Re: [PATCH v2 bpf-next 14/20] libbpf: Recognize __arena global varaibles.

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-02-12 at 18:08 -0800, Alexei Starovoitov wrote:
[...]

> 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.

Adjusting max_entries would be surprising indeed.
On the other hand, it would remove the error condition about
"Declared arena map size %zd is too small ...".
Probably either way is fine, as long as it is documented.
Don't have a strong opinion.