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

[Suren Baghdasaryan]

On Mon, Feb 14, 2022 at 11:01 PM Greg Kroah-Hartman
<gregkh at linuxfoundation.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?

>
> > So wouldn't userspace need to be compiled against the wrong
> > kernel headers for there to be a problem? In that case the allocation
> > would still succeed, but there would be no charge transfer and
> > unfortunately no error code.
>
> No error code is not good.  People upgrade their kernels all the time,
> and do not do a "rebuild the world" when doing so.
>
> > > And where is the userspace test for this new feature?
> >
> > I tested this on a Pixel after modifying the gralloc implementation to
> > mark allocated buffers as not used by the sender. This required
> > setting the BINDER_BUFFER_FLAG_SENDER_NO_NEED in libhwbinder. That
> > code can be found here:
> > https://android-review.googlesource.com/c/platform/system/libhwbinder/+/1910752/1/Parcel.cpp
> > https://android-review.googlesource.com/c/platform/system/libhidl/+/1910611/
> >
> > Then by inspecting gpu.memory.current files in sysfs I was able to see
> > the memory attributed to processes other than the graphics allocator
> > service. Before this change, several megabytes of memory were
> > attributed to the graphics allocator service but those buffers are
> > actually used by other processes like surfaceflinger, the camera, etc.
> > After the change, the gpu.memory.current amount for the graphics
> > allocator service was 0 and the charges showed up in the
> > gpu.memory.current files for those other processes like this:
> >
> > PID: 764 Process Name: zygote64
> > system 8192
> > system-uncached 23191552
> >
> > PID: 529 Process Name: /system/bin/surfaceflinger
> > system-uncached 109535232
> > system 92196864
> >
> > PID: 530 Process Name:
> > /vendor/bin/hw/android.hardware.graphics.allocator at 4.0-service
> > system-uncached 0
> > system 0
> > sensor_direct_heap 0
> >
> > PID: 806 Process Name:
> > /apex/com.google.pixel.camera.hal/bin/hw/android.hardware.camera.provider at 2.7-service-google
> > system 1196032
> >
> > PID: 4608 Process Name: com.google.android.GoogleCamera
> > system 2408448
> > system-uncached 38887424
> > sensor_direct_heap 0
> >
> > PID: 32102 Process Name: com.google.android.googlequicksearchbox:search
> > system-uncached 91279360
> > system 20480
> >
> > PID: 2758 Process Name: com.google.android.youtube
> > system-uncached 1662976
> > system 8192
> >
> > PID: 2517 Process Name: com.google.android.apps.nexuslauncher
> > system-uncached 115662848
> > system 122880
> >
> > PID: 2066 Process Name: com.android.systemui
> > system 86016
> > system-uncached 37957632
> >
> > >  Isn't there a binder test framework somewhere?
> >
> > Android has the Vendor Test Suite where automated tests could be added
> > for this. Is that what you're thinking of?
>
> tools/testing/selftests/ is a good start.  VTS is the worst-case as no
> one can really run that on their own, but it is better than nothing.
> Having no test at all for this is not ok.
>
> thanks,
>
> greg k-h