On 6/25/24 03:47, 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_frags, 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.
Reviewed-by: Pavel Begunkov asml.silence@gmail.com