Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to allocate more pages per call

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

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