Re: [PATCH bpf-next 0/5] bpf: BPF specific memory allocator.

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jun 27, 2022 at 12:03:08AM -0700, Christoph Hellwig wrote:
> I'd suggest you discuss you needs with the slab mainainers and the mm
> community firs.
> 
> On Wed, Jun 22, 2022 at 05:32:25PM -0700, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@kernel.org>
> > 
> > Introduce any context BPF specific memory allocator.
> > 
> > Tracing BPF programs can attach to kprobe and fentry. Hence they
> > run in unknown context where calling plain kmalloc() might not be safe.
> > Front-end kmalloc() with per-cpu per-bucket cache of free elements.
> > Refill this cache asynchronously from irq_work.

I can't tell from your description whether a bump allocator would work
for you.  That is, can you tell which allocations need to persist past
program execution (and use kmalloc for them) and which can be freed as
soon as the program has finished (and can use the bump allocator)?

If so, we already have one for you, the page_frag allocator
(Documentation/vm/page_frags.rst).  It might need to be extended to meet
your needs, but it's certainly faster than the kmalloc allocator.

> > There is a lot more work ahead, but this set is useful base.
> > Future work:
> > - get rid of call_rcu in hash map
> > - get rid of atomic_inc/dec in hash map
> > - tune watermarks per allocation size
> > - adopt this approach alloc_percpu_gfp
> > - expose bpf_mem_alloc as uapi FD to be used in dynptr_alloc, kptr_alloc
> > - add sysctl to force bpf_mem_alloc in hash map when safe even if pre-alloc
> >   requested to reduce memory consumption
> > - convert lru map to bpf_mem_alloc
> > 
> > Alexei Starovoitov (5):
> >   bpf: Introduce any context BPF specific memory allocator.
> >   bpf: Convert hash map to bpf_mem_alloc.
> >   selftests/bpf: Improve test coverage of test_maps
> >   samples/bpf: Reduce syscall overhead in map_perf_test.
> >   bpf: Relax the requirement to use preallocated hash maps in tracing
> >     progs.
> > 
> >  include/linux/bpf_mem_alloc.h           |  26 ++
> >  kernel/bpf/Makefile                     |   2 +-
> >  kernel/bpf/hashtab.c                    |  16 +-
> >  kernel/bpf/memalloc.c                   | 512 ++++++++++++++++++++++++
> >  kernel/bpf/verifier.c                   |  31 +-
> >  samples/bpf/map_perf_test_kern.c        |  22 +-
> >  tools/testing/selftests/bpf/test_maps.c |  38 +-
> >  7 files changed, 610 insertions(+), 37 deletions(-)
> >  create mode 100644 include/linux/bpf_mem_alloc.h
> >  create mode 100644 kernel/bpf/memalloc.c
> > 
> > -- 
> > 2.30.2
> > 
> ---end quoted text---
>