On Tue, Jan 11, 2022 at 02:01:17PM +0000, Matthew Wilcox wrote:

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

quoted

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*().

And changing the bio_vec for *that* purpose was not really acceptable.

But now that you're looking to change it in a big way (and with some
spare bits avaiable...oohh!), maybe I can go that direction after all.

Or, are you looking at a design in which any phyr is implicitly FOLL_PIN'd
if it exists at all?

That.  I think there's still good reasons to keep a single-page (or
maybe dual-page) GUP around, but no reason to mix it with ranges.

quoted

Or any other thoughts in this area are very welcome.

That's there's no support for unpinning part of a range.  You pin it,
do the IO, unpin it.  That simplifies the accounting.

VFIO wouldn't like this :(

Jason