On Tue, Jan 11, 2022 at 12:17:18AM -0800, John Hubbard wrote:

Zooming in on the pinning aspect for a moment: last time I attempted to
convert O_DIRECT callers from gup to pup, I recall wanting very much to
record, in each bio_vec, whether these pages were acquired via FOLL_PIN,
or some non-FOLL_PIN method. Because at the end of the IO, it is not
easy to disentangle which pages require put_page() and which require
unpin_user_page*().

I don't think that is a problem.  Pinning only need to happen for
ITER_IOVEC, and the only non-user pages there is the ZERO_PAGE added
for padding that can be special cased.