linux-kernel - Re: [PATCH net-next v6 5/6] net: devmem: document SO_DEVMEM

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Message-ID: <aQva8v22RVQEgPi_@mini-arch>
Date: Wed, 5 Nov 2025 15:17:06 -0800
From: Stanislav Fomichev <stfomichev@...il.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Bobby Eshleman <bobbyeshleman@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, 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>, Arnd Bergmann <arnd@...db.de>,
	Jonathan Corbet <corbet@....net>,
	Andrew Lunn <andrew+netdev@...n.ch>, Shuah Khan <shuah@...nel.org>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Stanislav Fomichev <sdf@...ichev.me>,
	Bobby Eshleman <bobbyeshleman@...a.com>
Subject: Re: [PATCH net-next v6 5/6] net: devmem: document
 SO_DEVMEM_AUTORELEASE socket option

On 11/05, Stanislav Fomichev wrote:
> On 11/05, Mina Almasry wrote:
> > On Wed, Nov 5, 2025 at 9:34 AM Stanislav Fomichev <stfomichev@...il.com> wrote:
> > >
> > > On 11/04, Bobby Eshleman wrote:
> > > > From: Bobby Eshleman <bobbyeshleman@...a.com>
> > > >
> > >
> > > [..]
> > >
> > > > +Autorelease Control
> > > > +~~~~~~~~~~~~~~~~~~~
> > >
> > > Have you considered an option to have this flag on the dmabuf binding
> > > itself? This will let us keep everything in ynl and not add a new socket
> > > option. I think also semantically, this is a property of the binding
> > > and not the socket? (not sure what's gonna happen if we have
> > > autorelease=on and autorelease=off sockets receiving to the same
> > > dmabuf)
> > 
> > I think this thread (and maybe other comments on that patch) is the
> > context that missed your inbox:
> > 
> > https://lore.kernel.org/netdev/aQIoxVO3oICd8U8Q@devvm11784.nha0.facebook.com/
> > 
> > Let us know if you disagree.
> 
> Thanks, I did miss that whole v5 because I was OOO, let me take a look!

Thank you for the context!

I think that the current approach is ok, we can go with that, but I
wonder whether we can simplify things a bit? What if we prohibit the
co-existence of autorelease=on and autorelease=off sockets on the
system? The first binding basically locks the kernel path into one way or
the other (presumably by using static-branch) and prohibits new bindings
that use a different mode. It will let us still keep the mode on the binding
and will help us not think about the co-existance (we can also still keep
things like one-dmabuf-per-socket restrictions in the new mode, etc).

I think for you, Mina, this should still work? You have a knob to go back
to the old mode if needed. At the same time, we keep the UAPI surface
smaller and keep the path more simple. Ideally, we can also deprecate
the old mode at some point (if you manage to successfully migrate of
course). WDYT?