io_uring/notif: implement notification stacking

The network stack allows only one ubuf_info per skb, and unlike
MSG_ZEROCOPY, each io_uring zerocopy send will carry a separate
ubuf_info. That means that send requests can't reuse a previosly
allocated skb and need to get one more or more of new ones. That's fine
for large sends, but otherwise it would spam the stack with lots of skbs
carrying just a little data each.

To help with that implement linking notification (i.e. an io_uring wrapper
around ubuf_info) into a list. Each is refcounted by skbs and the stack
as usual. additionally all non head entries keep a reference to the
head, which they put down when their refcount hits 0. When the head have
no more users, it'll efficiently put all notifications in a batch.

As mentioned previously about ->io_link_skb, the callback implementation
always allows to bind to an skb without a ubuf_info.

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/bf1e7f9b72f9ecc99999fdc0d2cded5eea87fd0b.1713369317.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 3 insertions, 0 deletions

diff --git a/io_uring/notif.h b/io_uring/notif.h
index 2cf9ff6abd7a..f3589cfef4a9 100644
--- a/io_uring/notif.h
+++ b/io_uring/notif.h
@@ -14,6 +14,9 @@ struct io_notif_data {
 	struct file		*file;
 	struct ubuf_info	uarg;
 
+	struct io_notif_data	*next;
+	struct io_notif_data	*head;
+
 	unsigned		account_pages;
 	bool			zc_report;
 	bool			zc_used;