Re: [PATCH] mm/page_alloc: return 0 in case this node has no page within the zone

From: Michal Hocko
Date: Wed Feb 22 2017 - 03:51:07 EST

On Thu 09-02-17 21:59:29, Wei Yang wrote:
> On Tue, Feb 07, 2017 at 04:41:21PM +0100, Michal Hocko wrote:
> >On Tue 07-02-17 23:32:47, Wei Yang wrote:
> >> On Tue, Feb 07, 2017 at 10:45:57AM +0100, Michal Hocko wrote:
> >[...]
> >> >Is there any reason why for_each_mem_pfn_range cannot be changed to
> >> >honor the given start/end pfns instead? I can imagine that a small zone
> >> >would see a similar pointless iterations...
> >> >
> >>
> >> Hmm... No special reason, just not thought about this implementation. And
> >> actually I just do the similar thing as in zone_spanned_pages_in_node(), in
> >> which also return 0 when there is no overlap.
> >>
> >> BTW, I don't get your point. You wish to put the check in
> >> for_each_mem_pfn_range() definition?
> >
> >My point was that you are handling one special case (an empty zone) but
> >the underlying problem is that __absent_pages_in_range might be wasting
> >cycles iterating over memblocks that are way outside of the given pfn
> >range. At least this is my understanding. If you fix that you do not
> >need the special case, right?
> >--
> >Michal Hocko
> >SUSE Labs
> > Not really, sorry, this area is full of awkward and subtle code when new
> > changes build on top of previous awkwardness/surprises. Any cleanup
> > would be really appreciated. That is the reason I didn't like the
> > initial check all that much.
> Looks my fetchmail failed to get your last reply. So I copied it here.
> Yes, the change here looks not that nice, while currently this is what I can't
> come up with.

THen I will suggest dropping this patch from the mmotm tree because it
doesn't sound like a big improvement and I would encourage you or
anybody else to take a deeper look and unclutter this area to be more
readable and better maintainable.
Michal Hocko