On Thu, Jul 19, 2018 at 12:37:13PM +0200, Jan Kara wrote:

On Thu 19-07-18 18:21:23, Ming Lei wrote:

quoted

On Thu, Jul 19, 2018 at 11:39:18AM +0200, Martin Wilck wrote:

quoted

bio_iov_iter_get_pages() returns only pages for a single non-empty
segment of the input iov_iter's iovec. This may be much less than the number
of pages __blkdev_direct_IO_simple() is supposed to process. Call

In bio_iov_iter_get_pages(), iov_iter_get_pages() supposes to retrieve
as many as possible pages since both 'maxsize' and 'maxpages' are provided
to cover all.

Huh?  Why does having a way to say "I (the caller) don't want more than <amount>"
implies anything of that sort?  It never had been promised, explicitly or not...

quoted

So the question is why iov_iter_get_pages() doesn't work as expected?

Well, there has never been a promise that it will grab *all* pages in the
iter AFAIK. Practically, I think that it was just too hairy to implement in
the macro magic that iter processing is... Al might know more (added to
CC).

Not really - it's more that VM has every right to refuse letting you pin
an arbitrary amount of pages anyway.