Re: [PATCH] mm: softdirty: unmapped addresses between VMAs are clean

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 10 Sep 2014 22:41:04 -0700 Peter Feiner <pfeiner@google.com> wrote:

> On Wed, Sep 10, 2014 at 04:36:28PM -0700, Andrew Morton wrote:
> > On Wed, 10 Sep 2014 16:24:46 -0700 Peter Feiner <pfeiner@google.com> wrote:
> > > @@ -1048,32 +1048,51 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> > > +	while (1) {
> > > +		unsigned long vm_start = end;
> > 
> > Did you really mean to do that?  If so, perhaps a little comment to
> > explain how it works?
> 
> It's the same idea that I used in the pagemap_pte_hole patch that I submitted
> today: if vma is NULL, then we fill in the pagemap from (addr) to (end) with
> non-present pagemap entries. 
> 
> Should I submit a v2 with a comment?

I spent quite some time staring at that code wondering wtf, so anything
you can do to clarify it would be good.

I think a better name would be plain old "start", to communicate that
it's just a local convenience variable.  "vm_start" means "start of a
vma" and that isn't accurate in this context; in fact it is misleading.