Re: [RFC] bpf: account ring buffer backing pages separately from Lost RAM

On Mon, Aug 17, 2026 at 11:10 AM David Hildenbrand (Arm)
<[email protected]> wrote:
>
> On 8/15/26 11:18, Xiang Gao wrote:
> > Hi,
>
> Hi,
>
> >
> > I would like to discuss accounting BPF ring buffer backing pages in
> > system-wide memory reports.
> >
> > BPF ring buffers allocate their data and metadata as order-0 pages directly
> > from the buddy allocator, and then map those pages with vmap().
>
> I assume there is a reason the slab isn't used, right? Are these pages mapped
> into user space such that page->mapcount would get used?
>
> Can you point me at relevant code?

See code in [0]. And yes, these pages are meant to be mapped into user space.

  [0] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/bpf/ringbuf.c#n93

>
> >
> > Because vmap() maps caller-owned pages, these backing pages are not counted
> > by VmallocUsed. They are also not slab pages. As a result, most BPF ring
> > buffer memory is not represented by an existing named /proc/meminfo category
> > and appears as Lost RAM in Android memory reports.
> >
> > We measured this on an Android 6.18 kernel.
> >
> > Test case:
> >
> >   32 BPF ring buffer maps
> >   16 MiB data area per map
> >   512 MiB total data area
> >
> > Observed changes:
> >
> >   Lost RAM:       approximately +529 MiB
> >   VmallocUsed:      approximately +2 MiB
> >   Slab:        approximately unchanged
> >
> > After destroying all maps, the values returned close to baseline.
> >
> > The question is whether the kernel should expose the unique physical backing
> > pages of live BPF ring buffers through a dedicated global counter and a
> > /proc/meminfo entry, for example:
> >
> >   BpfRingbuf: <value in kB>
>
> This looks a bit too specific for my taste. And I think we should try to no
> inflate these statistics here too much.
>

+1, way too specific

> >
> > The proposed counter would include:
> >
> >   * ring buffer data pages;
> >   * metadata pages;
> >   * consumer and producer position pages.
> >
> > It would exclude:
> >
> >   * the second virtual mapping of data pages;
> >   * the pages[] pointer array;
> >   * map metadata allocations;
> >   * vmap page tables.
> >
> > The goal is to account for the currently unclassified direct backing pages.
> > Slab- and vmalloc-backed auxiliary allocations are already represented by
> > existing memory categories and should not be counted again.
> >
> > A possible implementation is an NR_BPF_RINGBUF vmstat counter maintained by
> > the ring buffer allocation and free paths, with the aggregate exposed through
> > /proc/meminfo.
> >
> > Questions:
> >
> > 1. Is a dedicated BPF ring buffer counter appropriate?
>
> I don't think so.
>
> See [1] where we just had the same discussion for tracing buffers. For them,
> Steve [2] had an idea on how to expose them more fine-grained and tracing specific.
>
> [1] https://lore.kernel.org/r/[email protected]
> [2] https://lore.kernel.org/r/[email protected]
>

We already report per-BPF ringbuf memory usage either through bpf()
syscall or map's fdinfo. E.g., with `sudo bpftool map show` you'll
see"

1455733: ringbuf  name event_ringbuf  flags 0x0
        key 0B  value 0B  max_entries 262144  memlock 275776B
        btf_id 2193434
        pids tcpeventd(2549812)

where memlock is how much memory is allocated for the ringbuf data area.

> > 2. Should this be represented as an NR_* vmstat counter?
>
> I don't think so.
>
> > 3. Is /proc/meminfo an acceptable interface for this information?
>
> Again, I don't think so. "Lost RAM" really is just "excessive memory allocated
> by some other subsystem".
>
> I agree that some users might want to figure out what is consuming that much
> memory, but I don't think growing /proc/meminfo in that way is really what we want.
>
> > 4. Is counting only unique physical backing pages the correct accounting unit?
>
> I'd assume the "It would exclude" part above should not be accounted there, if
> that's what you mean.
>
> --
> Cheers,
>
> David