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

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Message-ID: <20260121173512.748e2155@kernel.org>
Date: Wed, 21 Jan 2026 17:35:12 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Bobby Eshleman <bobbyeshleman@...il.com>
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, 20 Jan 2026 21:44:09 -0800 Bobby Eshleman wrote:
> 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.

My question was more why the system-wide policy exists, rather than
binding-by-binding. Naively I'd think that a single socket must pick
but system wide there could easily be multiple bindings not bothering
each other, doing different things?

> > > +- 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)?

TBH I read the doc before reading the code, which I guess may actually
be better since we don't expect users to read the code first either..

Now after reading the code I'm not sure the doc explains things
properly. AFAIU there's no association of token <> socket within the
same binding. User can close socket A and return the tokens via socket
B. As written the doc made me think that there will be a leak if socket
is closed without releasing tokens, or that there may be a race with
data queued but not read. Neither is true, really?