linux-kernel - Re: [PATCH 2/4] bpf: Add dmabuf iterator

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Message-ID: <CABdmKX0P1tpa-jxzN1_TCyk6Cw6drYM+KRZQ5YQcjNOBFtOFJw@mail.gmail.com>
Date: Thu, 17 Apr 2025 09:04:48 -0700
From: "T.J. Mercier" <tjmercier@...gle.com>
To: Song Liu <song@...nel.org>
Cc: sumit.semwal@...aro.org, christian.koenig@....com, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	skhan@...uxfoundation.org, linux-kernel@...r.kernel.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, linux-doc@...r.kernel.org, 
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, android-mm@...gle.com, 
	simona@...ll.ch, corbet@....net, eddyz87@...il.com, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me, 
	jolsa@...nel.org, mykolal@...com
Subject: Re: [PATCH 2/4] bpf: Add dmabuf iterator

On Wed, Apr 16, 2025 at 9:56 PM Song Liu <song@...nel.org> wrote:
>
> On Wed, Apr 16, 2025 at 7:09 PM T.J. Mercier <tjmercier@...gle.com> wrote:
> >
> > On Wed, Apr 16, 2025 at 6:26 PM Song Liu <song@...nel.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.

> 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.