linux-kernel - Re: [PATCH 3/3] overlayfs: Report writeback errors on upper

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Date:   Mon, 28 Dec 2020 11:25:18 -0800
From:   Sargun Dhillon <sargun@...gun.me>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Amir Goldstein <amir73il@...il.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>, Jan Kara <jack@...e.cz>,
        NeilBrown <neilb@...e.com>, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>,
        Chengguang Xu <cgxu519@...ernel.net>
Subject: Re: [PATCH 3/3] overlayfs: Report writeback errors on upper

On Mon, Dec 28, 2020 at 9:26 AM Jeff Layton <jlayton@...nel.org> wrote:
>
> On Mon, 2020-12-28 at 15:56 +0000, Matthew Wilcox wrote:
> > On Mon, Dec 28, 2020 at 08:25:50AM -0500, Jeff Layton wrote:
> > > To be clear, the main thing you'll lose with the method above is the
> > > ability to see an unseen error on a newly opened fd, if there was an
> > > overlayfs mount using the same upper sb before your open occurred.
> > >
> > > IOW, consider two overlayfs mounts using the same upper layer sb:
> > >
> > > ovlfs1                              ovlfs2
> > > ----------------------------------------------------------------------
> > > mount
> > > open fd1
> > > write to fd1
> > > <writeback fails>
> > >                             mount (upper errseq_t SEEN flag marked)
> > > open fd2
> > > syncfs(fd2)
> > > syncfs(fd1)
> > >
> > >
> > > On a "normal" (non-overlay) fs, you'd get an error back on both syncfs
> > > calls. The first one has a sample from before the error occurred, and
> > > the second one has a sample of 0, due to the fact that the error was
> > > unseen at open time.
> > >
> > > On overlayfs, with the intervening mount of ovlfs2, syncfs(fd1) will
> > > return an error and syncfs(fd2) will not. If we split the SEEN flag into
> > > two, then we can ensure that they both still get an error in this
> > > situation.
> >
> > But do we need to?  If the inode has been evicted we also lose the errno.
> > The guarantee we provide is that a fd that was open before the error
> > occurred will see the error.  An fd that's opened after the error occurred
> > may or may not see the error.
> >
>
> In principle, you can lose errors this way (which was the justification
> for making errseq_sample return 0 when there are unseen errors). E.g.,
> if you close fd1 instead of doing a syncfs on it, that error will be
> lost forever.
>
> As to whether that's OK, it's hard to say. It is a deviation from how
> this works in a non-containerized situation, and I'd argue that it's
> less than ideal. You may or may not see the error on fd2, but it's
> dependent on events that take place outside the container and that
> aren't observable from within it. That effectively makes the results
> non-deterministic, which is usually a bad thing in computing...
>
> --
> Jeff Layton <jlayton@...nel.org>
>

I agree that predictable behaviour outweighs any benefit of complexity
cutting we might do here.