[PATCH] mm: fix calculation accounting dirtyable highmem

[Minchan Kim]

When I tested vmscale in mmtest in 32bit, I found the benchmark
was slow down 0.5 times.

                base        node
                   1    global-1
User           12.98       16.04
System        147.61      166.42
Elapsed        26.48       38.08

With vmstat, I found IO wait avg is much increased compared to
base.

The reason was highmem_dirtyable_memory accumulates free pages
and highmem_file_pages from HIGHMEM to MOVABLE zones which was
wrong. With that, dirth_thresh in throtlle_vm_write is always
0 so that it calls congestion_wait frequently if writeback
starts.

With this patch, it is much recovered.

                base        node          fi
                   1    global-1         fix
User           12.98       16.04       13.78
System        147.61      166.42      143.92
Elapsed        26.48       38.08       29.64

Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/page-writeback.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 8db1db2..bf27594 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -307,27 +307,31 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
 {
 #ifdef CONFIG_HIGHMEM
 	int node;
-	unsigned long x = 0;
+	unsigned long x;
 	int i;
-	unsigned long dirtyable = highmem_file_pages;
+	unsigned long dirtyable = 0;
 
 	for_each_node_state(node, N_HIGH_MEMORY) {
 		for (i = ZONE_NORMAL + 1; i < MAX_NR_ZONES; i++) {
 			struct zone *z;
+			unsigned long nr_pages;
 
 			if (!is_highmem_idx(i))
 				continue;
 
 			z = &NODE_DATA(node)->node_zones[i];
-			dirtyable += zone_page_state(z, NR_FREE_PAGES);
+			if (!populated_zone(z))
+				continue;
 
+			nr_pages = zone_page_state(z, NR_FREE_PAGES);
 			/* watch for underflows */
-			dirtyable -= min(dirtyable, high_wmark_pages(z));
-
-			x += dirtyable;
+			nr_pages -= min(nr_pages, high_wmark_pages(z));
+			dirtyable += nr_pages;
 		}
 	}
 
+	x = dirtyable + highmem_file_pages;
+
 	/*
 	 * Unreclaimable memory (kernel memory or anonymous memory
 	 * without swap) can bring down the dirtyable pages below
-- 
1.9.1