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SubjectRe: [PATCH 04/10] iov_iter: Add mapping and discard iterator types
On Fri, 2018-09-14 at 05:18 +0100, Al Viro wrote:
> 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
> 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.

Another question that is relevant for most networked filesystems
(including AFS, I believe), is how will you deal with encryption of the
data you are transmitting? Encrypting and decrypting in-place directly
in the page cache or in a userspace O_DIRECT mapped buffer might not be
the best and most secure option, so won't you find yourself wanting to
copy the data anyway?

Trond Myklebust
Linux NFS client maintainer, Hammerspace

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