memory hotplug: fix page_zone() calculation in test_pages_isolated()

My last bugfix here (adding zone->lock) introduced a new problem: Using
page_zone(pfn_to_page(pfn)) to get the zone after the for() loop is wrong.
 pfn will then be >= end_pfn, which may be in a different zone or not
present at all.  This may lead to an addressing exception in page_zone()
or spin_lock_irqsave().

Now I use __first_valid_page() again after the loop to find a valid page
for page_zone().
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Acked-by: Nathan Fontenot <nfont@austin.ibm.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_isolation.c

@@ -130,10 +130,11 @@ int test_pages_isolated(unsigned long start_pfn, unsigned long end_pfn)
 		if (page && get_pageblock_migratetype(page) != MIGRATE_ISOLATE)
 			break;
 	}
-	if (pfn < end_pfn)
+	page = __first_valid_page(start_pfn, end_pfn - start_pfn);
+	if ((pfn < end_pfn) || !page)
 		return -EBUSY;
 	/* Check all pages are free or Marked as ISOLATED */
-	zone = page_zone(pfn_to_page(pfn));
+	zone = page_zone(page);
 	spin_lock_irqsave(&zone->lock, flags);
 	ret = __test_page_isolated_in_pageblock(start_pfn, end_pfn);
 	spin_unlock_irqrestore(&zone->lock, flags);