On 1/20/22 6:12 AM, Christoph Hellwig wrote:
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.
I am really glad to hear you say that. Because I just worked through it
again in detail yesterday (including your and others' old emails about
this), and tentatively reached the same conclusion from seeing the call
paths. But I wanted to confirm with someone who actually knows this code
well, and that's not me. :)
Things like dio_refill_pages() are mixing in the zero page, but like you
say, that can be handled. I have a few ideas for that.
Now that the goal is a considerably narrower as compared to in 2019
("convert DIO callers to pup", instead of "convert the world to pup",
ha), this looks quite feasible after all.
thanks,
--
John Hubbard
NVIDIA
