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

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Date:   Mon, 14 Feb 2022 13:14:51 -0800
From:   Hridya Valsaraju <hridya@...gle.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     Todd Kjos <tkjos@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "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>,
        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>,
        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 12:37 PM John Stultz <john.stultz@...aro.org> wrote:
>
> On Mon, Feb 14, 2022 at 12:19 PM Todd Kjos <tkjos@...gle.com> wrote:
> > On Mon, Feb 14, 2022 at 11:29 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> > > On Mon, Feb 14, 2022 at 10:33 AM Todd Kjos <tkjos@...gle.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