Re: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem_free()

Mike Rapoport
2024-04-19T06:56:55Z

On Thu, Apr 18, 2024 at 02:01:22PM -0700, Song Liu wrote:
> On Thu, Apr 18, 2024 at 10:54 AM 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 AM Mike Rapoport <rppt@kernel.org> wrote:
> > > > > >
> > > > > > I'm looking at execmem_types more as definition of the consumers, maybe 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, kprobe,
> > > > > 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 kprobes
> > > > and bpf to warrant a type for each.
> > >
> > > AFAICT, some of these constraints can be changed without too much work.
> >
> > But why?
> > I honestly don't understand what are you trying to optimize here. A few
> > 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, and
> 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 works.

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 cache 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

> Thanks,
> Song

--
Sincerely yours,
Mike.