memory hotplug: fix updating of num_physpages for hot plugged memory

Sizing of memory allocations shouldn't depend on the number of physical
pages found in a system, as that generally includes (perhaps a huge amount
of) non-RAM pages.  The amount of what actually is usable as storage
should instead be used as a basis here.

In line with that, the memory hotplug code should update num_physpages in
a way that it retains its original (post-boot) meaning; in particular,
decreasing the value should at best be done with great care - this patch
doesn't try to ever decrease this value at all as it doesn't really seem
meaningful to do so.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: 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/memory_hotplug.c

@@ -339,8 +339,11 @@ EXPORT_SYMBOL_GPL(__remove_pages);
 
 void online_page(struct page *page)
 {
+	unsigned long pfn = page_to_pfn(page);
+
 	totalram_pages++;
-	num_physpages++;
+	if (pfn >= num_physpages)
+		num_physpages = pfn + 1;
 
 #ifdef CONFIG_HIGHMEM
 	if (PageHighMem(page))
@@ -832,7 +835,6 @@ repeat:
 	zone->present_pages -= offlined_pages;
 	zone->zone_pgdat->node_present_pages -= offlined_pages;
 	totalram_pages -= offlined_pages;
-	num_physpages -= offlined_pages;
 
 	setup_per_zone_wmarks();
 	calculate_zone_inactive_ratio(zone);