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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Tue, 5 Jan 2021 18:57:21 +0200
Message-ID: <CAOQ4uxgA96sDca3YWAqXjkMS8YKmFv259cFS7aWNvhfC6MtL6w@mail.gmail.com>
Subject: Re: [PATCH 3/3] overlayfs: Report writeback errors on upper
To:     Vivek Goyal <vgoyal@redhat.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Sargun Dhillon <sargun@sargun.me>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        Jeff Layton <jlayton@kernel.org>,
        Miklos Szeredi <miklos@szeredi.hu>, Jan Kara <jack@suse.cz>,
        NeilBrown <neilb@suse.com>, Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@lst.de>,
        Chengguang Xu <cgxu519@mykernel.net>
Content-Type: text/plain; charset="UTF-8"
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On Tue, Jan 5, 2021 at 6:26 PM Vivek Goyal <vgoyal@redhat.com> wrote:
>
> On Tue, Jan 05, 2021 at 09:11:23AM +0200, Amir Goldstein wrote:
> > > >
> > > > What I would rather see is:
> > > > - Non-volatile: first syncfs in every container gets an error (nice to have)
> > >
> > > I am not sure why are we making this behavior per container. This should
> > > be no different from current semantics we have for syncfs() on regular
> > > filesystem. And that will provide what you are looking for. If you
> > > want single error to be reported in all ovleray mounts, then make
> > > sure you have one fd open in each mount after mount, then call syncfs()
> > > on that fd.
> > >
> >
> > Ok.
> >
> > > Not sure why overlayfs behavior/semantics should be any differnt
> > > than what regular filessytems like ext4/xfs are offering. Once we
> > > get page cache sharing sorted out with xfs reflink, then people
> > > will not even need overlayfs and be able to launch containers
> > > just using xfs reflink and share base image. In that case also
> > > they will need to keep an fd open per container they want to
> > > see an error in.
> > >
> > > So my patches exactly provide that. syncfs() behavior is same with
> > > overlayfs as application gets it on other filesystems. And to me
> > > its important to keep behavior same.
> > >
> > > > - Volatile: every syncfs and every fsync in every container gets an error
> > > >   (important IMO)
> > >
> > > For volatile mounts, I agree that we need to fail overlayfs instance
> > > as soon as first error is detected since mount. And this applies to
> > > not only syncfs()/fsync() but to read/write and other operations too.
> > >
> > > For that we will need additional patches which are floating around
> > > to keep errseq sample in overlay and check for errors in all
> > > paths syncfs/fsync/read/write/.... and fail fs.
> >
> > > But these patches build on top of my patches.
> >
> > Here we disagree.
> >
> > I don't see how Jeff's patch is "building on top of your patches"
> > seeing that it is perfectly well contained and does not in fact depend
> > on your patches.
>
> Jeff's patches are solving problem only for volatile mounts and they
> are propagating error to overlayfs sb.
>
> My patches are solving the issue both for volatile mount as well as
> non-volatile mounts and solve it using same method so there is no
> confusion.
>
> So there are multiple pieces to this puzzle and IMHO, it probably
> should be fixed in this order.
>
> A. First fix the syncfs() path to return error both for volatile as
>    as well non-volatile mounts.
>
> B. And then add patches to fail filesystem for volatile mount as soon
>    as first error is detected (either in syncfs path or in other paths
>    like read/write/...). This probably will require to save errseq
>    in ovl_fs, and then compare with upper_sb in critical paths and fail
>    filesystem as soon as error is detected.
>
> C. Finally fix the issues related to mount/remount error detection which
>    Sargun is wanting to fix. This will be largerly solved by B except
>    saving errseq on disk.
>
> My patches should fix the first problem. And more patches can be
> applied on top to fix issue B and issue C.
>
> Now if we agree with this, in this context I see that fixing problem
> B and C is building on top of my patches which fixes problem A.
>

That order is fine by me.

> >
> > And I do insist that the fix for volatile mounts syncfs/fsync error
> > reporting should be applied before your patches or at the very least
> > not heavily depend on them.
>
> I still don't understand that why volatile syncfs() error reporting
> is more important than non-volatile syncfs(). But I will stop harping
> on this point now.
>
> My issue with Jeff's patches is that syncfs() error reporting should
> be dealt in same way both for volatile and non-volatile mount. That
> is compare file->f_sb_err and upper_sb->s_wb_err to figure out if
> there is an error to report to user space. Currently this patches
> only solve the problem for volatile mounts and use propagation to
> overlay sb which is conflicting for non-volatile mounts.
>
> IIUC, your primary concern with volatile mount is that you want to
> detect as soon as writeback error happens, and flag it to container
> manager so that container manager can stop container, throw away
> upper layer and restart from scratch. If yes, what you want can
> be solved by solving problem B and backporting it to LTS kernel.
> I think patches for that will be well contained within overlayfs
> (And no VFS) changes and should be relatively easy to backport.
>
> IOW, backportability to LTS kernel should not be a concern/blocker
> for my patch series which fixes syncfs() issue for overlayfs.
>

That's all I wanted to know.

Thanks,
Amir.