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SubjectRe: [PATCH 04/10] iov_iter: Add mapping and discard iterator types
On Thu, Sep 13, 2018 at 04:52:09PM +0100, David Howells wrote:
> Add two new iterator types to iov_iter:
> 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.

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