On Fri, Jun 18, 2021 at 11:50:25AM -0700, Linus Torvalds wrote:

I think that you may need it to be based on Al's series for that to
work, which might be inconvenient, though.

One other non-code issue: particularly since you only handle a subset
of the iov_iter cases, it would be nice to have an explanation for
_why_ those particular cases.

IOW, have some trivial explanation for each of the cases. "iovec" is
for regular read/write, what triggers the kvec and bvec cases?

But also, the other way around. Why doesn't the pipe case trigger? No
splice support?

Pipe ones are strictly destinations - they can't be sources.  So if you
see it called for one of those, you've a bug.

Xarray ones are *not* - they can be sources, and that's missing here.

Much more unpleasant, though, is that this thing has hard dependency on
nr_seg == 1 *AND* openly suggests the use of iov_iter_single_seg_count(),
which is completely wrong.  That sucker has some weird users left (as
of #work.iov_iter), but all of them are actually due to API deficiencies
and I very much hope to kill that thing off.

Why not simply add iov_iter_check_zeroes(), that would be called after
copy_from_iter() and verified that all that's left in the iterator
consists of zeroes?  Then this copy_struct_from_...() would be
trivial to express through those two.  And check_zeroes would also
be trivial, especially on top of #work.iov_iter.  With no calls of
iov_iter_advance() at all, while we are at it...

IDGI... Omar, what semantics do you really want from that primitive?