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

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Message-ID: <aXgryaSx9CzFR7kv@devvm11784.nha0.facebook.com>
Date: Mon, 26 Jan 2026 19:06:49 -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 06:44:40PM -0800, Jakub Kicinski wrote:
> On Mon, 26 Jan 2026 18:30:45 -0800 Bobby Eshleman wrote:
> > > > I'm not a fan of the existing cmsg scheme, but we already have userspace
> > > > using it, so getting more performance out of it seems like an easy win?  
> > > 
> > > I don't like:
> > >  - the fact that we have to add the binding to a socket (extra field)
> > >    - single socket can only serve single binding, there's no technical
> > >      reason for this really, AFAICT, just the fact that we have a single
> > >      pointer in the sock struct  
> > 
> > The core reason is that sockets lose the ability to map a given token to
> > a given binding by no longer storing the niov ptr.
> > 
> > One proposal I had was to encode some number of bits in the token that
> > can be used to lookup the binding in an array, I could reboot that
> > approach.
> > 
> > With 32 bits, we can represent:
> > 
> > dmabuf max size = 512 GB, max dmabuf count = 8
> > dmabuf max size = 256 GB, max dmabuf count = 16
> > dmabuf max size = 128 GB, max dmabuf count = 32
> > 
> > etc...
> > 
> > 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.