[PATCH] vmscan: drive everything via nr_to_scan

Page reclaim is currently a bit schitzo: sometimes we say "go and scan this
many pages and tell me how many pages were freed" and at other times we say
"go and scan this many pages, but stop if you freed this many".

It makes the logic harder to control and to understand.  This patch coverts
everything into the "go and scan this many pages and tell me how many pages
were freed" model.

It doesn't seem to affect performance much either way.

mm/vmscan.c

@@ -468,39 +468,31 @@ shrink_list(struct list_head *page_list, unsigned int gfp_mask, int *nr_scanned)
  * a batch of pages and working on them outside the lock.  Any pages which were
  * not freed will be added back to the LRU.
  *
- * shrink_cache() is passed the number of pages to try to free, and returns
- * the number of pages which were reclaimed.
+ * shrink_cache() is passed the number of pages to scan and returns the number
+ * of pages which were reclaimed.
  *
  * For pagecache intensive workloads, the first loop here is the hottest spot
  * in the kernel (apart from the copy_*_user functions).
  */
 static int
-shrink_cache(const int nr_pages, struct zone *zone,
-		unsigned int gfp_mask, int max_scan, int *total_scanned)
+shrink_cache(struct zone *zone, unsigned int gfp_mask,
+		int max_scan, int *total_scanned)
 {
 	LIST_HEAD(page_list);
 	struct pagevec pvec;
-	int nr_to_process;
 	int ret = 0;
 
-	/*
-	 * Try to ensure that we free `nr_pages' pages in one pass of the loop.
-	 */
-	nr_to_process = nr_pages;
-	if (nr_to_process < SWAP_CLUSTER_MAX)
-		nr_to_process = SWAP_CLUSTER_MAX;
-
 	pagevec_init(&pvec, 1);
 
 	lru_add_drain();
 	spin_lock_irq(&zone->lru_lock);
-	while (max_scan > 0 && ret < nr_pages) {
+	while (max_scan > 0) {
 		struct page *page;
 		int nr_taken = 0;
 		int nr_scan = 0;
 		int nr_freed;
 
-		while (nr_scan++ < nr_to_process &&
+		while (nr_scan++ < SWAP_CLUSTER_MAX &&
 				!list_empty(&zone->inactive_list)) {
 			page = list_entry(zone->inactive_list.prev,
 						struct page, lru);
@@ -742,13 +734,12 @@ refill_inactive_zone(struct zone *zone, const int nr_pages_in,
 }
 
 /*
- * Try to reclaim `nr_pages' from this zone.  Returns the number of reclaimed
- * pages.  This is a basic per-zone page freer.  Used by both kswapd and
- * direct reclaim.
+ * Scan `nr_pages' from this zone.  Returns the number of reclaimed pages.
+ * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
  */
 static int
 shrink_zone(struct zone *zone, int max_scan, unsigned int gfp_mask,
-	const int nr_pages, int *total_scanned, struct page_state *ps)
+		int *total_scanned, struct page_state *ps)
 {
 	unsigned long ratio;
 
@@ -762,7 +753,7 @@ shrink_zone(struct zone *zone, int max_scan, unsigned int gfp_mask,
 	 * just to make sure that the kernel will slowly sift through the
 	 * active list.
 	 */
-	ratio = (unsigned long)nr_pages * zone->nr_active /
+	ratio = (unsigned long)SWAP_CLUSTER_MAX * zone->nr_active /
 				((zone->nr_inactive | 1) * 2);
 	atomic_add(ratio+1, &zone->refill_counter);
 	if (atomic_read(&zone->refill_counter) > SWAP_CLUSTER_MAX) {
@@ -780,8 +771,7 @@ shrink_zone(struct zone *zone, int max_scan, unsigned int gfp_mask,
 		atomic_set(&zone->refill_counter, 0);
 		refill_inactive_zone(zone, count, ps);
 	}
-	return shrink_cache(nr_pages, zone, gfp_mask,
-				max_scan, total_scanned);
+	return shrink_cache(zone, gfp_mask, max_scan, total_scanned);
 }
 
 /*
@@ -802,13 +792,12 @@ shrink_zone(struct zone *zone, int max_scan, unsigned int gfp_mask,
  */
 static int
 shrink_caches(struct zone **zones, int priority, int *total_scanned,
-		int gfp_mask, int nr_pages, struct page_state *ps)
+		int gfp_mask, struct page_state *ps)
 {
 	int ret = 0;
 	int i;
 
 	for (i = 0; zones[i] != NULL; i++) {
-		int to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX);
 		struct zone *zone = zones[i];
 		int max_scan;
 
