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

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Message-ID: <aXg2AQMMk3obScYD@devvm11784.nha0.facebook.com>
Date: Mon, 26 Jan 2026 19:50:25 -0800
From: Bobby Eshleman <bobbyeshleman@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Stanislav Fomichev <stfomichev@...il.com>,
	"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 Mon, Jan 26, 2026 at 07:43:59PM -0800, Jakub Kicinski wrote:
> On Mon, 26 Jan 2026 19:06:49 -0800 Bobby Eshleman wrote:
> > > > Then, if the dmabuf count encoding space is exhausted, the socket would
> > > > have to wait until the user returns all of the tokens from one of the
> > > > dmabufs and frees the ID (or err out is another option).
> > > > 
> > > > This wouldn't change adding a field to the socket, we'd have to add one
> > > > or two more for allocating the dmabuf ID and fetching the dmabuf with
> > > > it. But it does fix the single binding thing.  
> > > 
> > > I think the bigger problem (than space exhaustion) is that we'd also
> > > have some understanding of permissions. If an application guesses 
> > > the binding ID of another app it can mess up its buffers. ENOBUENO..  
> > 
> > I was thinking it would be per-socket, effectively:
> > 
> > sk->sk_devmem_info.bindings[binding_id_from_token(token)]
> > 
> > So sockets could only access those that they have already recv'd on.
> 
> Ah, missed that the array would be per socket. I guess it'd have to be
> reusing the token xarray otherwise we're taking up even more space in
> the socket struct? Dunno.

Yeah, unless we just want to break this all off into a malloc'd struct
we point to... or put into tcp_sock (not sure if either addresses the
unappealing bit of adding to struct sock)?