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=-3.9 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 C8D1AC2D0A3 for ; Mon, 16 Nov 2020 19:27:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8D75122265 for ; Mon, 16 Nov 2020 19:27:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="F85nO6tY" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727301AbgKPT1a (ORCPT ); Mon, 16 Nov 2020 14:27:30 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:26326 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727435AbgKPT13 (ORCPT ); Mon, 16 Nov 2020 14:27:29 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1605554848; 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=AEtwxSlqRNKPo5HyAHp1Up7hERgTeX4ubjUEG+IW1oA=; b=F85nO6tYbJTh2rrQaOZa1NDWsyGVZ24A8+QvsfcfgRhnYP6xESyEOunugxw3RkKwB+s15s biqRr2OYRA5yFnxX4/ZqeZXnPDAqDr8EaXY2s1/1Q7ibjOni76EjnxS7g3uZU6nKkP5E5Q UBbKJ7cLLYSh0UhOtsleAQ6IpvawVgg= 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-306-KoNy-GbUPtmE8cWoVQfWAg-1; Mon, 16 Nov 2020 14:27:26 -0500 X-MC-Unique: KoNy-GbUPtmE8cWoVQfWAg-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 CBEF21868421; Mon, 16 Nov 2020 19:27:24 +0000 (UTC) Received: from horse.redhat.com (ovpn-114-201.rdu2.redhat.com [10.10.114.201]) by smtp.corp.redhat.com (Postfix) with ESMTP id 63D3460C04; Mon, 16 Nov 2020 19:27:24 +0000 (UTC) Received: by horse.redhat.com (Postfix, from userid 10451) id E22D6220BCF; Mon, 16 Nov 2020 14:27:18 -0500 (EST) Date: Mon, 16 Nov 2020 14:27:18 -0500 From: Vivek Goyal To: Sargun Dhillon Cc: Amir Goldstein , overlayfs , Miklos Szeredi , Alexander Viro , Giuseppe Scrivano , Daniel J Walsh , David Howells , linux-fsdevel , Chengguang Xu Subject: Re: [RFC PATCH 3/3] overlay: Add the ability to remount volatile directories when safe Message-ID: <20201116192718.GC9190@redhat.com> References: <20201116045758.21774-1-sargun@sargun.me> <20201116045758.21774-4-sargun@sargun.me> <20201116144240.GA9190@redhat.com> <20201116163615.GA17680@redhat.com> <20201116182553.GB18698@ircssh-2.c.rugged-nimbus-611.internal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201116182553.GB18698@ircssh-2.c.rugged-nimbus-611.internal> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Nov 16, 2020 at 06:25:56PM +0000, Sargun Dhillon wrote: > On Mon, Nov 16, 2020 at 11:36:15AM -0500, Vivek Goyal wrote: > > On Mon, Nov 16, 2020 at 05:20:04PM +0200, Amir Goldstein wrote: > > > On Mon, Nov 16, 2020 at 4:42 PM Vivek Goyal wrote: > > > > > > > > On Sun, Nov 15, 2020 at 08:57:58PM -0800, Sargun Dhillon wrote: > > > > > Overlayfs added the ability to setup mounts where all syncs could be > > > > > short-circuted in (2a99ddacee43: ovl: provide a mount option "volatile"). > > > > > > > > > > A user might want to remount this fs, but we do not let the user because > > > > > of the "incompat" detection feature. In the case of volatile, it is safe > > > > > to do something like[1]: > > > > > > > > > > $ sync -f /root/upperdir > > > > > $ rm -rf /root/workdir/incompat/volatile > > > > > > > > > > There are two ways to go about this. You can call sync on the underlying > > > > > filesystem, check the error code, and delete the dirty file if everything > > > > > is clean. If you're running lots of containers on the same filesystem, or > > > > > you want to avoid all unnecessary I/O, this may be suboptimal. > > > > > > > > > > > > > Hi Sargun, > > > > > > > > I had asked bunch of questions in previous mail thread to be more > > > > clear on your requirements but never got any response. It would > > > > have helped understanding your requirements better. > > > > > > > > How about following patch set which seems to sync only dirty inodes of > > > > upper belonging to a particular overlayfs instance. > > > > > > > > https://lore.kernel.org/linux-unionfs/20201113065555.147276-1-cgxu519@mykernel.net/ > > > > > > > > So if could implement a mount option which ignores fsync but upon > > > > syncfs, only syncs dirty inodes of that overlayfs instance, it will > > > > make sure we