[v3,bpf-next,00/14] bpf: Introduce BPF arena.

Message

Alexei Starovoitov March 8, 2024, 1:07 a.m. UTC

From: Alexei Starovoitov <[e-mail]>

v2->v3:
- contains bpf bits only, but cc-ing past audience for continuity
- since prerequisite patches landed, this series focus on the main
  functionality of bpf_arena.
- adopted Andrii's approach to support arena in libbpf.
- simplified LLVM support. Instead of two instructions it's now only one.
- switched to cond_break (instead of open coded iters) in selftests
- implemented several follow-ups that will be sent after this set
  . remember first IP and bpf insn that faulted in arena.
    report to user space via bpftool
  . copy paste and tweak glob_match() aka mini-regex as a selftests/bpf
- see patch 1 for detailed description of bpf_arena

v1->v2:
- Improved commit log with reasons for using vmap_pages_range() in arena.
  Thanks to Johannes
- Added support for __arena global variables in bpf programs
- Fixed race conditions spotted by Barret
- Fixed wrap32 issue spotted by Barret
- Fixed bpf_map_mmap_sz() the way Andrii suggested

The work on bpf_arena was inspired by Barret's work:
https://github.com/google/ghost-userspace/blob/main/lib/queue.bpf.h
that implements queues, lists and AVL trees completely as bpf programs
using giant bpf array map and integer indices instead of pointers.
bpf_arena is a sparse array that allows to use normal C pointers to
build such data structures. Last few patches implement page_frag
allocator, link list and hash table as bpf programs.

v1:
bpf programs have multiple options to communicate with user space:
- Various ring buffers (perf, ftrace, bpf): The data is streamed
  unidirectionally from bpf to user space.
- Hash map: The bpf program populates elements, and user space consumes
  them via bpf syscall.
- mmap()-ed array map: Libbpf creates an array map that is directly
  accessed by the bpf program and mmap-ed to user space. It's the fastest
  way. Its disadvantage is that memory for the whole array is reserved at
  the start.

Alexei Starovoitov (13):
  bpf: Introduce bpf_arena.
  bpf: Disasm support for addr_space_cast instruction.
  bpf: Add x86-64 JIT support for PROBE_MEM32 pseudo instructions.
  bpf: Add x86-64 JIT support for bpf_addr_space_cast instruction.
  bpf: Recognize addr_space_cast instruction in the verifier.
  bpf: Recognize btf_decl_tag("arg:arena") as PTR_TO_ARENA.
  libbpf: Add __arg_arena to bpf_helpers.h
  libbpf: Add support for bpf_arena.
  bpftool: Recognize arena map type
  bpf: Add helper macro bpf_addr_space_cast()
  selftests/bpf: Add unit tests for bpf_arena_alloc/free_pages
  selftests/bpf: Add bpf_arena_list test.
  selftests/bpf: Add bpf_arena_htab test.

Andrii Nakryiko (1):
  libbpf: Recognize __arena global varaibles.

