On 11/05, Mina Almasry wrote:
> In tcp_recvmsg_locked(), detect if the skb being received by the user
> is a devmem skb. In this case - if the user provided the MSG_SOCK_DEVMEM
> flag - pass it to tcp_recvmsg_devmem() for custom handling.
>
> tcp_recvmsg_devmem() copies any data in the skb header to the linear
> buffer, and returns a cmsg to the user indicating the number of bytes
> returned in the linear buffer.
>
> tcp_recvmsg_devmem() then loops over the unaccessible devmem skb frags,
> and returns to the user a cmsg_devmem indicating the location of the
> data in the dmabuf device memory. cmsg_devmem contains this information:
>
> 1. the offset into the dmabuf where the payload starts. 'frag_offset'.
> 2. the size of the frag. 'frag_size'.
> 3. an opaque token 'frag_token' to return to the kernel when the buffer
> is to be released.
>
> The pages awaiting freeing are stored in the newly added
> sk->sk_user_pages, and each page passed to userspace is get_page()'d.
> This reference is dropped once the userspace indicates that it is
> done reading this page. All pages are released when the socket is
> destroyed.
>
> Signed-off-by: Willem de Bruijn <willemb@xxxxxxxxxx>
> Signed-off-by: Kaiyuan Zhang <kaiyuanz@xxxxxxxxxx>
> Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
>
> ---
>
> RFC v3:
> - Fixed issue with put_cmsg() failing silently.
>
> ---
> include/linux/socket.h | 1 +
> include/net/page_pool/helpers.h | 9 ++
> include/net/sock.h | 2 +
> include/uapi/asm-generic/socket.h | 5 +
> include/uapi/linux/uio.h | 6 +
> net/ipv4/tcp.c | 189 +++++++++++++++++++++++++++++-
> net/ipv4/tcp_ipv4.c | 7 ++
> 7 files changed, 214 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index cfcb7e2c3813..fe2b9e2081bb 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -326,6 +326,7 @@ struct ucred {
> * plain text and require encryption
> */
>
> +#define MSG_SOCK_DEVMEM 0x2000000 /* Receive devmem skbs as cmsg */
Sharing the feedback that I've been providing internally on the public list:
IMHO, we need a better UAPI to receive the tokens and give them back to
the kernel. CMSG + setsockopt(SO_DEVMEM_DONTNEED) get the job done,
but look dated and hacky :-(
We should either do some kind of user/kernel shared memory queue to
receive/return the tokens (similar to what Jonathan was doing in his
proposal?) or bite the bullet and switch to io_uring.
I was also suggesting to do it via netlink initially, but it's probably
a bit slow for these purpose, idk.
