On Tue, Jul 12, 2022 at 05:49:32AM +0000, Song Liu wrote:
> > On Jul 11, 2022, at 9:18 PM, Luis Chamberlain <mcgrof@xxxxxxxxxx> 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
