Re: [mm][RFC][PATCH 0/11] mm accessor updates.

Peter Zijlstra
2009-12-16T23:02:49Z

On Wed, 2009-12-16 at 10:27 -0600, Christoph Lameter wrote:
> On Wed, 16 Dec 2009, Andi Kleen wrote:
>
> > > Do you have alternative recommendation rather than wrapping all accesses by
> > > special functions ?
> >
> > Work out what changes need to be done for ranged mmap locks and do them all
> > in one pass.
>
> Locking ranges is already possible through the split ptlock and
> could be enhanced through placing locks in the vma structures.
>
> That does nothing solve the basic locking issues of mmap_sem. We need
> Kame-sans abstraction layer. A vma based lock or a ptlock still needs to
> ensure that the mm struct does not vanish while the lock is held.

It should, you shouldn't be able to remove a mm while there's still
vma's around, and you shouldn't be able to remove a vma when there's
still pagetables around. And if you rcu-free all of them you're stable
enough for lots of speculative behaviour.

No need to retain mmap_sem for any of that.

As for per-vma locks, those are pretty much useless too, there's plenty
applications doing lots of work on a few very large vmas.