[PATCH 2/4] powerpc/mm: Handle VDSO unmapping via close() rather than arch_unmap()

[Jeff Xu]

On Wed, Aug 7, 2024 at 8:21 PM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> On Wed, 7 Aug 2024 at 16:20, Liam R. Howlett <Liam.Howlett at oracle.com> wrote:
> >
> > Okay, I'm going to try one more time here.  You are suggesting to have a
> > conf flag to leave the vdso pointer unchanged when it is unmapped.
> > Having the close behind the conf option will not prevent it from being
> > unmapped or mapped over, so what you are suggesting is have a
> > configuration option that leaves a pointer, mm->context.vdso, to be
> > unsafe if it is unmapped if you disable checkpoint restore.
>
> We definitely do not want that kind of complexity. It makes the kernel
> just more complicated to have to deal with both cases.
>
> That said, I don't love how special powerpc is here.
>
> What we could do is to is
>
>  - stop calling these things "special mappings", and just admit that
> it's for different vdso mappings and nothing else (for some odd reason
> arm and nios2 calls it a "kuser helper" rather than vdso, but it's the
> exact same thing)
>
>  - don't do this whole indirect function pointer thing with mremap and
> close at all, and just do this all unapologetically and for all
> architectures in the generic VM layer together with "if (vma->vm_start
> == mm->context.vdso)" etc.
>
> that would get rid of the conceptual complexity of having different
> architectures doing different things (and the unnecessary overhead of
> having an indirect function pointer that just points to one single
> thing).
>
> But I think the current "clean up the existing mess" is probably the
> less invasive one over "make the existing mess be explicitly about
> vdso and avoid unnecessary per-architecture differences".
>
> If people want to, we can do the unification (and stop pretending the
> "special mappings" could be something else) later.

OK. Now this is clear to me (at last).

Treating vdso mapping, (maybe all the special mappings) as normal
mapping and handling mmap/mremap/munmap properly from userspace will
indeed make things clear conceptually. I'm OK with doing this later
since it is a big change.

Thanks
-Jeff

>
>          Linus