Re: [PATCH v6 bpf-next 0/5] bpf_prog_pack followup

[Luis Chamberlain <mcgrof@kernel.org>]

On Tue, Jul 12, 2022 at 05:49:32AM +0000, Song Liu wrote:
> > On Jul 11, 2022, at 9:18 PM, Luis Chamberlain <mcgrof@kernel.org> wrote:
> 
> > I believe you are mentioning requiring text_poke() because the way
> > eBPF code uses the module_alloc() is different. Correct me if I'm
> > wrong, but from what I gather is you use the text_poke_copy() as the data
> > is already RO+X, contrary module_alloc() use cases. You do this since your
> > bpf_prog_pack_alloc() calls set_memory_ro() and set_memory_x() after
> > module_alloc() and before you can use this memory. This is a different type
> > of allocator. And, again please correct me if I'm wrong but now you want to
> > share *one* 2 MiB huge-page for multiple BPF programs to help with the
> > impact of TLB misses.
> 
> Yes, sharing 1x 2MiB huge page is the main reason to require text_poke. 
> OTOH, 2MiB huge pages without sharing is not really useful. Both kprobe
> and ftrace only uses a fraction of a 4kB page. Most BPF programs and 
> modules cannot use 2MiB either. Therefore, vmalloc_rw_exec() doesn't add
> much value on top of current module_alloc(). 

Thanks for the clarification.

> > A vmalloc_ro_exec() by definition would imply a text_poke().
> > 
> > Can kprobes, ftrace and modules use it too? It would be nice
> > so to not have to deal with the loose semantics on the user to
> > have to use set_vm_flush_reset_perms() on ro+x later, but
> > I think this can be addressed separately on a case by case basis.
> 
> I am pretty confident that kprobe and ftrace can share huge pages with 
> BPF programs.

Then wonderful, we know where to go in terms of a new API then as it
can be shared in the future for sure and there are gains.

> I haven't looked into all the details with modules, but 
> given CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC, I think it is also 
> possible.

Sure.

> Once this is done, a regular system (without huge BPF program or huge
> modules) will just use 1x 2MB page for text from module, ftrace, kprobe, 
> and bpf programs. 

That would be nice, if possible, however modules will require likely its
own thing, on my system I see about 57 MiB used on coresize alone.

lsmod | grep -v Module | cut -f1 -d ' ' | \
	xargs sudo modinfo | grep filename | \
	grep -o '/.*' | xargs stat -c "%s - %n" | \
	awk 'BEGIN {sum=0} {sum+=$1} END {print sum}'
60001272

And so perhaps we need such a pool size to be configurable.

> > But a vmalloc_ro_exec() with a respective free can remove the
> > requirement to do set_vm_flush_reset_perms().
> 
> Removing the requirement to set_vm_flush_reset_perms() is the other
> reason to go directly to vmalloc_ro_exec(). 

Yes fantastic.

> My current version looks like this:
> 
> void *vmalloc_exec(unsigned long size);
> void vfree_exec(void *ptr, unsigned int size);
> 
> ro is eliminated as there is no rw version of the API. 

Alright.

I am not sure if 2 MiB will suffice given what I mentioned above, and
what to do to ensure this grows at a reasonable pace. Then, at least for
usage for all architectures since not all will support text_poke() we
will want to consider a way to make it easy to users to use non huge
page fallbacks, but that would be up to those users, so we can wait for
that.

> The ugly part is @size for vfree_exec(). We need it to share huge 
> pages. 

I suppose this will become evident during patch review.

> Under the hood, it looks similar to current bpf_prog_pack_alloc
> and bpf_prog_pack_free. 

Groovy.

  Luis