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From: Song Liu <song@kernel.org>
Date: Thu, 18 Apr 2024 09:13:27 -0700
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Subject: Re: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem_free()
To: Mike Rapoport <rppt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>, Peter Zijlstra <peterz@infradead.org>, 
	linux-kernel@vger.kernel.org, Alexandre Ghiti <alexghiti@rivosinc.com>, 
	Andrew Morton <akpm@linux-foundation.org>, Bjorn Topel <bjorn@kernel.org>, 
	Catalin Marinas <catalin.marinas@arm.com>, Christophe Leroy <christophe.leroy@csgroup.eu>, 
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On Thu, Apr 18, 2024 at 8:37=E2=80=AFAM Mike Rapoport <rppt@kernel.org> wro=
te:
>
[...]
> >
> > Is +/- 2G enough for all realistic use cases? If so, I guess we don't
> > really need
> > EXECMEM_ANYWHERE below?
> >
> > > >
> > > > * I'm not sure about BPF's requirements; it seems happy doing the s=
ame as
> > > >   modules.
> > >
> > > BPF are happy with vmalloc().
> > >
> > > > So if we *must* use a common execmem allocator, what we'd reall wan=
t is our own
> > > > types, e.g.
> > > >
> > > >       EXECMEM_ANYWHERE
> > > >       EXECMEM_NOPLT
> > > >       EXECMEM_PREL32
> > > >
> > > > ... and then we use those in arch code to implement module_alloc() =
and friends.
> > >
> > > I'm looking at execmem_types more as definition of the consumers, may=
be I
> > > should have named the enum execmem_consumer at the first place.
> >
> > I think looking at execmem_type from consumers' point of view adds
> > unnecessary complexity. IIUC, for most (if not all) archs, ftrace, kpro=
be,
> > and bpf (and maybe also module text) all have the same requirements.
> > Did I miss something?
>
> It's enough to have one architecture with different constrains for kprobe=
s
> and bpf to warrant a type for each.
>

AFAICT, some of these constraints can be changed without too much work.

> Where do you see unnecessary complexity?
>
> > IOW, we have
> >
> > enum execmem_type {
> >         EXECMEM_DEFAULT,
> >         EXECMEM_TEXT,
> >         EXECMEM_KPROBES =3D EXECMEM_TEXT,
> >         EXECMEM_FTRACE =3D EXECMEM_TEXT,
> >         EXECMEM_BPF =3D EXECMEM_TEXT,      /* we may end up without
> > _KPROBE, _FTRACE, _BPF */
> >         EXECMEM_DATA,  /* rw */
> >         EXECMEM_RO_DATA,
> >         EXECMEM_RO_AFTER_INIT,
> >         EXECMEM_TYPE_MAX,
> > };
> >
> > Does this make sense?
>
> How do you suggest to deal with e.g. riscv that has separate address spac=
es
> for modules, kprobes and bpf?

IIUC, modules and bpf use the same address space on riscv, while kprobes us=
e
vmalloc address. I haven't tried this yet, but I think we can let
kprobes use the
same space as modules and bpf, which is:

ffffffff00000000 |  -4     GB | ffffffff7fffffff |    2 GB | modules, BPF

Did I get this right?

Thanks,
Song