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

[Trond Myklebust]

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:
> > 
> >  (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.

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
trond.myklebust@xxxxxxxxxxxxxxx