 arch/x86/net/bpf_jit_comp.c                   | 231 +++++++-
 include/linux/bpf.h                           |  10 +-
 include/linux/bpf_types.h                     |   1 +
 include/linux/bpf_verifier.h                  |   1 +
 include/linux/filter.h                        |   4 +
 include/uapi/linux/bpf.h                      |  14 +
 kernel/bpf/Makefile                           |   3 +
 kernel/bpf/arena.c                            | 558 ++++++++++++++++++
 kernel/bpf/btf.c                              |  19 +-
 kernel/bpf/core.c                             |  16 +
 kernel/bpf/disasm.c                           |  10 +
 kernel/bpf/log.c                              |   3 +
 kernel/bpf/syscall.c                          |  42 ++
 kernel/bpf/verifier.c                         | 123 +++-
 .../bpf/bpftool/Documentation/bpftool-map.rst |   2 +-
 tools/bpf/bpftool/gen.c                       |  13 +
 tools/bpf/bpftool/map.c                       |   2 +-
 tools/include/uapi/linux/bpf.h                |  14 +
 tools/lib/bpf/bpf_helpers.h                   |   1 +
 tools/lib/bpf/libbpf.c                        | 163 ++++-
 tools/lib/bpf/libbpf.h                        |   2 +-
 tools/lib/bpf/libbpf_probes.c                 |   7 +
 tools/testing/selftests/bpf/DENYLIST.aarch64  |   2 +
 tools/testing/selftests/bpf/DENYLIST.s390x    |   2 +
 tools/testing/selftests/bpf/bpf_arena_alloc.h |  67 +++
 .../testing/selftests/bpf/bpf_arena_common.h  |  70 +++
 tools/testing/selftests/bpf/bpf_arena_htab.h  | 100 ++++
 tools/testing/selftests/bpf/bpf_arena_list.h  |  92 +++
 .../testing/selftests/bpf/bpf_experimental.h  |  43 ++
 .../selftests/bpf/prog_tests/arena_htab.c     |  88 +++
 .../selftests/bpf/prog_tests/arena_list.c     |  68 +++
 .../selftests/bpf/prog_tests/verifier.c       |   2 +
 .../testing/selftests/bpf/progs/arena_htab.c  |  48 ++
 .../selftests/bpf/progs/arena_htab_asm.c      |   5 +
 .../testing/selftests/bpf/progs/arena_list.c  |  87 +++
 .../selftests/bpf/progs/verifier_arena.c      | 146 +++++
 tools/testing/selftests/bpf/test_loader.c     |   9 +-
 37 files changed, 2028 insertions(+), 40 deletions(-)
 create mode 100644 kernel/bpf/arena.c
 create mode 100644 tools/testing/selftests/bpf/bpf_arena_alloc.h
 create mode 100644 tools/testing/selftests/bpf/bpf_arena_common.h
 create mode 100644 tools/testing/selftests/bpf/bpf_arena_htab.h
 create mode 100644 tools/testing/selftests/bpf/bpf_arena_list.h
 create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_htab.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_list.c
 create mode 100644 tools/testing/selftests/bpf/progs/arena_htab.c
 create mode 100644 tools/testing/selftests/bpf/progs/arena_htab_asm.c
 create mode 100644 tools/testing/selftests/bpf/progs/arena_list.c
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_arena.c

