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

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On Wed, Jul 06, 2022 at 06:55:36PM +0100, Matthew Wilcox wrote:
> On Wed, Jul 06, 2022 at 10:50:34AM -0700, Alexei Starovoitov wrote:
> > On Mon, Jul 04, 2022 at 09:34:23PM +0100, Matthew Wilcox wrote:
> > > 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@xxxxxxxxxx>
> > > > > 
> > > > > 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.
> > 
> > Already looked at it, and into mempool, and everything we could find.
> > All 'normal' allocators sooner or later synchornously call into page_alloc,
> 
> Today it does, yes.  But it might be adaptable to your needs if only I
> knew what those needs were.  

needs: fully reentrant, nmi safe, any context safe.
I feel you're still failing to grasp 'any context' part.
bpf prog can attach to all available_filter_functions and should be able
to alloc from there.
Or kprobe anywhere and everywhere in the kernel .text that is not marked notrace/nokprobe.

> For example, I assume that a BPF program
> has a fairly tight limit on how much memory it can cause to be allocated.
> Right?

No. It's constrained by memcg limits only. It can allocate gigabytes.



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