occurs. It is unfortunate that > f2fs apparently wasn't designed with that goal in mind. > > In any case, I don't think we can proceed with any other f2fs direct I/O > improvements until this data leakage bug can be solved one way or another. If > my patch to remove support for allocating writes isn't acceptable and the > desired solution is going to require some more invasive f2fs surgery, are you or > Chao going to work on it? I'm not sure there's much I can do here. I may have time to take look into the implementation as I proposed above, maybe just enabling this in FSYNC_MODE_STRICT mode if user concerns unwritten data? thoughts? > > - Eric >