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From:   Hridya Valsaraju <hridya@google.com>
Date:   Mon, 14 Feb 2022 13:14:51 -0800
Message-ID: <CA+wgaPM8AmQ4g6pDSwRABtc7hq11XMw+kH-rRkB8xNGJhM3OKA@mail.gmail.com>
Subject: Re: [RFC v2 6/6] android: binder: Add a buffer flag to relinquish
 ownership of fds
To:     John Stultz <john.stultz@linaro.org>
Cc:     Todd Kjos <tkjos@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "T.J. Mercier" <tjmercier@google.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Jonathan Corbet <corbet@lwn.net>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Christian Brauner <brauner@kernel.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
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On Mon, Feb 14, 2022 at 12:37 PM John Stultz <john.stultz@linaro.org> wrote:
>
> On Mon, Feb 14, 2022 at 12:19 PM Todd Kjos <tkjos@google.com> wrote:
> > On Mon, Feb 14, 2022 at 11:29 AM Suren Baghdasaryan <surenb@google.com> wrote:
> > > On Mon, Feb 14, 2022 at 10:33 AM Todd Kjos <tkjos@google.com> wrote:
> > > >
> > > > Since we are creating a new gpu cgroup abstraction, couldn't this
> > > > "transfer" be done in userspace by the target instead of in the kernel
> > > > driver? Then this patch would reduce to just a flag on the buffer
> > > > object.
> > >
> > > Are you suggesting to have a userspace accessible cgroup interface for
> > > transferring buffer charges and the target process to use that
> > > interface for requesting the buffer to be charged to its cgroup?
> >
> > Well, I'm asking why we need to do these cgroup-ish actions in the
> > kernel when it seems more natural to do it in userspace.

This was our plan originally i.e. to create a cgroup interface that
userspace could use for explicit charge transfer. However, in our
initial discussions with all interested parties and cgroup maintainers
we reached a conclusion that an explicit charge transfer UAPI for the
cgroup controller did not fit in with the current cgroup
charge/uncharge mechanisms. Like John mentioned, the charge transfer
during binder IPC was suggested by Daniel during LPC.

Regards,
Hridya

> >
>
> In case its useful, some additional context from some of the Linux
> Plumber's discussions last fall:
>
> Daniel Stone outlines some concerns with the cgroup userland handling
> for accounting:
>   https://youtu.be/3OqllZONTiQ?t=3430
>
> And the binder ownership transfer bit was suggested here by Daniel Vetter:
>   https://youtu.be/3OqllZONTiQ?t=3730
>
> thanks
> -john