Re: [PATCH]mmap: avoid unnecessary anon_vma lock

[Andi Kleen <andi@firstfloor.org>]

On Tue, Mar 29, 2011 at 03:35:17PM -0700, Andrew Morton wrote:
> On Mon, 28 Mar 2011 09:57:39 -0700
> Andi Kleen <andi@firstfloor.org> wrote:
> 
> > Shaohua Li <shaohua.li@intel.com> writes:
> > 
> > > If we only change vma->vm_end, we can avoid taking anon_vma lock even 'insert'
> > > isn't NULL, which is the case of split_vma.
> > > From my understanding, we need the lock before because rmap must get the
> > > 'insert' VMA when we adjust old VMA's vm_end (the 'insert' VMA is linked to
> > > anon_vma list in __insert_vm_struct before).
> > > But now this isn't true any more. The 'insert' VMA is already linked to
> > > anon_vma list in __split_vma(with anon_vma_clone()) instead of
> > > __insert_vm_struct. There is no race rmap can't get required VMAs.
> > > So the anon_vma lock is unnecessary, and this can reduce one locking in brk
> > > case and improve scalability.
> > 
> > Looks good to me.
> 
> Looks way too tricky to me.
> 
> Please review this code for maintainability.  Have we documented what
> we're doing as completely and as clearly as we are able?

I agree the comments could be improved, but the code change looked good
to me. I don't think it impacts maintainability by itself because
we already do similar magic.

-Andi