Re: [PATCH net-next] libceph: Partially revert changes to support MSG_SPLICE_PAGES

[Ilya Dryomov]

On Mon, Jun 26, 2023 at 7:53 PM David Howells <dhowells@xxxxxxxxxx> wrote:
>
>
> Fix the mishandling of MSG_DONTWAIT and also reinstates the per-page
> checking of the source pages (which might have come from a DIO write by
> userspace) by partially reverting the changes to support MSG_SPLICE_PAGES
> and doing things a little differently.  In messenger_v1:
>
>  (1) The ceph_tcp_sendpage() is resurrected and the callers reverted to use
>      that.
>
>  (2) The callers now pass MSG_MORE unconditionally.  Previously, they were
>      passing in MSG_MORE|MSG_SENDPAGE_NOTLAST and then degrading that to
>      just MSG_MORE on the last call to ->sendpage().
>
>  (3) Make ceph_tcp_sendpage() a wrapper around sendmsg() rather than
>      sendpage(), setting MSG_SPLICE_PAGES if sendpage_ok() returns true on
>      the page.
>
> In messenger_v2:
>
>  (4) Bring back do_try_sendpage() and make the callers use that.
>
>  (5) Make do_try_sendpage() use sendmsg() for both cases and set
>      MSG_SPLICE_PAGES if sendpage_ok() is set.
>
> Note that libceph is using sendpage_ok() for this, but it should not really
> be considered exhaustive in this regard - this is, however, what libceph is
> doing upstream.

Such use spans far beyond libceph: drbd, iSCSI, NVMe-over-TCP, etc.
sendpage_ok() the only tool we have got...

>
> Fixes: 40a8c17aa770 ("ceph: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage")
> Fixes: fa094ccae1e7 ("ceph: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage()")
> Reported-by: Ilya Dryomov <idryomov@xxxxxxxxx>
> Link: https://lore.kernel.org/r/CAOi1vP9vjLfk3W+AJFeexC93jqPaPUn2dD_4NrzxwoZTbYfOnw@xxxxxxxxxxxxxx/
> Link: https://lore.kernel.org/r/CAOi1vP_Bn918j24S94MuGyn+Gxk212btw7yWeDrRcW1U8pc_BA@xxxxxxxxxxxxxx/
> 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
> ---
>  net/ceph/messenger_v1.c |   57 +++++++++++++++++++------------
>  net/ceph/messenger_v2.c |   87 ++++++++++++++++++++++++++++++++++++++----------
>  2 files changed, 105 insertions(+), 39 deletions(-)
>
> diff --git a/net/ceph/messenger_v1.c b/net/ceph/messenger_v1.c
> index 814579f27f04..1ee58cbaf151 100644
> --- a/net/ceph/messenger_v1.c
> +++ b/net/ceph/messenger_v1.c
> @@ -74,6 +74,38 @@ static int ceph_tcp_sendmsg(struct socket *sock, struct kvec *iov,
>         return r;
>  }
>
> +/*
> + * @more: either or both of MSG_MORE and MSG_SENDPAGE_NOTLAST

MSG_SENDPAGE_NOTLAST flag doesn't exist anymore.

> + */
> +static int ceph_tcp_sendpage(struct socket *sock, struct page *page,
> +                            int offset, size_t size, int more)
> +{
> +       struct msghdr msg = {
> +               .msg_flags = MSG_DONTWAIT | MSG_NOSIGNAL | more,
> +       };
> +       struct bio_vec bvec;
> +       int ret;
> +
> +       /*
> +        * sendpage cannot properly handle pages with page_count == 0,

I would suggest sendpage -> MSG_SPLICE_PAGES here like was done in
the other file.

> +        * 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))
> +               msg.msg_flags |= MSG_SPLICE_PAGES;
> +
> +       bvec_set_page(&bvec, page, size, offset);

The bvec is not added to the message, so I don't think this actually
sends anything...

