bcachefs: Make bch2_btree_cache_scan() try harder

Previously, when bch2_btree_cache_scan() attempted to reclaim a node but
failed (because trylock failed, because it was dirty, etc.), it would
count that against the number of nodes it was scanning and attempting to
free. This patch changes that behaviour, so that now we only count nodes
that we then don't free if they have the accessed bit (which we also
clear).
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_cache.c

@@ -327,17 +327,13 @@ static unsigned long bch2_btree_cache_scan(struct shrinker *shrink,
 	}
 restart:
 	list_for_each_entry_safe(b, t, &bc->live, list) {
-		touched++;
-
-		if (touched >= nr) {
-			/* Save position */
-			if (&t->list != &bc->live)
-				list_move_tail(&bc->live, &t->list);
-			break;
+		/* tweak this */
+		if (btree_node_accessed(b)) {
+			clear_btree_node_accessed(b);
+			goto touched;
 		}
 
-		if (!btree_node_accessed(b) &&
-		    !btree_node_reclaim(c, b)) {
+		if (!btree_node_reclaim(c, b)) {
 			/* can't call bch2_btree_node_hash_remove under lock  */
 			freed++;
 			if (&t->list != &bc->live)
@@ -358,8 +354,18 @@ static unsigned long bch2_btree_cache_scan(struct shrinker *shrink,
 			else if (!mutex_trylock(&bc->lock))
 				goto out;
 			goto restart;
-		} else
-			clear_btree_node_accessed(b);
+		} else {
+			continue;
+		}
+touched:
+		touched++;
+
+		if (touched >= nr) {
+			/* Save position */
+			if (&t->list != &bc->live)
+				list_move_tail(&bc->live, &t->list);
+			break;
+		}
 	}
 
 	mutex_unlock(&bc->lock);