On Thu, Feb 02, 2017 at 02:56:51AM -0800, Christoph Hellwig wrote:

quoted

I really wonder if this is the right approach.  Most of the users of
iov_iter_get_pages()/iov_iter_get_pages_alloc() look like they want
something like
	iov_iter_for_each_page(iter, size, f, data)
with int (*f)(struct page *page, size_t from, size_t size, void *data)
passed as callback.  Not everything fits that model, but there's a whole
lot of things that do.

I was planning to do that, mostly because of the iomap dio code that
would not only get a lot cleaner with this, but also support multi-page
bvecs that we hope to have in the block layer soon.  The issue with it
is that we need to touch all the arch get_user_pages_fast
implementations, so it's going to be a relatively invasive change that I
didn't want to fix with just introducing the new direct I/O code.

I'm not sure we need to touch any get_user_pages_fast() at all; let it
fill a medium-sized array and use that as a buffer.  In particular,
I *really* don't like the idea of having the callbacks done in an
inconsistent locking environment - sometimes under ->mmap_sem, sometimes
not.

I played with "let it fill bio_vec array", but it doesn't really fit the
users; variant with callbacks is cleaner, IMO.