> +
> +       ret = sock_sendmsg(sock, &msg);
> +       if (ret == -EAGAIN)
> +               ret = 0;
> +
> +       return ret;
> +}
> +
>  static void con_out_kvec_reset(struct ceph_connection *con)
>  {
>         BUG_ON(con->v1.out_skip);
> @@ -450,10 +482,6 @@ 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,
> -               };
>                 struct page *page;
>                 size_t page_offset;
>                 size_t length;
> @@ -465,13 +493,8 @@ static int write_partial_message_data(struct ceph_connection *con)
>                 }
>
>                 page = ceph_msg_data_next(cursor, &page_offset, &length);
> -               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);
> +               ret = ceph_tcp_sendpage(con->sock, page, page_offset, length,
> +                                       MSG_MORE);
>                 if (ret <= 0) {
>                         if (do_datacrc)
>                                 msg->footer.data_crc = cpu_to_le32(crc);
> @@ -501,22 +524,14 @@ static int write_partial_message_data(struct ceph_connection *con)
>   */
>  static int write_partial_skip(struct ceph_connection *con)
>  {
> -       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);
>         while (con->v1.out_skip > 0) {
>                 size_t size = min(con->v1.out_skip, (int)PAGE_SIZE);
>
> -               if (size == con->v1.out_skip)
> -                       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);
> +               ret = ceph_tcp_sendpage(con->sock, ceph_zero_page, 0, size,
> +                                       MSG_MORE);
>                 if (ret <= 0)
>                         goto out;
>                 con->v1.out_skip -= ret;
> diff --git a/net/ceph/messenger_v2.c b/net/ceph/messenger_v2.c
> index 87ac97073e75..139b61598f4f 100644
> --- a/net/ceph/messenger_v2.c
> +++ b/net/ceph/messenger_v2.c
> @@ -117,38 +117,89 @@ static int ceph_tcp_recv(struct ceph_connection *con)
>         return ret;
>  }
>
> +static int do_sendmsg(struct socket *sock, struct iov_iter *it)
> +{
> +       struct msghdr msg = { .msg_flags = CEPH_MSG_FLAGS };
> +       int ret;
> +
> +       msg.msg_iter = *it;
> +       while (iov_iter_count(it)) {
> +               ret = sock_sendmsg(sock, &msg);
> +               if (ret <= 0) {
> +                       if (ret == -EAGAIN)
> +                               ret = 0;
> +                       return ret;
> +               }
> +
> +               iov_iter_advance(it, ret);
> +       }
> +
> +       WARN_ON(msg_data_left(&msg));
> +       return 1;
> +}
> +
> +static int do_try_sendpage(struct socket *sock, struct iov_iter *it)
> +{
> +       struct msghdr msg = {};
> +       struct bio_vec bv;
> +       int ret;
> +
> +       if (WARN_ON(!iov_iter_is_bvec(it)))
> +               return -EINVAL;
> +
> +       while (iov_iter_count(it)) {
> +               msg.msg_flags = CEPH_MSG_FLAGS;
> +               if (sendpage_ok(bv.bv_page))
> +                       msg.msg_flags |= MSG_SPLICE_PAGES;
> +
> +               /* iov_iter_iovec() for ITER_BVEC */
> +               bvec_set_page(&bv, it->bvec->bv_page,
> +                             min(iov_iter_count(it),
> +                                 it->bvec->bv_len - it->iov_offset),
> +                             it->bvec->bv_offset + it->iov_offset);
> +
> +               /*
> +                * MSG_SPLICE_PAGES 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.
> +                */

This comment is now misplaced.  I would suggest

  if (sendpage_ok(bv.bv_page))
          msg.msg_flags |= MSG_SPLICE_PAGES;
  else
          msg.msg_flags &= ~MSG_SPLICE_PAGES;

here and keeping the initialization

  struct msghdr msg = { .msg_flags = CEPH_MSG_FLAGS };

above thus staying closer to the original.

Thanks,

                Ilya