Re: [PATCH v7 04/19] iov_iter: Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable

[Al Viro]

On Fri, Aug 27, 2021 at 06:49:11PM +0200, Andreas Gruenbacher wrote:
> Turn iov_iter_fault_in_readable into a function that returns the number
> of bytes not faulted in (similar to copy_to_user) instead of returning a
> non-zero value when any of the requested pages couldn't be faulted in.
> This supports the existing users that require all pages to be faulted in
> as well as new users that are happy if any pages can be faulted in at
> all.
> 
> Rename iov_iter_fault_in_readable to fault_in_iov_iter_readable to make
> sure that this change doesn't silently break things.

I really disagree with these calling conventions.  "Number not faulted in"
is bloody useless; make it "nothing could be faulted in"/"something had
been faulted in" and it would make sense.  Failure several pages into the
area should not be treated as a hard error, for one thing, and ANY user
of that thing will have to cope with short copies anyway, no matter how
much you've managed to fault in.