From: Sargun Dhillon <sargun@sargun.me>
To: Jeff Layton <jlayton@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>,
Amir Goldstein <amir73il@gmail.com>,
Vivek Goyal <vgoyal@redhat.com>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
overlayfs <linux-unionfs@vger.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>
Subject: Re: [PATCH 3/3] overlayfs: Report writeback errors on upper
Date: Mon, 28 Dec 2020 11:25:18 -0800 [thread overview]
Message-ID: <CAMp4zn92-WLFkDPVCUX=e+oyHb--7thDDwFEqyvBTGm64biyDw@mail.gmail.com> (raw)
In-Reply-To: <5bc11eb2e02893e7976f89a888221c902c11a2b4.camel@kernel.org>
On Mon, Dec 28, 2020 at 9:26 AM Jeff Layton <jlayton@kernel.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@kernel.org>
>
I agree that predictable behaviour outweighs any benefit of complexity
cutting we might do here.
next prev parent reply other threads:[~2020-12-28 19:26 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-21 19:50 [RFC PATCH 0/3][v3] vfs, overlayfs: Fix syncfs() to return correct errors Vivek Goyal
2020-12-21 19:50 ` [PATCH 1/3] vfs: Do not ignore return code from s_op->sync_fs Vivek Goyal
2020-12-22 1:23 ` NeilBrown
2020-12-22 15:17 ` Vivek Goyal
2020-12-21 19:50 ` [PATCH 2/3] vfs: Add a super block operation to check for writeback errors Vivek Goyal
2020-12-22 16:19 ` Matthew Wilcox
2020-12-22 16:25 ` Vivek Goyal
2020-12-23 12:44 ` Jeff Layton
2020-12-23 12:48 ` Jeff Layton
2021-01-04 19:41 ` Vivek Goyal
2020-12-21 19:50 ` [PATCH 3/3] overlayfs: Report writeback errors on upper Vivek Goyal
2020-12-22 16:20 ` Matthew Wilcox
2020-12-22 16:29 ` Vivek Goyal
2020-12-22 17:46 ` Matthew Wilcox
2020-12-22 17:55 ` Vivek Goyal
2020-12-23 12:53 ` Jeff Layton
2020-12-23 18:20 ` Sargun Dhillon
2020-12-23 18:50 ` Matthew Wilcox
2020-12-23 19:29 ` Sargun Dhillon
2020-12-23 20:07 ` Matthew Wilcox
2020-12-23 20:21 ` Sargun Dhillon
2020-12-23 20:44 ` Matthew Wilcox
2020-12-24 9:32 ` Amir Goldstein
2020-12-24 10:12 ` Sargun Dhillon
2020-12-24 12:13 ` Matthew Wilcox
2020-12-25 6:50 ` Amir Goldstein
2020-12-28 13:25 ` Jeff Layton
2020-12-28 15:51 ` Amir Goldstein
2021-01-04 15:51 ` Vivek Goyal
2020-12-28 15:56 ` Matthew Wilcox
2020-12-28 17:26 ` Jeff Layton
2020-12-28 19:25 ` Sargun Dhillon [this message]
2020-12-28 19:37 ` Amir Goldstein
2020-12-28 20:48 ` Matthew Wilcox
2021-01-02 13:25 ` Jeff Layton
2021-01-04 16:59 ` Vivek Goyal
2021-01-04 15:14 ` Vivek Goyal
2021-01-04 15:22 ` Amir Goldstein
2021-01-04 15:40 ` Vivek Goyal
2021-01-04 21:42 ` Amir Goldstein
2021-01-04 22:44 ` Vivek Goyal
2021-01-05 7:11 ` Amir Goldstein
2021-01-05 16:26 ` Vivek Goyal
2021-01-05 16:57 ` Amir Goldstein
2020-12-23 19:00 ` Jeff Layton
2021-01-04 20:00 ` Vivek Goyal
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