Subject: Re: [RFC PATCH v3 10/12] tcp: RX path for devmem TCP

From: Stanislav Fomichev <[email-masked]>
Date: Mon, 6 Nov 2023 10:44:36 -0800

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 <[email-masked]>
> Signed-off-by: Kaiyuan Zhang <[email-masked]>
> Signed-off-by: Mina Almasry <[email-masked]>
> 
> ---
> 
> 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.