mm: page_alloc: generalize the dirty balance reserve

The dirty balance reserve that dirty throttling has to consider is
merely memory not available to userspace allocations.  There is nothing
writeback-specific about it.  Generalize the name so that it's reusable
outside of that context.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 3b6fb71..33bb1b1 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -356,10 +356,10 @@
 	struct per_cpu_pageset __percpu *pageset;
 
 	/*
-	 * This is a per-zone reserve of pages that should not be
-	 * considered dirtyable memory.
+	 * This is a per-zone reserve of pages that are not available
+	 * to userspace allocations.
 	 */
-	unsigned long		dirty_balance_reserve;
+	unsigned long		totalreserve_pages;
 
 #ifndef CONFIG_SPARSEMEM
 	/*
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 7ba7dcca..066bd21 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -287,7 +287,6 @@
 /* linux/mm/page_alloc.c */
 extern unsigned long totalram_pages;
 extern unsigned long totalreserve_pages;
-extern unsigned long dirty_balance_reserve;
 extern unsigned long nr_free_buffer_pages(void);
 extern unsigned long nr_free_pagecache_pages(void);