Re: [PATCH 22/23] fs: default to generic_file_splice_read for files having ->read_iter

[Al Viro]

On Tue, Jul 07, 2020 at 07:48:00PM +0200, Christoph Hellwig wrote:
> If a file implements the ->read_iter method, the iter based splice read
> works and is always preferred over the ->read based one.  Use it by
> default in do_splice_to and remove all the direct assignment of
> generic_file_splice_read to file_operations.

The worst problem here is the assumption that all ->read_iter() instances
will take pipe-backed destination; that's _not_ automatically true.
In particular, it's almost certainly false for tap_read_iter() (as
well as tun_chr_read_iter() in IFF_VNET_HDR case).

Other potentially interesting cases: cuse and hugetlbfs.

But in any case, that blind assertion ("iter based splice read works")
really needs to be backed by something.