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

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2024-02-13 at 17:02 -0800, Alexei Starovoitov wrote:
[...]
> > First is regarding initialization data.
> > In bpf_object__init_internal_map() the amount of bpf_map_mmap_sz(map)
> > bytes is mmaped and only data_sz bytes are copied,
> > then bpf_map_mmap_sz(map) bytes are copied in bpf_object__create_maps().
> > Is Linux/libc smart enough to skip action on pages which were mmaped but
> > never touched?
> 
> kernel gives zeroed out pages to user space.
> So it's ok to mmap a page, copy data_sz bytes into it
> and later copy the full page from one addr to another.
> No garbage copy.
> In this case there will be data by llvm construction of ".arena.1"
> It looks to me that even .bss-like __arena vars have zero-s in data
> and non-zero data_sz.

I was actually worried about second memcpy increasing RSS unnecessarily,
but I missed that internal map does:

  def->max_entries = roundup(data_sz, page_sz) / page_sz;

So that is not an issue as bpf_map_mmap_sz() for internal map is
proportional to data_sz, not full arena.
Sorry for the noise.