[PATCH 2/4] bpf: Add dmabuf iterator

[Song Liu]

On Thu, Apr 17, 2025 at 9:05 AM T.J. Mercier <tjmercier at google.com> wrote:
>
> On Wed, Apr 16, 2025 at 9:56 PM Song Liu <song at kernel.org> wrote:
> >
> > On Wed, Apr 16, 2025 at 7:09 PM T.J. Mercier <tjmercier at google.com> wrote:
> > >
> > > On Wed, Apr 16, 2025 at 6:26 PM Song Liu <song at kernel.org> wrote:
> > [...]
> > > >
> > > > Here is another rookie question, it appears to me there is a file descriptor
> > > > associated with each DMA buffer, can we achieve the same goal with
> > > > a task-file iterator?
> > >
> > > That would find almost all of them, but not the kernel-only
> > > allocations. (kernel_rss in the dmabuf_dump output I attached earlier.
> > > If there's a leak, it's likely to show up in kernel_rss because some
> > > driver forgot to release its reference(s).) Also wouldn't that be a
> > > ton more iterations since we'd have to visit every FD to find the
> > > small portion that are dmabufs? I'm not actually sure if buffers that
> > > have been mapped, and then have had their file descriptors closed
> > > would show up in task_struct->files; if not I think that would mean
> > > scanning both files and vmas for each task.
> >
> > I don't think scanning all FDs to find a small portion of specific FDs
> > is a real issue. We have a tool that scans all FDs in the system and
> > only dump data for perf_event FDs. I think it should be easy to
> > prototype a tool by scanning all files and all vmas. If that turns out
> > to be very slow, which I highly doubt will be, we can try other
> > approaches.
>
> But this will not find *all* the buffers, and that defeats the purpose
> of having the iterator.

Do you mean this approach cannot get kernel only allocations? If
that's the case, we probably do need a separate iterator. I am
interested in other folks thoughts on this.

> > OTOH, I am wondering whether we can build a more generic iterator
> > for a list of objects. Adding a iterator for each important kernel lists
> > seems not scalable in the long term.
>
> I think the wide variety of differences in locking for different
> objects would make this difficult to do in a generic way.

Agreed it is not easy to build a generic solution. But with the
help from BTF, we can probably build something that covers
a large number of use cases.

Thanks,
Song