On Mon, Feb 12, 2024 at 4:49 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
> On Mon, 2024-02-12 at 16:44 -0800, Alexei Starovoitov wrote:
> > > I hit a strange bug when playing with patch. Consider a simple example [0].
> > > When the following BPF global variable:
> > >
> > > int __arena * __arena bar;
> > >
> > > - is commented -- the test passes;
> > > - is uncommented -- in the test fails because global variable 'shared' is NULL.
> >
> > Right. That's expected, because __uint(max_entries, 1);
> > The test creates an area on 1 page and it's consumed
> > by int __arena * __arena bar; variable.
> > Of course, one variable doesn't take the whole page.
> > There could have been many arena global vars.
> > But that page is not available anymore to bpf_arena_alloc_pages,
> > so it returns NULL.
>
> My bad, thank you for explaining.
Since it was a surprising behavior we can make libbpf
to auto-extend max_entries with the number of pages necessary
for arena global vars, but it will be surprising too.
struct {
__uint(type, BPF_MAP_TYPE_ARENA);
__uint(map_flags, BPF_F_MMAPABLE);
__ulong(map_extra, 2ull << 44); // this is start of user VMA
__uint(max_entries, 1000); // this is length of user VMA in pages
} arena SEC(".maps");
if libbpf adds extra pages to max_entries the user_vm_end shifts too
and libbpf would need to mmap() it with that size.
When all is hidden in libbpf it's fine, but still can be a surprise
to see a different max_entries in map_info and bpftool map list.
Not sure which way is user friendlier.
