On Mon, Nov 6, 2023 at 10:44 AM Stanislav Fomichev sdf@google.com wrote:
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:
There may have been a miscommunication. I don't recall hearing this specific feedback from you, at least in the last few months. Sorry if it seemed like I'm ignoring feedback :)
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?)
I'll take a look at Jonathan's proposal, sorry, I'm not immediately familiar but I wanted to respond :-) But is the suggestion here to build a new kernel-user communication channel primitive for the purpose of passing the information in the devmem cmsg? IMHO that seems like an overkill. Why add 100-200 lines of code to the kernel to add something that can already be done with existing primitives? I don't see anything concretely wrong with cmsg & setsockopt approach, and if we switch to something I'd prefer to switch to an existing primitive for simplicity?
The only other existing primitive to pass data outside of the linear buffer is the MSG_ERRQUEUE that is used for zerocopy. Is that preferred? Any other suggestions or existing primitives I'm not aware of?
or bite the bullet and switch to io_uring.
IMO io_uring & socket support are orthogonal, and one doesn't preclude the other. As you know we like to use sockets and I believe there are issues with io_uring adoption at Google that I'm not familiar with (and could be wrong). I'm interested in exploring io_uring support as a follow up but I think David Wei will be interested in io_uring support as well anyway.
I was also suggesting to do it via netlink initially, but it's probably a bit slow for these purpose, idk.
Yeah, I hear netlink is reserved for control paths and is inappropriate for data path, but I'll let folks correct me if wrong.