Re: [PATCH] mm: memmap_init_zone() performance improvement

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

On Tue, 06 Nov 2012 09:03:26 -0700
Mike Yoknis <mike.yoknis@hp.com> wrote:

> On Tue, 2012-10-30 at 09:14 -0600, Dave Hansen wrote:
> > On 10/20/2012 01:29 AM, Mel Gorman wrote:
> > > I'm travelling at the moment so apologies that I have not followed up on
> > > this. My problem is still the same with the patch - it changes more
> > > headers than is necessary and it is sparsemem specific. At minimum, try
> > > the suggestion of
> > >
> > > if (!early_pfn_valid(pfn)) {
> > >       pfn = ALIGN(pfn + MAX_ORDER_NR_PAGES, MAX_ORDER_NR_PAGES) - 1;
> > >       continue;
> > > }
> > 
> > Sorry I didn't catch this until v2...
> > 
> > Is that ALIGN() correct?  If pfn=3, then it would expand to:
> > 
> > (3+MAX_ORDER_NR_PAGES+MAX_ORDER_NR_PAGES-1) & ~(MAX_ORDER_NR_PAGES-1)
> > 
> > You would end up skipping the current MAX_ORDER_NR_PAGES area, and then
> > one _extra_ because ALIGN() aligns up, and you're adding
> > MAX_ORDER_NR_PAGES too.  It doesn't matter unless you run in to a
> > !early_valid_pfn() in the middle of a MAX_ORDER area, I guess.
> > 
> > I think this would work, plus be a bit smaller:
> > 
> >         pfn = ALIGN(pfn + 1, MAX_ORDER_NR_PAGES) - 1;
> > 
> Dave,
> I see your point about "rounding-up".  But, I favor the way Mel
> suggested it.  It more clearly shows the intent, which is to move up by
> MAX_ORDER_NR_PAGES.  The "pfn+1" may suggest that there is some
> significance to the next pfn, but there is not.
> I find Mel's way easier to understand.

I don't think that really answers Dave's question.  What happens if we
"run in to a !early_valid_pfn() in the middle of a MAX_ORDER area"?