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

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Message-ID: <aXgjVb5PVqdPM10J@devvm11784.nha0.facebook.com>
Date: Mon, 26 Jan 2026 18:30:45 -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 05:26:46PM -0800, Jakub Kicinski wrote:
> On Wed, 21 Jan 2026 20:07:11 -0800 Stanislav Fomichev wrote:
> > On 01/21, Jakub Kicinski wrote:
> > > On Wed, 21 Jan 2026 19:25:27 -0800 Bobby Eshleman wrote:  
> > > > Good point. The only real use case for autorelease=on is for backwards
> > > > compatibility... so I thought maybe DEVMEM_A_DMABUF_COMPAT_TOKEN
> > > > or DEVMEM_A_DMABUF_COMPAT_DONTNEED would be clearer?  
> > > 
> > > Hm. Maybe let's return to naming once we have consensus on the uAPI.
> > > 
> > > Does everyone think that pushing this via TCP socket opts still makes
> > > sense, even tho in practice the TCP socket is just how we find the
> > > binding?  
> > 
> > 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.

>  - the 7 levels of indentation in tcp_recvmsg_dmabuf()

For sure, it is getting hairy.

> 
> I understand your argument, but as is this series feels closer to a PoC
> than an easy win (the easy part should imply minor changes and no
> detrimental effect on code quality IMHO).

Sure, let's try to find a way to minimize the changes.

Best,
Bobby