Re: [PATCH bpf-next v2 0/5] execmem_alloc for BPF programs

[Song Liu <song@kernel.org>]

On Sun, Nov 20, 2022 at 3:41 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Thu, Nov 17, 2022 at 10:36:43AM -0800, Song Liu wrote:
> > On Thu, Nov 17, 2022 at 12:50 AM Mike Rapoport <rppt@kernel.org> wrote:
> > >
> > > On Mon, Nov 14, 2022 at 12:30:49PM -0800, Song Liu wrote:
> > > > On Sun, Nov 13, 2022 at 2:35 AM Mike Rapoport <rppt@kernel.org> wrote:
> > > > >
> > > > > My concern is that the proposed execmem_alloc() cannot be used for
> > > > > centralized handling of loading text. I'm not familiar enough with
> > > > > modules/ftrace/kprobes/BPF to clearly identify the potential caveats, but
> > > > > my gut feeling is that the proposed execmem_alloc() won't be an improvement
> > > > > but rather a hindrance for moving to centralized handling of loading text.
> > > >
> > > > I don't follow why this could ever be a hindrance. Luis is very excited about
> > > > this, and I am very sure it works for ftrace, kprobe, and BPF.
> > >
> > > Again, it's a gut feeling. But for execmem_alloc() to be a unified place of
> > > code allocation, it has to work for all architectures. If architectures
> > > have to override it, then where is the unification?
> > >
> > > The implementation you propose if great for x86, but to see it as unified
> > > solution it should be good at least for the major architectures.
> >
> > As I mentioned earlier, folks are working on using bpf_prog_pack for BPF
> > JIT on powerpc. We will also work on something similar for ARM.
>
> Does "something similar" mean that it won't use execmem_alloc() as is?

"Something similar" means it will use execmem_alloc as is. We still need
changes to the ARM JIT code, just like we need it for powerpc and x86.

>
> > I guess these are good enough for major architectures?
>
> Sorry if I wasn't clear, I referred for unified solution for all code
> allocations, not only BPF, so that execmem_alloc() will eventually replace
> module_alloc(). And that means it has to be able to deal with with
> architecture specific requirements at least on ARM and powerpc, probably
> others as well.
>
> > > > > It feels to me that a lot of ground work is needed to get to the point
> > > > > where we can use centralized handling of loading text.
> > > >
> > > > Could you please be more specific on what is needed?
> > >
> > > The most obvious one to implement Peter's suggestion with VM_TOPDOWN_VMAP
> > > so that execmem_alloc() can be actually used by modules.
> >
> > Current implementation is an alternative to VM_TOPDOWN_VMAP. I am
> > very sure it works for modules just like VM_TOPDOWN_VMAP solution.
>
> It might, but it still does not. And until they do I consider these
> patches as an optimization for BFP rather than unification of code
> allocations.

We haven't got module to use execmem_alloc yet, that's true. But
this has nothing to do with VM_TOPDOWN_VMAP at all.

Thanks,
Song