Re: [PATCH v18 09/15] iov_iter: Kill ITER_PIPE

[Al Viro]

On Thu, Apr 20, 2023 at 11:36:57PM +0100, Al Viro wrote:

> Don't get me wrong - I'd love to kill ITER_PIPE off; it's just that tons
> of ->splice_read() duplicating the corresponding ->read_iter() up to the
> point where it would call generic_file_read_iter(), modulo ignoring
> O_DIRECT case and then calling filemap_splice_read() instead...  Painful
> and asking for trouble down the road.

FWIW, broken in this series:
	* a bunch of 9p variants (those could probably switch to direct)
	* afs
	* ceph (very likely)
	* ecryptfs (atime propagation)
	* dax on various filesystems (probably)
	* f2fs
	* gfs2
	* nfs
	* ocfs2
	* orangefs
	* xfs
	* zonefs (probably)
	* splice from UDP sockets, unless I'm misreading something.
Your sock_splice_read() still falls back to generic_file_splice_read(),
rather than to direct_splice_read() and sockets don't have O_DIRECT
in flags.  Neither do they have associated page cache ;-)

Sure, we could provide a bunch of ->splice_read(), but then we'd have to make
sure that all subsequent changes to matching ->read_iter() get duplicated -
unless they are on O_DIRECT-only paths, that is...