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

[Matthew Wilcox]

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.