Re: [PATCH net-next v5 03/16] ceph: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage

[Ilya Dryomov]

On Sat, Jun 24, 2023 at 12:55 AM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Use sendmsg() and MSG_SPLICE_PAGES rather than sendpage in ceph when
> transmitting data.  For the moment, this can only transmit one page at a
> time because of the architecture of net/ceph/, but if
> write_partial_message_data() can be given a bvec[] at a time by the
> iteration code, this would allow pages to be sent in a batch.
>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> cc: Ilya Dryomov <idryomov@xxxxxxxxx>
> cc: Xiubo Li <xiubli@xxxxxxxxxx>
> cc: Jeff Layton <jlayton@xxxxxxxxxx>
> cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> cc: Jens Axboe <axboe@xxxxxxxxx>
> cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> cc: ceph-devel@xxxxxxxxxxxxxxx
> cc: netdev@xxxxxxxxxxxxxxx
> ---
>
> Notes:
>     ver #5)
>      - Switch condition for setting MSG_MORE in write_partial_message_data()
>
>  net/ceph/messenger_v1.c | 60 ++++++++++++++---------------------------
>  1 file changed, 20 insertions(+), 40 deletions(-)
>
> diff --git a/net/ceph/messenger_v1.c b/net/ceph/messenger_v1.c
> index d664cb1593a7..814579f27f04 100644
> --- a/net/ceph/messenger_v1.c
> +++ b/net/ceph/messenger_v1.c
> @@ -74,37 +74,6 @@ static int ceph_tcp_sendmsg(struct socket *sock, struct kvec *iov,
>         return r;
>  }
>
> -/*
> - * @more: either or both of MSG_MORE and MSG_SENDPAGE_NOTLAST
> - */
> -static int ceph_tcp_sendpage(struct socket *sock, struct page *page,
> -                            int offset, size_t size, int more)
> -{
> -       ssize_t (*sendpage)(struct socket *sock, struct page *page,
> -                           int offset, size_t size, int flags);
> -       int flags = MSG_DONTWAIT | MSG_NOSIGNAL | more;
> -       int ret;
> -
> -       /*
> -        * sendpage cannot properly handle pages with page_count == 0,
> -        * we need to fall back to sendmsg if that's the case.
> -        *
> -        * Same goes for slab pages: skb_can_coalesce() allows
> -        * coalescing neighboring slab objects into a single frag which
> -        * triggers one of hardened usercopy checks.
> -        */
> -       if (sendpage_ok(page))
> -               sendpage = sock->ops->sendpage;
> -       else
> -               sendpage = sock_no_sendpage;
> -
> -       ret = sendpage(sock, page, offset, size, flags);
> -       if (ret == -EAGAIN)
> -               ret = 0;
> -
> -       return ret;
> -}
> -
>  static void con_out_kvec_reset(struct ceph_connection *con)
>  {
>         BUG_ON(con->v1.out_skip);
> @@ -464,7 +433,6 @@ static int write_partial_message_data(struct ceph_connection *con)
>         struct ceph_msg *msg = con->out_msg;
>         struct ceph_msg_data_cursor *cursor = &msg->cursor;
>         bool do_datacrc = !ceph_test_opt(from_msgr(con->msgr), NOCRC);
> -       int more = MSG_MORE | MSG_SENDPAGE_NOTLAST;
>         u32 crc;
>
>         dout("%s %p msg %p\n", __func__, con, msg);
> @@ -482,6 +450,10 @@ static int write_partial_message_data(struct ceph_connection *con)
>          */
>         crc = do_datacrc ? le32_to_cpu(msg->footer.data_crc) : 0;
>         while (cursor->total_resid) {
> +               struct bio_vec bvec;
> +               struct msghdr msghdr = {
> +                       .msg_flags = MSG_SPLICE_PAGES,

Hi David,

This appears to be losing MSG_DONTWAIT | MSG_NOSIGNAL flags which were
set previously?

> +               };
>                 struct page *page;
>                 size_t page_offset;
>                 size_t length;
> @@ -493,10 +465,13 @@ static int write_partial_message_data(struct ceph_connection *con)
>                 }
>
>                 page = ceph_msg_data_next(cursor, &page_offset, &length);
> -               if (length == cursor->total_resid)
> -                       more = MSG_MORE;
> -               ret = ceph_tcp_sendpage(con->sock, page, page_offset, length,
> -                                       more);
> +               if (length != cursor->total_resid)
> +                       msghdr.msg_flags |= MSG_MORE;
> +
> +               bvec_set_page(&bvec, page, length, page_offset);
> +               iov_iter_bvec(&msghdr.msg_iter, ITER_SOURCE, &bvec, 1, length);
> +
> +               ret = sock_sendmsg(con->sock, &msghdr);
>                 if (ret <= 0) {

And this is losing munging -EAGAIN -> 0?

>                         if (do_datacrc)
>                                 msg->footer.data_crc = cpu_to_le32(crc);
> @@ -526,7 +501,10 @@ static int write_partial_message_data(struct ceph_connection *con)
>   */
>  static int write_partial_skip(struct ceph_connection *con)
>  {
> -       int more = MSG_MORE | MSG_SENDPAGE_NOTLAST;
> +       struct bio_vec bvec;
> +       struct msghdr msghdr = {
> +               .msg_flags = MSG_SPLICE_PAGES | MSG_MORE,
> +       };
>         int ret;
>
>         dout("%s %p %d left\n", __func__, con, con->v1.out_skip);
> @@ -534,9 +512,11 @@ static int write_partial_skip(struct ceph_connection *con)
>                 size_t size = min(con->v1.out_skip, (int)PAGE_SIZE);
>
>                 if (size == con->v1.out_skip)
> -                       more = MSG_MORE;
> -               ret = ceph_tcp_sendpage(con->sock, ceph_zero_page, 0, size,
> -                                       more);
> +                       msghdr.msg_flags &= ~MSG_MORE;
> +               bvec_set_page(&bvec, ZERO_PAGE(0), size, 0);
> +               iov_iter_bvec(&msghdr.msg_iter, ITER_SOURCE, &bvec, 1, size);
> +
> +               ret = sock_sendmsg(con->sock, &msghdr);
>                 if (ret <= 0)

Same here...  I would suggest that you keep ceph_tcp_sendpage() function
and make only minimal modifications to avoid regressions.

Thanks,

                Ilya