Re: [PATCH 04/10] iov_iter: Add mapping and discard iterator types

[Al Viro]

On Thu, Sep 13, 2018 at 04:52:09PM +0100, David Howells wrote:
> Add two new iterator types to iov_iter:
> 
>  (1) ITER_MAPPING
> 
>      This walks through a set of pages attached to an address_space that
>      are pinned or locked, starting at a given page and offset and walking
>      for the specified amount of space.  A facility to get a callback each
>      time a page is entirely processed is provided.
> 
>      This is useful for copying data from socket buffers to inodes in
>      network filesystems.

Interesting...  Questions:
	* what will hold those pages?  IOW, where will you unlock/drop/whatnot
those sucker?
	* "callback" sounds dangerous - it appears to imply that you won't
copy to/from the same page twice.  Not true for a lot of iov_iter users; what
happens if you pass such a beast to them?
	* why not simply "build and populate ITER_BVEC aliasing a piece of mapping",
possibly in "grab" and "grab+lock" variants?  Those ITER_MAPPING do seem to be
related to ITER_BVEC, at the very least.  Note, BTW, that iov_iter_get_pages...()
might mutate into something similar - "build and populate ITER_BVEC aliasing a piece
of given iov_iter".  Or, perhaps, a nicer-on-memory analogue of ITER_BVEC -
with <offset, bytes, pointer to pages array> instead of <offset, bytes, page> as
elements, with the same "populate from mapping" to get something similar to your
functionality and "populate from iov_iter" for iov_iter_get_pages... replacement.