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Date:   Mon, 28 Dec 2020 15:56:18 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     Amir Goldstein <amir73il@gmail.com>,
        Sargun Dhillon <sargun@sargun.me>,
        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
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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.