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

[Liam R. Howlett]

* Jeff Xu <jeffxu at google.com> [240808 14:37]:
> On Thu, Aug 8, 2024 at 11:08 AM Liam R. Howlett <Liam.Howlett at oracle.com> wrote:
> >
> > * Jeff Xu <jeffxu at google.com> [240807 23:37]:
> > > 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.
> > > >
> > > This is a new point that I didn't realize before, if we are going to handle
> > > unmap vdso safely, yes, this is a bugfix that should be applied everywhere
> > > for all arch, without CHECKPOINT_RESTORE config.
> > >
> > > Do we need to worry about mmap(fixed) ? which can have the same effect
> > > as mremap.
> >
> > Yes, but it should be handled by vm_ops->close() when MAP_FIXED unmaps
> > the vdso.  Note that you cannot MAP_FIXED over half of the vma as the
> > vm_ops->may_split() is special_mapping_split(), which just returns
> > -EINVAL.
> >
> The may_split() failure logic is specific to vm_special_mapping, right ?

Not really, it's just what exists for these vmas vm_ops struct.  It's
called on every vma for every split in __split_vma().

> 
> Do we still need to keep vm_special_mapping struct , if we are going to
> treat  special vma as normal vma ?

No, just set the vm_ops may_split to something that returns -EINVAL.