are not syncing whole of the upper fs. And we could > > > > do this syncing on unmount of overlayfs and remove dirty file upon > > > > successful sync. > > > > > > > > Looks like this will be much simpler method and should be able to > > > > meet your requirements (As long as you are fine with syncing dirty > > > > upper inodes of this overlay instance on unmount). > > > > > > > > > > Do note that the latest patch set by Chengguang not only syncs dirty > > > inodes of this overlay instance, but also waits for in-flight writeback on > > > all the upper fs inodes and I think that with !ovl_should_sync(ofs) > > > we will not re-dirty the ovl inodes and lose track of the list of dirty > > > inodes - maybe that can be fixed. > > > > > > Also, I am not sure anymore that we can safely remove the dirty file after > > > sync dirty inodes sync_fs and umount. If someone did sync_fs before us > > > and consumed the error, we may have a copied up file in upper whose > > > data is not on disk, but when we sync_fs on unmount we won't get an > > > error? not sure. > > > > May be we can save errseq_t when mounting overlay and compare with > > errseq_t stored in upper sb after unmount. That will tell us whether > > error has happened since we mounted overlay. (Similar to what Sargun > > is doing). > > > > In fact, if this is a concern, we have this issue with user space > > "sync " too? Other sync might fail and this one succeeds > > and we will think upper is just fine. May be container tools can > > keep a file/dir open at the time of mount and call syncfs using > > that fd instead. (And that should catch errors since that fd > > was opened, I am assuming). > > > > > > > > I am less concerned about ways to allow re-mount of volatile > > > overlayfs than I am about turning volatile overlayfs into non-volatile. > > > > If we are not interested in converting volatile containers into > > non-volatile, then whole point of these patch series is to detect > > if any writeback error has happened or not. If writeback error has > > happened, then we detect that at remount and possibly throw away > > container. > > > > What happens today if writeback error has happened. Is that page thrown > > away from page cache and read back from disk? IOW, will user lose > > the data it had written in page cache because writeback failed. I am > > assuming we can't keep the dirty page around for very long otherwise > > it has potential to fill up all the available ram with dirty pages which > > can't be written back. > > > > Why is it important to detect writeback error only during remount. What > > happens if container overlay instance is already mounted and writeback > > error happens. We will not detct that, right? > > > > IOW, if capturing writeback error is important for volatile containers, > > then capturing it only during remount time is not enough. Normally > > fsync/syncfs should catch it and now we have skipped those, so in > > the process we lost mechanism to detect writeback errrors for > > volatile containers? > > > > Thanks > > Vivek > > > > At least for my use case, any kind of syncing is generally bad unless > it can be controlled, and: > 1. Generate a limited set of IOPs > 2. Not block metadata operatons > ---- > > This is a challenge that is left up to the filesystem developers that hasn't > really been addressed yet. The closest we've seen is individual block devices > per upper dir using something like device mapper, and throttling at that level. I think I have heard complaints about cloud providers throttling kicking in as well and that can result in long stalls. But throttling can kick in due to memory pressure writebacks also, so disabling sync/fsync is no guarantee that this will not happen? Help me understand one thing. Say a volatile container is running (hence overlay is mounted). Now a writeback error happens (say on the page which was written by container app). How are you detecting that writeback error? fsync now has been disabled so overlay will return success. Thanks Vivek