Comments

Andrii Nakryiko March 11, 2024, 10:45 p.m. UTC | #1

On Thu, Mar 7, 2024 at 5:08 PM Alexei Starovoitov
<[e-mail]> wrote:
>
> From: Alexei Starovoitov <[e-mail]>
>
> v2->v3:
> - contains bpf bits only, but cc-ing past audience for continuity
> - since prerequisite patches landed, this series focus on the main
>   functionality of bpf_arena.
> - adopted Andrii's approach to support arena in libbpf.
> - simplified LLVM support. Instead of two instructions it's now only one.
> - switched to cond_break (instead of open coded iters) in selftests
> - implemented several follow-ups that will be sent after this set
>   . remember first IP and bpf insn that faulted in arena.
>     report to user space via bpftool
>   . copy paste and tweak glob_match() aka mini-regex as a selftests/bpf
> - see patch 1 for detailed description of bpf_arena
>
> v1->v2:
> - Improved commit log with reasons for using vmap_pages_range() in arena.
>   Thanks to Johannes
> - Added support for __arena global variables in bpf programs
> - Fixed race conditions spotted by Barret
> - Fixed wrap32 issue spotted by Barret
> - Fixed bpf_map_mmap_sz() the way Andrii suggested
>
> The work on bpf_arena was inspired by Barret's work:
> https://github.com/google/ghost-userspace/blob/main/lib/queue.bpf.h
> that implements queues, lists and AVL trees completely as bpf programs
> using giant bpf array map and integer indices instead of pointers.
> bpf_arena is a sparse array that allows to use normal C pointers to
> build such data structures. Last few patches implement page_frag
> allocator, link list and hash table as bpf programs.
>
> v1:
> bpf programs have multiple options to communicate with user space:
> - Various ring buffers (perf, ftrace, bpf): The data is streamed
>   unidirectionally from bpf to user space.
> - Hash map: The bpf program populates elements, and user space consumes
>   them via bpf syscall.
> - mmap()-ed array map: Libbpf creates an array map that is directly
>   accessed by the bpf program and mmap-ed to user space. It's the fastest
>   way. Its disadvantage is that memory for the whole array is reserved at
>   the start.
>
> Alexei Starovoitov (13):
>   bpf: Introduce bpf_arena.
>   bpf: Disasm support for addr_space_cast instruction.
>   bpf: Add x86-64 JIT support for PROBE_MEM32 pseudo instructions.
>   bpf: Add x86-64 JIT support for bpf_addr_space_cast instruction.
>   bpf: Recognize addr_space_cast instruction in the verifier.
>   bpf: Recognize btf_decl_tag("arg:arena") as PTR_TO_ARENA.
>   libbpf: Add __arg_arena to bpf_helpers.h
>   libbpf: Add support for bpf_arena.
>   bpftool: Recognize arena map type
>   bpf: Add helper macro bpf_addr_space_cast()
>   selftests/bpf: Add unit tests for bpf_arena_alloc/free_pages
>   selftests/bpf: Add bpf_arena_list test.
>   selftests/bpf: Add bpf_arena_htab test.
>
> Andrii Nakryiko (1):
>   libbpf: Recognize __arena global varaibles.
>
>  arch/x86/net/bpf_jit_comp.c                   | 231 +++++++-
>  include/linux/bpf.h                           |  10 +-
>  include/linux/bpf_types.h                     |   1 +
>  include/linux/bpf_verifier.h                  |   1 +
>  include/linux/filter.h                        |   4 +
>  include/uapi/linux/bpf.h                      |  14 +
>  kernel/bpf/Makefile                           |   3 +
>  kernel/bpf/arena.c                            | 558 ++++++++++++++++++
>  kernel/bpf/btf.c                              |  19 +-
>  kernel/bpf/core.c                             |  16 +
>  kernel/bpf/disasm.c                           |  10 +
>  kernel/bpf/log.c                              |   3 +
>  kernel/bpf/syscall.c                          |  42 ++
>  kernel/bpf/verifier.c                         | 123 +++-
>  .../bpf/bpftool/Documentation/bpftool-map.rst |   2 +-
>  tools/bpf/bpftool/gen.c                       |  13 +
>  tools/bpf/bpftool/map.c                       |   2 +-
>  tools/include/uapi/linux/bpf.h                |  14 +
>  tools/lib/bpf/bpf_helpers.h                   |   1 +
>  tools/lib/bpf/libbpf.c                        | 163 ++++-
>  tools/lib/bpf/libbpf.h                        |   2 +-
>  tools/lib/bpf/libbpf_probes.c                 |   7 +
>  tools/testing/selftests/bpf/DENYLIST.aarch64  |   2 +
>  tools/testing/selftests/bpf/DENYLIST.s390x    |   2 +
>  tools/testing/selftests/bpf/bpf_arena_alloc.h |  67 +++
>  .../testing/selftests/bpf/bpf_arena_common.h  |  70 +++
>  tools/testing/selftests/bpf/bpf_arena_htab.h  | 100 ++++
>  tools/testing/selftests/bpf/bpf_arena_list.h  |  92 +++
>  .../testing/selftests/bpf/bpf_experimental.h  |  43 ++
>  .../selftests/bpf/prog_tests/arena_htab.c     |  88 +++
>  .../selftests/bpf/prog_tests/arena_list.c     |  68 +++
>  .../selftests/bpf/prog_tests/verifier.c       |   2 +
>  .../testing/selftests/bpf/progs/arena_htab.c  |  48 ++
>  .../selftests/bpf/progs/arena_htab_asm.c      |   5 +
>  .../testing/selftests/bpf/progs/arena_list.c  |  87 +++
>  .../selftests/bpf/progs/verifier_arena.c      | 146 +++++
>  tools/testing/selftests/bpf/test_loader.c     |   9 +-
>  37 files changed, 2028 insertions(+), 40 deletions(-)
>  create mode 100644 kernel/bpf/arena.c
>  create mode 100644 tools/testing/selftests/bpf/bpf_arena_alloc.h
>  create mode 100644 tools/testing/selftests/bpf/bpf_arena_common.h
>  create mode 100644 tools/testing/selftests/bpf/bpf_arena_htab.h
>  create mode 100644 tools/testing/selftests/bpf/bpf_arena_list.h
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_htab.c
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_list.c
>  create mode 100644 tools/testing/selftests/bpf/progs/arena_htab.c
>  create mode 100644 tools/testing/selftests/bpf/progs/arena_htab_asm.c
>  create mode 100644 tools/testing/selftests/bpf/progs/arena_list.c
>  create mode 100644 tools/testing/selftests/bpf/progs/verifier_arena.c
>
> --
> 2.43.0
>

Besides a few comments on patch #1 (and maybe one or two potential
corner case issues I mentioned, which can be easily fixed), the series
looked good. So I've applied patches as is. I fixed typo ("varaibles")
in one of the commit subjects while applying.

