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:
- the offset into the dmabuf where the payload starts. 'frag_offset'.
- the size of the frag. 'frag_size'.
- 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@google.com Signed-off-by: Kaiyuan Zhang kaiyuanz@google.com Signed-off-by: Mina Almasry almasrymina@google.com
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.