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=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 594AAC433DB for ; Thu, 7 Jan 2021 13:46:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2D1AF224BD for ; Thu, 7 Jan 2021 13:46:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728091AbhAGNq2 (ORCPT ); Thu, 7 Jan 2021 08:46:28 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:38223 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727562AbhAGNq2 (ORCPT ); Thu, 7 Jan 2021 08:46:28 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1610027101; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=D52G95pEfQcjTOfA+HqwOJkz9BoqmXBTifvDL2eir00=; b=Pea4XcftPTEw5QKbwOn7qon1UXmXi00NTaMC6mLlvUQQUlVAo4Ia2w6/Ra2cNFqptoCDGe N9Ioq40JjNnrobyrWngWxDynBUoqzSWyiBkc6dtN6zBJ/d+cXkJn0fkUAAzPUVDWdBpzi+ scb2O4aS6VI9zQxglgULd5X5UP0Bcas= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-359-FRzcgvvdNr6IJbE3MgO-4Q-1; Thu, 07 Jan 2021 08:44:59 -0500 X-MC-Unique: FRzcgvvdNr6IJbE3MgO-4Q-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5DE14107ACFA; Thu, 7 Jan 2021 13:44:57 +0000 (UTC) Received: from horse.redhat.com (ovpn-116-86.rdu2.redhat.com [10.10.116.86]) by smtp.corp.redhat.com (Postfix) with ESMTP id ABD0E60C0F; Thu, 7 Jan 2021 13:44:56 +0000 (UTC) Received: by horse.redhat.com (Postfix, from userid 10451) id 3DD0822054F; Thu, 7 Jan 2021 08:44:56 -0500 (EST) Date: Thu, 7 Jan 2021 08:44:56 -0500 From: Vivek Goyal To: Amir Goldstein Cc: Sargun Dhillon , overlayfs , Miklos Szeredi , Alexander Viro , Giuseppe Scrivano , Daniel J Walsh , linux-fsdevel , David Howells , Chengguang Xu , Christoph Hellwig , NeilBrown , Jan Kara , Jeff Layton , Matthew Wilcox Subject: Re: [PATCH v3] overlay: Implement volatile-specific fsync error behaviour Message-ID: <20210107134456.GA3439@redhat.com> References: <20210106083546.4392-1-sargun@sargun.me> <20210106194658.GA3290@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: linux-unionfs@vger.kernel.org On Thu, Jan 07, 2021 at 09:02:00AM +0200, Amir Goldstein wrote: > On Wed, Jan 6, 2021 at 9:47 PM Vivek Goyal wrote: > > > > On Wed, Jan 06, 2021 at 12:35:46AM -0800, Sargun Dhillon wrote: > > > Overlayfs's volatile option allows the user to bypass all forced sync calls > > > to the upperdir filesystem. This comes at the cost of safety. We can never > > > ensure that the user's data is intact, but we can make a best effort to > > > expose whether or not the data is likely to be in a bad state. > > > > > > The best way to handle this in the time being is that if an overlayfs's > > > upperdir experiences an error after a volatile mount occurs, that error > > > will be returned on fsync, fdatasync, sync, and syncfs. This is > > > contradictory to the traditional behaviour of VFS which fails the call > > > once, and only raises an error if a subsequent fsync error has occurred, > > > and been raised by the filesystem. > > > > > > One awkward aspect of the patch is that we have to manually set the > > > superblock's errseq_t after the sync_fs callback as opposed to just > > > returning an error from syncfs. This is because the call chain looks > > > something like this: > > > > > > sys_syncfs -> > > > sync_filesystem -> > > > __sync_filesystem -> > > > /* The return value is ignored here > > > sb->s_op->sync_fs(sb) > > > _sync_blockdev > > > /* Where the VFS fetches the error to raise to userspace */ > > > errseq_check_and_advance > > > > > > Because of this we call errseq_set every time the sync_fs callback occurs. > > > > Why not start capturing return code of ->sync_fs and then return error > > from ovl->sync_fs. And then you don't have to do errseq_set(ovl_sb). > > > > I already posted a patch to capture retrun code from ->sync_fs. > > > > https://lore.kernel.org/linux-fsdevel/20201221195055.35295-2-vgoyal@redhat.com/ > > > > > > Vivek, > > IMO the more important question is "Why not?". > > Your patches will undoubtedly get to mainline in the near future and they do > make the errseq_set(ovl_sb) in this patch a bit redundant, I thought my patch of capturing ->sync_fs is really simple (just few lines), so backportability should not be an issue. That's why I asked for it. > but I really see no > harm in it. It is very simple for you to remove this line in your patch. > I do see the big benefit of an independent patch that is easy to apply to fix > a fresh v5.10 feature. > > I think it is easy for people to dismiss the importance of "syncfs on volatile" > which sounds like a contradiction, but it is not. > The fact that the current behavior is documented doesn't make it right either. > It just makes our review wrong. > The durability guarantee (that volatile does not provide) is very different > from the "reliability" guarantee that it CAN provide. > We do not want to have to explain to people that "volatile" provided different > guarantees depending on the kernel they are running. > Fixing syncfs/fsync of volatile is much more important IMO than erroring > on other fs ops post writeback error, because other fs ops are equally > unreliable on any filesystem in case application did not do fsync. > > Ignoring the factor of "backporting cost" when there is no engineering > justification to do so is just ignoring the pain of others. > Do you have an engineering argument for objecting this patch is > applied before your fixes to syncfs vfs API? Carrying ->sync_fs return code patch is definitely not a blocker. It is just nice to have. Anyway, I you don't want to carry that ->sync_fs return patch in stable, I am fine with this patch. I will follow up on that fix separately. Vivek > > Sargun, > > Please add Fixes/Stable #v5.10 tags. > > Thanks, > Amir. >