Re: [PATCH v2 bpf-next 00/20] bpf: Introduce BPF arena.

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On 09.02.24 05:05, Alexei Starovoitov wrote:
From: Alexei Starovoitov <ast@xxxxxxxxxx>

v1->v2:
- Improved commit log with reasons for using vmap_pages_range() in bpf_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.

Introduce bpf_arena, which is a sparse shared memory region between the bpf
program and user space.

Use cases:
1. User space mmap-s bpf_arena and uses it as a traditional mmap-ed anonymous
    region, like memcached or any key/value storage. The bpf program implements an
    in-kernel accelerator. XDP prog can search for a key in bpf_arena and return a
    value without going to user space.

Just so I understand it correctly: this is all backed by unmovable and unswappable memory.

Is there any (existing?) way to restrict/cap the memory consumption via this interface? How easy is this to access+use by unprivileged userspace?

arena_vm_fault() seems to allocate new pages simply via alloc_page(GFP_KERNEL | __GFP_ZERO); No memory accounting, mlock limit checks etc.

We certainly don't want each and every application to be able to break page compaction, swapping etc, that's why I am asking.

--
Cheers,

David / dhildenb





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