netdev - Re: [net-next v1 13/16] tcp: RX path for devmem TCP

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Message-ID: <dd47a2a4-cb80-4164-8855-045999931a8e@kernel.org>
Date: Fri, 8 Dec 2023 10:55:04 -0700
From: David Ahern <dsahern@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>,
 Shailend Chand <shailend@...gle.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
 bpf@...r.kernel.org, linux-media@...r.kernel.org,
 dri-devel@...ts.freedesktop.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Jonathan Corbet <corbet@....net>,
 Jeroen de Borst <jeroendb@...gle.com>,
 Praveen Kaligineedi <pkaligineedi@...gle.com>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>, Arnd Bergmann
 <arnd@...db.de>, Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Shuah Khan <shuah@...nel.org>, Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 Yunsheng Lin <linyunsheng@...wei.com>,
 Harshitha Ramamurthy <hramamurthy@...gle.com>,
 Shakeel Butt <shakeelb@...gle.com>, Willem de Bruijn <willemb@...gle.com>,
 Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: Re: [net-next v1 13/16] tcp: RX path for devmem TCP

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:
> 
> 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 <willemb@...gle.com>
> Signed-off-by: Kaiyuan Zhang <kaiyuanz@...gle.com>
> Signed-off-by: Mina Almasry <almasrymina@...gle.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?