Also, in one of the selftests you hard-coded PAGE_SIZE to 4096, which
isn't correct on some architectures, so please see how you can make it
not hard-coded (but still work for both bpf and user code). It seemed
minor enough to not delay patches (either way those architectures
don't support ARENA just yet).

[e-mail] March 11, 2024, 10:50 p.m. UTC | #2

Hello:

This series was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <[e-mail]>:

On Thu,  7 Mar 2024 17:07:58 -0800 you wrote:
> From: Alexei Starovoitov <[e-mail]>
> 
> v2->v3:
> - contains bpf bits only, but cc-ing past audience for continuity
> - since prerequisite patches landed, this series focus on the main
>   functionality of bpf_arena.
> - adopted Andrii's approach to support arena in libbpf.
> - simplified LLVM support. Instead of two instructions it's now only one.
> - switched to cond_break (instead of open coded iters) in selftests
> - implemented several follow-ups that will be sent after this set
>   . remember first IP and bpf insn that faulted in arena.
>     report to user space via bpftool
>   . copy paste and tweak glob_match() aka mini-regex as a selftests/bpf
> - see patch 1 for detailed description of bpf_arena
> 
> [...]

Here is the summary with links:
  - [v3,bpf-next,01/14] bpf: Introduce bpf_arena.
    https://git.kernel.org/bpf/bpf-next/c/317460317a02
  - [v3,bpf-next,02/14] bpf: Disasm support for addr_space_cast instruction.
    https://git.kernel.org/bpf/bpf-next/c/667a86ad9b71
  - [v3,bpf-next,03/14] bpf: Add x86-64 JIT support for PROBE_MEM32 pseudo instructions.
    https://git.kernel.org/bpf/bpf-next/c/2fe99eb0ccf2
  - [v3,bpf-next,04/14] bpf: Add x86-64 JIT support for bpf_addr_space_cast instruction.
    https://git.kernel.org/bpf/bpf-next/c/142fd4d2dcf5
  - [v3,bpf-next,05/14] bpf: Recognize addr_space_cast instruction in the verifier.
    https://git.kernel.org/bpf/bpf-next/c/6082b6c328b5
  - [v3,bpf-next,06/14] bpf: Recognize btf_decl_tag("arg:arena") as PTR_TO_ARENA.
    https://git.kernel.org/bpf/bpf-next/c/2edc3de6fb65
  - [v3,bpf-next,07/14] libbpf: Add __arg_arena to bpf_helpers.h
    https://git.kernel.org/bpf/bpf-next/c/4d2b56081c32
  - [v3,bpf-next,08/14] libbpf: Add support for bpf_arena.
    https://git.kernel.org/bpf/bpf-next/c/79ff13e99169
  - [v3,bpf-next,09/14] bpftool: Recognize arena map type
    https://git.kernel.org/bpf/bpf-next/c/eed512e8ac64
  - [v3,bpf-next,10/14] libbpf: Recognize __arena global varaibles.
    https://git.kernel.org/bpf/bpf-next/c/2e7ba4f8fd1f
  - [v3,bpf-next,11/14] bpf: Add helper macro bpf_addr_space_cast()
    https://git.kernel.org/bpf/bpf-next/c/204c628730c6
  - [v3,bpf-next,12/14] selftests/bpf: Add unit tests for bpf_arena_alloc/free_pages
    https://git.kernel.org/bpf/bpf-next/c/80a4129fcf20
  - [v3,bpf-next,13/14] selftests/bpf: Add bpf_arena_list test.
    https://git.kernel.org/bpf/bpf-next/c/9f2c156f90a4
  - [v3,bpf-next,14/14] selftests/bpf: Add bpf_arena_htab test.
    https://git.kernel.org/bpf/bpf-next/c/8df839ae23b8

You are awesome, thank you!

