linux-kernel - Re: [PATCH net-next v10 4/5] net: devmem: document NETDEV_A_DMABUF

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Message-ID: <aXBnqYQdomzH9bT/@devvm11784.nha0.facebook.com>
Date: Tue, 20 Jan 2026 21:44:09 -0800
From: Bobby Eshleman <bobbyeshleman@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Kuniyuki Iwashima <kuniyu@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	David Ahern <dsahern@...nel.org>,
	Mina Almasry <almasrymina@...gle.com>,
	Arnd Bergmann <arnd@...db.de>, Jonathan Corbet <corbet@....net>,
	Andrew Lunn <andrew+netdev@...n.ch>, Shuah Khan <shuah@...nel.org>,
	Donald Hunter <donald.hunter@...il.com>,
	Stanislav Fomichev <sdf@...ichev.me>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
	asml.silence@...il.com, matttbe@...nel.org, skhawaja@...gle.com,
	Bobby Eshleman <bobbyeshleman@...a.com>
Subject: Re: [PATCH net-next v10 4/5] net: devmem: document
 NETDEV_A_DMABUF_AUTORELEASE netlink attribute

On Tue, Jan 20, 2026 at 04:36:50PM -0800, Jakub Kicinski wrote:
> On Thu, 15 Jan 2026 21:02:15 -0800 Bobby Eshleman wrote:
> > +- Once a system-wide autorelease mode is selected (via the first binding),
> > +  all subsequent bindings must use the same mode. Attempts to create bindings
> > +  with a different mode will be rejected with -EBUSY.
> 
> Why?
> 

Originally I was using EINVAL, but when writing the tests I noticed this
might be a confusing case for users to interpret EINVAL (i.e., some
binding possibly made by someone else is in a different mode). I thought
EBUSY could capture the semantic "the system is locked up in a different
mode, try again when it isn't".

I'm not married to it though. Happy to go back to EINVAL or another
errno.

> > +- Applications using manual release mode (autorelease=0) must ensure all tokens
> > +  are returned via SO_DEVMEM_DONTNEED before socket close to avoid resource
> > +  leaks during the lifetime of the dmabuf binding. Tokens not released before
> > +  close() will only be freed when all RX queues are unbound AND all sockets
> > +  that called recvmsg() are closed.
> 
> Could you add a short example on how? by calling shutdown()?

Show an example of the three steps: returning the tokens, unbinding, and closing the
sockets (TCP/NL)?

Best,
Bobby