Re: [PATCH v3 0/2] libceph: add new iov_iter msg_data type and use it for reads

[Jeff Layton <jlayton@kernel.org>]

On Sun, 2022-07-03 at 22:56 -0700, Christoph Hellwig wrote:
> On Fri, Jul 01, 2022 at 06:30:11AM -0400, Jeff Layton wrote:
> > Currently, we take an iov_iter from the netfs layer, turn that into an
> > array of pages, and then pass that to the messenger which eventually
> > turns that back into an iov_iter before handing it back to the socket.
> > 
> > This patchset adds a new ceph_msg_data_type that uses an iov_iter
> > directly instead of requiring an array of pages or bvecs. This allows
> > us to avoid an extra allocation in the buffered read path, and should
> > make it easier to plumb in write helpers later.
> > 
> > For now, this is still just a slow, stupid implementation that hands
> > the socket layer a page at a time like the existing messenger does. It
> > doesn't yet attempt to pass through the iov_iter directly.
> > 
> > I have some patches that pass the cursor's iov_iter directly to the
> > socket in the receive path, but it requires some infrastructure that's
> > not in mainline yet (iov_iter_scan(), for instance). It should be
> > possible to something similar in the send path as well.
> 
> Btw, is there any good reason to not simply replace ceph_msg_data
> with an iov_iter entirely?
> 

Not really, no.

What I'd probably do is change the existing osd_req_op_* callers to use
the new iov_iter msg_data type first, and then once they all were you
could phase out the use of struct ceph_msg_data altogether.
-- 
Jeff Layton <jlayton@kernel.org>