Alexei Starovoitov March 11, 2024, 11:02 p.m. UTC | #3

On Mon, Mar 11, 2024 at 3:45 PM Andrii Nakryiko
<[e-mail]> wrote:
>
> On Thu, Mar 7, 2024 at 5:08 PM Alexei Starovoitov
> <[e-mail]> wrote:
> >
> > From: Alexei Starovoitov <[e-mail]>
> >
> > v2->v3:
> > - contains bpf bits only, but cc-ing past audience for continuity
> > - since prerequisite patches landed, this series focus on the main
> >   functionality of bpf_arena.
> > - adopted Andrii's approach to support arena in libbpf.
> > - simplified LLVM support. Instead of two instructions it's now only one.
> > - switched to cond_break (instead of open coded iters) in selftests
> > - implemented several follow-ups that will be sent after this set
> >   . remember first IP and bpf insn that faulted in arena.
> >     report to user space via bpftool
> >   . copy paste and tweak glob_match() aka mini-regex as a selftests/bpf
> > - see patch 1 for detailed description of bpf_arena
> >
> > v1->v2:
> > - Improved commit log with reasons for using vmap_pages_range() in arena.
> >   Thanks to Johannes
> > - Added support for __arena global variables in bpf programs
> > - Fixed race conditions spotted by Barret
> > - Fixed wrap32 issue spotted by Barret
> > - Fixed bpf_map_mmap_sz() the way Andrii suggested
> >
> > The work on bpf_arena was inspired by Barret's work:
> > https://github.com/google/ghost-userspace/blob/main/lib/queue.bpf.h
> > that implements queues, lists and AVL trees completely as bpf programs
> > using giant bpf array map and integer indices instead of pointers.
> > bpf_arena is a sparse array that allows to use normal C pointers to
> > build such data structures. Last few patches implement page_frag
> > allocator, link list and hash table as bpf programs.
> >
> > v1:
> > bpf programs have multiple options to communicate with user space:
> > - Various ring buffers (perf, ftrace, bpf): The data is streamed
> >   unidirectionally from bpf to user space.
> > - Hash map: The bpf program populates elements, and user space consumes
> >   them via bpf syscall.
> > - mmap()-ed array map: Libbpf creates an array map that is directly
> >   accessed by the bpf program and mmap-ed to user space. It's the fastest
> >   way. Its disadvantage is that memory for the whole array is reserved at
> >   the start.
> >
> > Alexei Starovoitov (13):
> >   bpf: Introduce bpf_arena.
> >   bpf: Disasm support for addr_space_cast instruction.
> >   bpf: Add x86-64 JIT support for PROBE_MEM32 pseudo instructions.
> >   bpf: Add x86-64 JIT support for bpf_addr_space_cast instruction.
> >   bpf: Recognize addr_space_cast instruction in the verifier.
> >   bpf: Recognize btf_decl_tag("arg:arena") as PTR_TO_ARENA.
> >   libbpf: Add __arg_arena to bpf_helpers.h
> >   libbpf: Add support for bpf_arena.
> >   bpftool: Recognize arena map type
> >   bpf: Add helper macro bpf_addr_space_cast()
> >   selftests/bpf: Add unit tests for bpf_arena_alloc/free_pages
> >   selftests/bpf: Add bpf_arena_list test.
> >   selftests/bpf: Add bpf_arena_htab test.
> >
> > Andrii Nakryiko (1):
> >   libbpf: Recognize __arena global varaibles.
> >
> >  arch/x86/net/bpf_jit_comp.c                   | 231 +++++++-
> >  include/linux/bpf.h                           |  10 +-
> >  include/linux/bpf_types.h                     |   1 +
> >  include/linux/bpf_verifier.h                  |   1 +
> >  include/linux/filter.h                        |   4 +
> >  include/uapi/linux/bpf.h                      |  14 +
> >  kernel/bpf/Makefile                           |   3 +
> >  kernel/bpf/arena.c                            | 558 ++++++++++++++++++
> >  kernel/bpf/btf.c                              |  19 +-
> >  kernel/bpf/core.c                             |  16 +
> >  kernel/bpf/disasm.c                           |  10 +
> >  kernel/bpf/log.c                              |   3 +
> >  kernel/bpf/syscall.c                          |  42 ++
> >  kernel/bpf/verifier.c                         | 123 +++-
> >  .../bpf/bpftool/Documentation/bpftool-map.rst |   2 +-
> >  tools/bpf/bpftool/gen.c                       |  13 +
> >  tools/bpf/bpftool/map.c                       |   2 +-
> >  tools/include/uapi/linux/bpf.h                |  14 +
> >  tools/lib/bpf/bpf_helpers.h                   |   1 +
> >  tools/lib/bpf/libbpf.c                        | 163 ++++-
> >  tools/lib/bpf/libbpf.h                        |   2 +-
> >  tools/lib/bpf/libbpf_probes.c                 |   7 +
> >  tools/testing/selftests/bpf/DENYLIST.aarch64  |   2 +
> >  tools/testing/selftests/bpf/DENYLIST.s390x    |   2 +
> >  tools/testing/selftests/bpf/bpf_arena_alloc.h |  67 +++
> >  .../testing/selftests/bpf/bpf_arena_common.h  |  70 +++
> >  tools/testing/selftests/bpf/bpf_arena_htab.h  | 100 ++++
> >  tools/testing/selftests/bpf/bpf_arena_list.h  |  92 +++
> >  .../testing/selftests/bpf/bpf_experimental.h  |  43 ++
> >  .../selftests/bpf/prog_tests/arena_htab.c     |  88 +++
> >  .../selftests/bpf/prog_tests/arena_list.c     |  68 +++
> >  .../selftests/bpf/prog_tests/verifier.c       |   2 +
> >  .../testing/selftests/bpf/progs/arena_htab.c  |  48 ++
> >  .../selftests/bpf/progs/arena_htab_asm.c      |   5 +
> >  .../testing/selftests/bpf/progs/arena_list.c  |  87 +++
> >  .../selftests/bpf/progs/verifier_arena.c      | 146 +++++
> >  tools/testing/selftests/bpf/test_loader.c     |   9 +-
> >  37 files changed, 2028 insertions(+), 40 deletions(-)
> >  create mode 100644 kernel/bpf/arena.c
> >  create mode 100644 tools/testing/selftests/bpf/bpf_arena_alloc.h
> >  create mode 100644 tools/testing/selftests/bpf/bpf_arena_common.h
> >  create mode 100644 tools/testing/selftests/bpf/bpf_arena_htab.h
> >  create mode 100644 tools/testing/selftests/bpf/bpf_arena_list.h
> >  create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_htab.c
> >  create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_list.c
> >  create mode 100644 tools/testing/selftests/bpf/progs/arena_htab.c
> >  create mode 100644 tools/testing/selftests/bpf/progs/arena_htab_asm.c
> >  create mode 100644 tools/testing/selftests/bpf/progs/arena_list.c
> >  create mode 100644 tools/testing/selftests/bpf/progs/verifier_arena.c
> >
> > --
> > 2.43.0
> >
>
> Besides a few comments on patch #1 (and maybe one or two potential
> corner case issues I mentioned, which can be easily fixed),
> the series
> looked good. So I've applied patches as is. I fixed typo ("varaibles")
> in one of the commit subjects while applying.

