[PATCH] jffs2: Hook up splice_write callback

[Al Viro]

On Tue, Mar 30, 2021 at 06:17:15PM +0200, Christoph Hellwig wrote:
> On Wed, Mar 31, 2021 at 12:15:37AM +1030, Joel Stanley wrote:
> > overlayfs using jffs2 as the upper filesystem would fail in some cases
> > since moving to v5.10. The test case used was to run 'touch' on a file
> > that exists in the lower fs, causing the modification time to be
> > updated. It returns EINVAL when the bug is triggered.
> > 
> > A bisection showed this was introduced in v5.9-rc1, with commit
> > 36e2c7421f02 ("fs: don't allow splice read/write without explicit ops").
> > Reverting that commit restores the expected behaviour.
> > 
> > Some digging showed that this was due to jffs2 lacking an implementation
> > of splice_write. (For unknown reasons the warn_unsupported that should
> > trigger was not displaying any output).
> > 
> > Adding this patch resolved the issue and the test now passes.
> 
> Looks good:
> 
> Reviewed-by: Christoph Hellwig <hch at lst.de>

The same goes for quite a few other filesystems, actually - at least
adfs, affs, bfs, hfs, hfsplus, hostfs, hpfs, minix, omfs, sysv, ufs 
and vboxsf are in the same boat, and I suspect that ecryptfs and ntfs
might be too.

Christoph, do you see any problems with doing the same thing for that
bunch as well?