On Tue, Feb 1, 2022 at 12:43 AM Hou Tao <hotforest@xxxxxxxxx> wrote:
>
> Hi Andrii,
>
> > >
> > > 4096 is OK for x86-64, but for other archs with greater than 4KB
> > > page size (e.g. 64KB under arm64), test_verifier for test case
> > > "check valid spill/fill, ptr to mem" will fail, so just use
> > > getpagesize() to initialize the ring buffer size. Do this for
> > > test_progs as well.
> > >
> [...]
>
> > > diff --git a/tools/testing/selftests/bpf/progs/ima.c b/tools/testing/selftests/bpf/progs/ima.c
> > > index 96060ff4ffc6..e192a9f16aea 100644
> > > --- a/tools/testing/selftests/bpf/progs/ima.c
> > > +++ b/tools/testing/selftests/bpf/progs/ima.c
> > > @@ -13,7 +13,6 @@ u32 monitored_pid = 0;
> > >
> > > struct {
> > > __uint(type, BPF_MAP_TYPE_RINGBUF);
> > > - __uint(max_entries, 1 << 12);
> >
> > Should we just bump it to 64/128/256KB instead? It's quite annoying to
> > do a split open and then load just due to this...
> >
> Agreed.
>
> > I'm also wondering if we should either teach kernel to round up to
> > closes power-of-2 of page_size internally, or teach libbpf to do this
> > for RINGBUF maps. Thoughts?
> >
> It seems that max_entries doesn't need to be page-aligned. For example
> if max_entries is 4096 and page size is 65536, we can allocate a
> 65536-sized page and set rb->mask 4095 and it will work. The only
> downside is 60KB memory is waster, but it is the implementation
> details and can be improved if subpage mapping can be supported.
>
> So how about removing the page-aligned restraint in kernel ?
>
No, if you read BPF ringbuf code carefully you'll see that we map the
entire ringbuf data twice in the memory (see [0] for lame ASCII
diagram), so that records that are wrapped at the end of the ringbuf
and go back to the start are still accessible as a linear array. It's
a very important guarantee, so it has to be page size multiple. But
auto-increasing it to the closest power-of-2 of page size seems like a
pretty low-impact change. Hard to imagine breaking anything except
some carefully crafted tests for ENOSPC behavior.
[0] https://github.com/torvalds/linux/blob/master/kernel/bpf/ringbuf.c#L73-L89
> Regards,
> Tao
