netdev - Re: [PATCH bpf-next] selftests/bpf: use getpagesize() to initialize ring buffer size

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Date:   Tue,  1 Feb 2022 16:43:13 +0800
From:   Hou Tao <hotforest@...il.com>
To:     andrii.nakryiko@...il.com
Cc:     andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
        daniel@...earbox.net, davem@...emloft.net, houtao1@...wei.com,
        kafai@...com, kuba@...nel.org, netdev@...r.kernel.org, yhs@...com
Subject: Re: [PATCH bpf-next] selftests/bpf: use getpagesize() to initialize ring buffer size

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 ?

Regards,
Tao