transferring bvecs over the network in drbd

[Christoph Hellwig <hch@infradead.org>]

Hi all,

I recently went over code that directly access the bio_vec bv_page/
bv_offset members and the code in _drbd_send_bio/_drbd_send_zc_bio
came to my attention.

It iterates the bio to kmap all segments, and then either does a
sock_sendmsg on a newly created kvec iter, or one one a new bvec iter
for each segment.  The former can't work on highmem systems and both
versions are rather inefficient.

What is preventing drbd from doing a single sock_sendmsg with the
bvec payload?  nvme-tcp (nvme_tcp_init_iter0 is a good example for
doing that, or the sunrpc svcsock code using it's local bvec list
(svc_tcp_sendmsg).