Re: [PATCH bpf-next 04/16] bpf: Introduce bpf_arena.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Feb 7, 2024 at 1:12 PM Barret Rhoden <brho@google.com> wrote:
>
> On 2/7/24 15:55, Alexei Starovoitov wrote:
> >> instead of uaddr, can you change this to take an address relative to the
> >> arena ("arena virtual address"?)?  the caller of this is in BPF, and
> >> they don't easily know the user virtual address.  maybe even just pgoff
> >> directly.
> > I thought about it, but it doesn't quite make sense.
> > bpf prog only sees user addresses.
> > All load/store returns them. If it bpf_printk-s an address it will be
> > user address.
> > bpf_arena_alloc_pages() also returns a user address.
>
> Yeah, makes sense to keep them all in the same address space.
>
> >
> > Kernel addresses are not seen by bpf prog at all.
> > kern_vm_base is completely hidden.
> > Only at JIT time, it's added to pointers.
> > So passing uaddr to arena_alloc_pages() matches mmap style.
> >
> > uaddr = bpf_arena_alloc_pages(... uaddr ...)
> > uaddr = mmap(uaddr, ...MAP_FIXED)
> >
> > Passing pgoff would be weird.
> > Also note that there is no extra flag for bpf_arena_alloc_pages().
> > uaddr == full 64-bit of zeros is not a valid addr to use.
>
> The problem I had with uaddr was that when I'm writing a BPF program, I
> don't know which address to use for a given page, e.g. the beginning of
> the arena.  I needed some way to tell me the user address "base" of the
> arena.  Though now that I can specify the user_vm_start through the
> map_extra, I think I'm ok.
>
> Specifically, say I want to break up my arena into two, 2GB chunks, one
> for each numa node, and I want to bump-allocate from each chunk.  When I
> want to allocate the first page from either segment, I'll need to know
> what user address is offset 0 or offset 2GB.

bump allocate... you mean like page_frag alloc does?
I've implemented one on top of arena:
https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/tree/tools/testing/selftests/bpf/bpf_arena_alloc.h?h=arena&id=36d78b0f1c14c959d907d68cd7d54439b9213d0c

Also I believe I addressed all issues with missing mutex and wrap around,
and pushed to:
https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/commit/?h=arena&id=e1cb522fee661e7346e8be567eade9cf607eaf11
Please take a look.

Including the wrap around test in the last commit:
https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/commit/?h=arena&id=01653c393a4167ccca23dc5a69aa9cf34a46eabd

Will wait a bit before sending v2.