Re: [RFC][PATCH] iov_iter: Add extraction functions

[Christoph Hellwig <hch@infradead.org>]

On Tue, Oct 18, 2022 at 03:29:42PM +0100, David Howells wrote:
> Note that iov_iter_get_pages2() doesn't handle KVEC-class iterators, which
> this code does - for kmalloc'd, vmalloc'd and vmap'd memory and for global and
> stack variables.  What I've written gets the physical addresses but
> doesn't/can't pin it (which probably means I can't just move my code into
> iov_iter_get_pages2()).

Please look at the long discussion on Ira's patches for
iov_iter_pin_pages where we went through this over and over again.
We need to handle all of these in the common helper.

> Further, my code also treats multipage folios as single units which
> iov_iter_get_pages2() also doesn't - at least from XARRAY-class iterators.
> 
> The UBUF-/IOVEC-extraction code doesn't handle multipage folios because
> get_user_pages_fast() doesn't - though perhaps it will need to at some point.

Yes, we need to be smarter about folios.  But that is nothing magic
about network file systems.  I have old patches that added a (barely
working) variant of get_user_pages_fast that exported bio_vecs and I
think willy planned to look into a folio based version of get/pin
user pages.  But one thing at a time.  I think the work from Ira to
switch to proper pinning and sort out the weirdnesses for non-users
buffer types is the first priority.  After that proper folio or large
contiguous range support is another item.  But none of that is helped
by parallel infrastructure.