@@ -818,15 +807,9 @@ shrink_caches(struct zone **zones, int priority, int *total_scanned,
 		if (zone->all_unreclaimable && priority != DEF_PRIORITY)
 			continue;	/* Let kswapd poll it */
 
-		/*
-		 * If we cannot reclaim `nr_pages' pages by scanning twice
-		 * that many pages then fall back to the next zone.
-		 */
-		max_scan = zone->nr_inactive >> priority;
-		if (max_scan < to_reclaim * 2)
-			max_scan = to_reclaim * 2;
-		ret += shrink_zone(zone, max_scan, gfp_mask,
-				to_reclaim, total_scanned, ps);
+		max_scan = max(zone->nr_inactive >> priority,
+				SWAP_CLUSTER_MAX * 2UL);
+		ret += shrink_zone(zone, max_scan, gfp_mask, total_scanned, ps);
 	}
 	return ret;
 }
@@ -853,7 +836,6 @@ int try_to_free_pages(struct zone **zones,
 {
 	int priority;
 	int ret = 0;
-	const int nr_pages = SWAP_CLUSTER_MAX;
 	int nr_reclaimed = 0;
 	struct reclaim_state *reclaim_state = current->reclaim_state;
 	int i;
@@ -869,13 +851,13 @@ int try_to_free_pages(struct zone **zones,
 
 		get_page_state(&ps);
 		nr_reclaimed += shrink_caches(zones, priority, &total_scanned,
-						gfp_mask, nr_pages, &ps);
+						gfp_mask, &ps);
 		shrink_slab(total_scanned, gfp_mask);
 		if (reclaim_state) {
 			nr_reclaimed += reclaim_state->reclaimed_slab;
 			reclaim_state->reclaimed_slab = 0;
 		}
-		if (nr_reclaimed >= nr_pages) {
+		if (nr_reclaimed >= SWAP_CLUSTER_MAX) {
 			ret = 1;
 			goto out;
 		}
@@ -939,27 +921,21 @@ static int balance_pgdat(pg_data_t *pgdat, int nr_pages, struct page_state *ps)
 			struct zone *zone = pgdat->node_zones + i;
 			int total_scanned = 0;
 			int max_scan;
-			int to_reclaim;
 			int reclaimed;
 
 			if (zone->all_unreclaimable && priority != DEF_PRIORITY)
 				continue;
 
-			if (nr_pages) {		/* Software suspend */
-				to_reclaim = min(to_free, SWAP_CLUSTER_MAX*8);
-			} else {		/* Zone balancing */
-				to_reclaim = zone->pages_high-zone->free_pages;
-				if (to_reclaim <= 0)
-					continue;
+			if (nr_pages == 0) {	/* Not software suspend */
+				if (zone->free_pages <= zone->pages_high)
+					all_zones_ok = 0;
 			}
 			zone->temp_priority = priority;
 			max_scan = zone->nr_inactive >> priority;
-			if (max_scan < to_reclaim * 2)
-				max_scan = to_reclaim * 2;
 			if (max_scan < SWAP_CLUSTER_MAX)
 				max_scan = SWAP_CLUSTER_MAX;
 			reclaimed = shrink_zone(zone, max_scan, GFP_KERNEL,
-					to_reclaim, &total_scanned, ps);
+					&total_scanned, ps);
 			reclaim_state->reclaimed_slab = 0;
 			shrink_slab(total_scanned, GFP_KERNEL);
 			reclaimed += reclaim_state->reclaimed_slab;
@@ -968,16 +944,6 @@ static int balance_pgdat(pg_data_t *pgdat, int nr_pages, struct page_state *ps)
 				continue;
 			if (zone->pages_scanned > zone->present_pages * 2)
 				zone->all_unreclaimable = 1;
-			/*
-			 * If this scan failed to reclaim `to_reclaim' or more
-			 * pages, we're getting into trouble.  Need to scan
-			 * some more, and throttle kswapd.   Note that this zone
-			 * may now have sufficient free pages due to freeing
-			 * activity by some other process.   That's OK - we'll
-			 * pick that info up on the next pass through the loop.
-			 */
-			if (reclaimed < to_reclaim)
-				all_zones_ok = 0;
 		}
 		if (nr_pages && to_free > 0)
 			continue;	/* swsusp: need to do more work */