Re: [PATCH 2/2] blkdev: __blkdev_direct_IO_simple: make sure to fill up the bio

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Jul 19, 2018 at 12:37:13PM +0200, Jan Kara wrote:
> On Thu 19-07-18 18:21:23, Ming Lei wrote:
> > On Thu, Jul 19, 2018 at 11:39:18AM +0200, Martin Wilck wrote:
> > > 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...

> > 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.