On Fri, Dec 8, 2023 at 9:55 AM David Ahern dsahern@kernel.org wrote:
On 12/7/23 5:52 PM, 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
Changes in v1:
- Added dmabuf_id to dmabuf_cmsg (David/Stan).
- Devmem -> dmabuf (David).
- Change tcp_recvmsg_dmabuf() check to skb->dmabuf (Paolo).
- Use __skb_frag_ref() & napi_pp_put_page() for refcounting (Yunsheng).
RFC v3:
- Fixed issue with put_cmsg() failing silently.
What happens if a retransmitted packet is received or an rx window is closed and a probe is received where the kernel drops the skb - is the iov reference(s) in the skb returned to the pool by the stack and ready for use again?
When an skb is dropped, skb_frag_unref() is called on the frags, which calls napi_pp_put_page(), drops the references, and the iov is recycled, yes.