Re: Regression due to /sys/kernel/dmabuf/buffers removal

[T.J. Mercier]

On Tue, May 5, 2026 at 6:00 AM Julian Orth <ju.orth@xxxxxxxxx> wrote:
>
> On Tue, May 5, 2026 at 2:41 PM Christian König <christian.koenig@xxxxxxx> wrote:
> >
> > Hi Julian,
> >
> > On 5/5/26 14:25, Julian Orth wrote:
> > > In ab4c3dcf9a71582503b4fb25aeab884c696cab25 ("dma-buf: Remove DMA-BUF
> > > sysfs stats") the /sys/kernel/dmabuf/buffer directory was removed.
> > >
> > > I've been using this interface, specifically the exporter_name file,
> > > to detect dmabufs created via udmabuf. Such dmabufs show "udmabuf" in
> > > exporter_name. I've been doing this for two reasons: 1) to detect that
> > > mmap on such buffers will be fast and 2) to detect that GPU access to
> > > such buffers will be slow.
> >
> > Crap, I really hoped that Android was the only user of that sysfs interface since that approach turned out to be quite broken.
> >
> > It's number one rule on Linux that we don't break userspace. So I hope that you don't insist on bringing that interface back, but if you do I will just revert the removal until we found a better solution.
>
> Bringing it back shouldn't be necessary.
>
> >
> > > With the removal of that file, that detection mechanism no longer works.
> > >
> > > I'm not particularly fond of that mechanism but it was the only one
> > > providing that functionality that I could find at the time. If there
> > > is another one, ideally an ioctl on the dmabuf, please let me know.
> >
> > The virtual fdinfo file you can find under /proc/$pid/fdinfo/$fd also contains the exporter name for the DMA-buf.
> >
> > You can find the full documentation here: https://docs.kernel.org/filesystems/proc.html#dma-buffer-files
> >
> > Is that sufficient?
>
> I think that is sufficient. I probably didn't use fdinfo initially
> because 1) it's a lot more work to parse and 2) I wasn't sure if it
> was intended to be machine-readable or if there could sometimes be
> newlines in the values and such.
>
> >
> > Additional to that the debugfs for DMA-buf also contains that information and I'm open to the suggestion with the IOCTL.
>
> My application runs as a regular user so it cannot access /sys/kernel/debug.
>
> Having an IOCTL would be ideal if it is not too much work. I'll fall
> back to fdinfo for now.
>
> Thanks, Julian

Phew, I'm glad fdinfo suits your needs.

Adding an ioctl would introduce new UAPI so I think we'd want to avoid
that unless absolutely necessary.

Thanks,
T.J.

> >
> > Regards,
> > Christian.
> >
> > >
> > > Shipping an entire BPF compiler in my application, which the original
> > > patch suggests as the replacement, is not an option when the removed
> > > alternative was simply reading a file.
> > >
> > > Thanks, Julian
> >