RFC improve io_uring's ubuf_info refcounting

There is a couple of tricks we can do with io_uring to improve ubuf_info refcounting. First, we ammortise reference grabbing and then give them away to the network layer, which is implemented in 8 and 11. Also, we don't need need additional pinning for TCP, which is removed by 7.

1-4 are needed because otherwise we're out of space in io_notif_data and using ->desc or some other field of ubuf_info would be ugly. It'll also facilitate further ideas like adding a simpler notification model for UDP.

liburing/examples/io_uring-sendzc benchmark using a branch containing the patchset and some more [1] showed ~1.6% qps improvement for UDP (dummy dev), and ~1% for TCP (localhost + hacks enabling zc).

I didn't specifically test xen and vhost and not sure how, would love some help with that.

[1] https://github.com/isilence/linux/tree/net/zc-ref-optimisation

Pavel Begunkov (11):
net: introduce struct ubuf_info_msgzc xen/netback: use struct ubuf_info_msgzc vhost/net: use struct ubuf_info_msgzc
net: shrink struct ubuf_info
net: rename ubuf_info's flags
net: add flags for controlling ubuf_info net/tcp: optimise tcp ubuf refcounting
net: let callers provide ->msg_ubuf refs io_uring/notif: add helper for flushing refs io_uring/notif: mark notifs with UARGFL_CALLER_PINNED io_uring/notif: add ubuf_info ref caching

 drivers/net/xen-netback/common.h    |  2 +-
 drivers/net/xen-netback/interface.c |  4 +--
 drivers/net/xen-netback/netback.c   |  7 +++---
 drivers/vhost/net.c                 | 17 +++++++------
 include/linux/skbuff.h              | 35 +++++++++++++++++++++++---
 io_uring/net.c                      |  8 +++++-
 io_uring/notif.c                    | 21 ++++++++++------
 io_uring/notif.h                    | 22 +++++++++++++++-
 net/core/skbuff.c                   | 39 ++++++++++++++++-------------
 net/ipv4/ip_output.c                |  3 ++-
 net/ipv4/tcp.c                      | 11 +++++---
 net/ipv6/ip6_output.c               |  3 ++-
 12 files changed, 123 insertions(+), 49 deletions(-)

2.37.0