Thanks! That subj typo survived two months of reviews in v1,v2,v3 and
I swear I use ./scripts/checkpatch.pl --codespell all the time.
I guess it got drowned in all of the messages like:
WARNING: 'mmaped' may be misspelled - perhaps 'mapped'?
#356: FILE: tools/lib/bpf/libbpf.c:13666:
+            *mmaped = map->mmaped;
              ^^^^^^

WARNING: 'mmaped' may be misspelled - perhaps 'mapped'?
#356: FILE: tools/lib/bpf/libbpf.c:13666:
+            *mmaped = map->mmaped;
                            ^^^^^^
> Also, in one of the selftests you hard-coded PAGE_SIZE to 4096, which
> isn't correct on some architectures, so please see how you can make it
> not hard-coded (but still work for both bpf and user code). It seemed
> minor enough to not delay patches (either way those architectures
> don't support ARENA just yet).

yes. It's on todo list already.
I've added #define PAGE_SIZE 4096 to user space side of bpf selftest,
because it's used in bpf_arena_*.h code which is dual
compiled as bpf prog (and then it's using PAGE_SIZE from vmlinux.h)
and compiled as native code.
So bpf side gets correct PAGE_SIZE automatically as a nice constant
at compile time, but for user space there is no good PAGE_SIZE
constant to use.
Just doing
#define PAGE_SIZE sysconf(_SC_PAGE_SIZE)
produces inefficient code.
Hence I left it as a todo to figure out later.