linux-kernel - Re: [RFC v2 6/6] android: binder: Add a buffer flag to relinquish ownership of fds

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Date:   Tue, 15 Feb 2022 08:30:43 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     "T.J. Mercier" <tjmercier@...gle.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Corbet <corbet@....net>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <brauner@...nel.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Laura Abbott <labbott@...hat.com>,
        Brian Starkey <Brian.Starkey@....com>,
        John Stultz <john.stultz@...aro.org>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kalesh Singh <kaleshsingh@...gle.com>, Kenny.Ho@....com,
        DRI mailing list <dri-devel@...ts.freedesktop.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-media <linux-media@...r.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        cgroups mailinglist <cgroups@...r.kernel.org>
Subject: Re: [RFC v2 6/6] android: binder: Add a buffer flag to relinquish
 ownership of fds

On Mon, Feb 14, 2022 at 11:19:35PM -0800, Suren Baghdasaryan wrote:
> On Mon, Feb 14, 2022 at 11:01 PM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> >
> > On Mon, Feb 14, 2022 at 02:25:47PM -0800, T.J. Mercier wrote:
> > > On Fri, Feb 11, 2022 at 11:19 PM Greg Kroah-Hartman
> > > > > --- a/include/uapi/linux/android/binder.h
> > > > > +++ b/include/uapi/linux/android/binder.h
> > > > > @@ -137,6 +137,7 @@ struct binder_buffer_object {
> > > > >
> > > > >  enum {
> > > > >       BINDER_BUFFER_FLAG_HAS_PARENT = 0x01,
> > > > > +     BINDER_BUFFER_FLAG_SENDER_NO_NEED = 0x02,
> > > > >  };
> > > > >
> > > > >  /* struct binder_fd_array_object - object describing an array of fds in a buffer
> > > > > --
> > > > > 2.35.1.265.g69c8d7142f-goog
> > > > >
> > > >
> > > > How does userspace know that binder supports this new flag?
> > >
> > > Sorry, I don't completely follow even after Todd's comment. Doesn't
> > > the presence of BINDER_BUFFER_FLAG_SENDER_NO_NEED in the header do
> > > this?
> >
> > There is no "header" when running a new kernel on an old userspace,
> > right?  How about the other way around, old kernel, new userspace?
> 
> 1. new kernel + old userspace = kernel supports the feature but
> userspace does not use it. The old userspace won't even mount the new
> cgroup controller, accounting is not performed, charge is not
> transferred.
> 2. old kernel + new userspace = the new cgroup controller is not
> supported by the kernel, accounting is not performed, charge is not
> transferred.
> 3. old kernel + old userspace = same as #2
> 4. new kernel + new userspace = cgroup is mounted, feature is
> supported and used.
> Does that work or do we need a separate indication of whether binder
> driver supports the charge transfer feature?

Ok, if that's all working, this is fine, it just seemed odd to add a new
type like this.  Perhaps this can go into the changelog text...

thanks,

greg k-h