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From: Song Liu <song@kernel.org>
Date: Fri, 19 Apr 2024 08:54:40 -0700
Message-ID: <CAPhsuW7WoU+a46FhqqH8f-3=ehxeD4wSgKDWegMin1pT49OSWw@mail.gmail.com>
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>, 
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On Thu, Apr 18, 2024 at 11:56=E2=80=AFPM Mike Rapoport <rppt@kernel.org> wr=
ote:
>
> On Thu, Apr 18, 2024 at 02:01:22PM -0700, Song Liu wrote:
> > On Thu, Apr 18, 2024 at 10:54=E2=80=AFAM Mike Rapoport <rppt@kernel.org=
> wrote:
> > >
> > > On Thu, Apr 18, 2024 at 09:13:27AM -0700, Song Liu wrote:
> > > > On Thu, Apr 18, 2024 at 8:37=E2=80=AFAM Mike Rapoport <rppt@kernel.=
org> wrote:
> > > > > > >
> > > > > > > I'm looking at execmem_types more as definition of the consum=
ers, maybe I
> > > > > > > should have named the enum execmem_consumer at the first plac=
e.
> > > > > >
> > > > > > I think looking at execmem_type from consumers' point of view a=
dds
> > > > > > unnecessary complexity. IIUC, for most (if not all) archs, ftra=
ce, kprobe,
> > > > > > and bpf (and maybe also module text) all have the same requirem=
ents.
> > > > > > Did I miss something?
> > > > >
> > > > > It's enough to have one architecture with different constrains fo=
r kprobes
> > > > > and bpf to warrant a type for each.
> > > >
> > > > AFAICT, some of these constraints can be changed without too much w=
ork.
> > >
> > > But why?
> > > I honestly don't understand what are you trying to optimize here. A f=
ew
> > > lines of initialization in execmem_info?
> >
> > IIUC, having separate EXECMEM_BPF and EXECMEM_KPROBE makes it
> > harder for bpf and kprobe to share the same ROX page. In many use cases=
,
> > a 2MiB page (assuming x86_64) is enough for all BPF, kprobe, ftrace, an=
d
> > module text. It is not efficient if we have to allocate separate pages =
for each
> > of these use cases. If this is not a problem, the current approach work=
s.
>
> The caching of large ROX pages does not need to be per type.
>
> In the POC I've posted for caching of large ROX pages on x86 [1], the cac=
he is
> global and to make kprobes and bpf use it it's enough to set a flag in
> execmem_info.
>
> [1] https://lore.kernel.org/all/20240411160526.2093408-1-rppt@kernel.org

For the ROX to work, we need different users (module text, kprobe, etc.) to=
 have
the same execmem_range. From [1]:

static void *execmem_cache_alloc(struct execmem_range *range, size_t size)
{
..
       p =3D __execmem_cache_alloc(size);
       if (p)
               return p;
      err =3D execmem_cache_populate(range, size);
..
}

We are calling __execmem_cache_alloc() without range. For this to work,
we can only call execmem_cache_alloc() with one execmem_range.

Did I miss something?

Thanks,
Song