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>

include/linux/mmzone.h

@@ -356,10 +356,10 @@ struct zone {
 	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
 	/*

include/linux/swap.h

@@ -287,7 +287,6 @@ static inline void workingset_node_shadows_dec(struct radix_tree_node *node)
 /* 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);
 

mm/page-writeback.c

@@ -278,7 +278,12 @@ static unsigned long zone_dirtyable_memory(struct zone *zone)
 	unsigned long nr_pages;
 
 	nr_pages = zone_page_state(zone, NR_FREE_PAGES);
-	nr_pages -= min(nr_pages, zone->dirty_balance_reserve);
+	/*
+	 * Pages reserved for the kernel should not be considered
+	 * dirtyable, to prevent a situation where reclaim has to
+	 * clean pages in order to balance the zones.
+	 */
+	nr_pages -= min(nr_pages, zone->totalreserve_pages);
 
 	nr_pages += zone_page_state(zone, NR_INACTIVE_FILE);
 	nr_pages += zone_page_state(zone, NR_ACTIVE_FILE);
@@ -332,7 +337,12 @@ static unsigned long global_dirtyable_memory(void)
 	unsigned long x;
 
 	x = global_page_state(NR_FREE_PAGES);
-	x -= min(x, dirty_balance_reserve);
+	/*
+	 * Pages reserved for the kernel should not be considered
+	 * dirtyable, to prevent a situation where reclaim has to
+	 * clean pages in order to balance the zones.
+	 */
+	x -= min(x, totalreserve_pages);
 
 	x += global_page_state(NR_INACTIVE_FILE);
 	x += global_page_state(NR_ACTIVE_FILE);

mm/page_alloc.c

@@ -114,13 +114,6 @@ static DEFINE_SPINLOCK(managed_page_count_lock);
 unsigned long totalram_pages __read_mostly;
 unsigned long totalreserve_pages __read_mostly;
 unsigned long totalcma_pages __read_mostly;
-/*
- * When calculating the number of globally allowed dirty pages, there
- * is a certain number of per-zone reserves that should not be
- * considered dirtyable memory.  This is the sum of those reserves
- * over all existing zones that contribute dirtyable memory.
- */
-unsigned long dirty_balance_reserve __read_mostly;
 
 int percpu_pagelist_fraction;
 gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
@@ -5942,20 +5935,12 @@ static void calculate_totalreserve_pages(void)
 
 			if (max > zone->managed_pages)
 				max = zone->managed_pages;
+
+			zone->totalreserve_pages = max;
+
 			reserve_pages += max;
-			/*
-			 * Lowmem reserves are not available to
-			 * GFP_HIGHUSER page cache allocations and
-			 * kswapd tries to balance zones to their high
-			 * watermark.  As a result, neither should be
-			 * regarded as dirtyable memory, to prevent a
-			 * situation where reclaim has to clean pages
-			 * in order to balance the zones.
-			 */
-			zone->dirty_balance_reserve = max;
 		}
 	}
-	dirty_balance_reserve = reserve_pages;
 	totalreserve_pages = reserve_pages;
 }