Re: [PATCH 5/6] Have ia64 use add_active_range() and free_area_init_nodes

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 15 May 2006 19:29:18 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Mon, 15 May 2006 11:19:27 +0100
> Andy Whitcroft <apw@shadowen.org> wrote:
> 
> > Nick Piggin wrote:
> > > Andy Whitcroft wrote:
> > > 
> > >> Interesting.  You are correct there was no config component, at the time
> > >> I didn't have direct evidence that any architecture needed it, only that
> > >> we had an unchecked requirement on zones, a requirement that had only
> > >> recently arrived with the changes to free buddy detection.  I note that
> > > 
> > > 
> > > Recently arrived? Over a year ago with the no-buddy-bitmap patches,
> > > right? Just checking because I that's what I'm assuming broke it...
> > 
> > Yep, sorry I forget I was out of the game for 6 months!  And yes that
> > was when the requirements were altered.
> > 
> When no-bitmap-buddy patches was included,
> 
> 1. bad_range() is not covered by CONFIG_VM_DEBUG. It always worked.
> ==
> static int bad_range(struct zone *zone, struct page *page)
> {
>         if (page_to_pfn(page) >= zone->zone_start_pfn + zone->spanned_pages)
>                 return 1;
>         if (page_to_pfn(page) < zone->zone_start_pfn)
>                 return 1;
> ==
> And , this code
> ==
>                 buddy = __page_find_buddy(page, page_idx, order);
> 
>                 if (bad_range(zone, buddy))
>                         break;
> ==
> 
> checked whether buddy is in zone and guarantees it to have page struct.
> 
> 
> But clean-up/speed-up codes vanished these checks. (I don't know when this occurs)
> Sorry for misses these things.
> 

One more point
When above no-bitmap patches was included, the user of not-aligned zones
are only ia64, I think. Because ia64 used virtual mem_map, page_to_pfn(page)
on  CONFIG_DISCONTIG_MEM doesn't access page struct itself. 

#define page_to_pfn(page)	(page - vmemmap)

So, it didn't  panic. ia64/vmemmap was safe.

If other archs used not-aligned zone + CONFIG_DISCONTIGMEM,
not-aligned-zones problem would come out earlier.

-Kame

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