Re: [PATCH 00/13] mm: jit/text allocator

[Song Liu]

On Fri, Jun 2, 2023 at 2:35 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Thu, Jun 01, 2023 at 02:14:56PM -0400, Kent Overstreet wrote:
> > On Thu, Jun 01, 2023 at 05:12:03PM +0100, Mark Rutland wrote:
> > > For a while I have wanted to give kprobes its own allocator so that it can work
> > > even with CONFIG_MODULES=n, and so that it doesn't have to waste VA space in
> > > the modules area.
> > >
> > > Given that, I think these should have their own allocator functions that can be
> > > provided independently, even if those happen to use common infrastructure.
> >
> > How much memory can kprobes conceivably use? I think we also want to try
> > to push back on combinatorial new allocators, if we can.
>
> That depends on who's using it, and how (e.g. via BPF).
>
> To be clear, I'm not necessarily asking for entirely different allocators, but
> I do thinkg that we want wrappers that can at least pass distinct start+end
> parameters to a common allocator, and for arm64's modules code I'd expect that
> we'd keep the range falblack logic out of the common allcoator, and just call
> it twice.
>
> > > > Several architectures override module_alloc() because of various
> > > > constraints where the executable memory can be located and this causes
> > > > additional obstacles for improvements of code allocation.
> > > >
> > > > This set splits code allocation from modules by introducing
> > > > jit_text_alloc(), jit_data_alloc() and jit_free() APIs, replaces call
> > > > sites of module_alloc() and module_memfree() with the new APIs and
> > > > implements core text and related allocation in a central place.
> > > >
> > > > Instead of architecture specific overrides for module_alloc(), the
> > > > architectures that require non-default behaviour for text allocation must
> > > > fill jit_alloc_params structure and implement jit_alloc_arch_params() that
> > > > returns a pointer to that structure. If an architecture does not implement
> > > > jit_alloc_arch_params(), the defaults compatible with the current
> > > > modules::module_alloc() are used.
> > >
> > > As above, I suspect that each of the callsites should probably be using common
> > > infrastructure, but I don't think that a single jit_alloc_arch_params() makes
> > > sense, since the parameters for each case may need to be distinct.
> >
> > I don't see how that follows. The whole point of function parameters is
> > that they may be different :)
>
> What I mean is that jit_alloc_arch_params() tries to aggregate common
> parameters, but they aren't actually common (e.g. the actual start+end range
> for allocation).
>
> > Can you give more detail on what parameters you need? If the only extra
> > parameter is just "does this allocation need to live close to kernel
> > text", that's not that big of a deal.
>
> My thinking was that we at least need the start + end for each caller. That
> might be it, tbh.

IIUC, arm64 uses VMALLOC address space for BPF programs. The reason
is each BPF program uses at least 64kB (one page) out of the 128MB
address space. Puranjay Mohan (CC'ed) is working on enabling
bpf_prog_pack for arm64. Once this work is done, multiple BPF programs
will be able to share a page. Will this improvement remove the need to
specify a different address range for BPF programs?

Thanks,
Song