mm/zsmalloc: add `freeable' column to pool stat

Add a new column to pool stats, which will tell how many pages ideally
can be freed by class compaction, so it will be easier to analyze
zsmalloc fragmentation.

At the moment, we have only numbers of FULL and ALMOST_EMPTY classes,
but they don't tell us how badly the class is fragmented internally.

The new /sys/kernel/debug/zsmalloc/zramX/classes output look as follows:

 class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage freeable
[..]
    12   224           0            2           146          5          8                4        4
    13   240           0            0             0          0          0                1        0
    14   256           1           13          1840       1672        115                1       10
    15   272           0            0             0          0          0                1        0
[..]
    49   816           0            3           745        735        149                1        2
    51   848           3            4           361        306         76                4        8
    52   864          12           14           378        268         81                3       21
    54   896           1           12           117         57         26                2       12
    57   944           0            0             0          0          0                3        0
[..]
 Total                26          131         12709      10994       1071                       134

For example, from this particular output we can easily conclude that
class-896 is heavily fragmented -- it occupies 26 pages, 12 can be freed
by compaction.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -494,6 +494,8 @@ static void __exit zs_stat_exit(void)
 	debugfs_remove_recursive(zs_stat_root);
 }
 
+static unsigned long zs_can_compact(struct size_class *class);
+
 static int zs_stats_size_show(struct seq_file *s, void *v)
 {
 	int i;
@@ -501,14 +503,15 @@ static int zs_stats_size_show(struct seq_file *s, void *v)
 	struct size_class *class;
 	int objs_per_zspage;
 	unsigned long class_almost_full, class_almost_empty;
-	unsigned long obj_allocated, obj_used, pages_used;
+	unsigned long obj_allocated, obj_used, pages_used, freeable;
 	unsigned long total_class_almost_full = 0, total_class_almost_empty = 0;
 	unsigned long total_objs = 0, total_used_objs = 0, total_pages = 0;
+	unsigned long total_freeable = 0;
 
-	seq_printf(s, " %5s %5s %11s %12s %13s %10s %10s %16s\n",
+	seq_printf(s, " %5s %5s %11s %12s %13s %10s %10s %16s %8s\n",
 			"class", "size", "almost_full", "almost_empty",
 			"obj_allocated", "obj_used", "pages_used",
-			"pages_per_zspage");
+			"pages_per_zspage", "freeable");
 
 	for (i = 0; i < zs_size_classes; i++) {
 		class = pool->size_class[i];
@@ -521,6 +524,7 @@ static int zs_stats_size_show(struct seq_file *s, void *v)
 		class_almost_empty = zs_stat_get(class, CLASS_ALMOST_EMPTY);
 		obj_allocated = zs_stat_get(class, OBJ_ALLOCATED);
 		obj_used = zs_stat_get(class, OBJ_USED);
+		freeable = zs_can_compact(class);
 		spin_unlock(&class->lock);
 
 		objs_per_zspage = get_maxobj_per_zspage(class->size,
@@ -528,23 +532,25 @@ static int zs_stats_size_show(struct seq_file *s, void *v)
 		pages_used = obj_allocated / objs_per_zspage *
 				class->pages_per_zspage;
 
-		seq_printf(s, " %5u %5u %11lu %12lu %13lu %10lu %10lu %16d\n",
+		seq_printf(s, " %5u %5u %11lu %12lu %13lu"
+				" %10lu %10lu %16d %8lu\n",
 			i, class->size, class_almost_full, class_almost_empty,
 			obj_allocated, obj_used, pages_used,
-			class->pages_per_zspage);
+			class->pages_per_zspage, freeable);
 
 		total_class_almost_full += class_almost_full;
 		total_class_almost_empty += class_almost_empty;
 		total_objs += obj_allocated;
 		total_used_objs += obj_used;
 		total_pages += pages_used;
+		total_freeable += freeable;
 	}
 
 	seq_puts(s, "\n");
-	seq_printf(s, " %5s %5s %11lu %12lu %13lu %10lu %10lu\n",
+	seq_printf(s, " %5s %5s %11lu %12lu %13lu %10lu %10lu %16s %8lu\n",
 			"Total", "", total_class_almost_full,
 			total_class_almost_empty, total_objs,
-			total_used_objs, total_pages);
+			total_used_objs, total_pages, "", total_freeable);
 
 	return 0;
 }