zsmalloc/zram: introduce zs_pool_stats api

`zs_compact_control' accounts the number of migrated objects but it has
a limited lifespan -- we lose it as soon as zs_compaction() returns back
to zram. It worked fine, because (a) zram had it's own counter of
migrated objects and (b) only zram could trigger compaction. However,
this does not work for automatic pool compaction (not issued by zram).
To account objects migrated during auto-compaction (issued by the
shrinker) we need to store this number in zs_pool.

Define a new `struct zs_pool_stats' structure to keep zs_pool's stats
there. It provides only `num_migrated', as of this writing, but it
surely can be extended.

A new zsmalloc zs_pool_stats() symbol exports zs_pool's stats back to
caller.

Use zs_pool_stats() in zram and remove `num_migrated' from zram_stats.

Signed-off-by: Sergey Senozhatsky <[e-mail]>
Suggested-by: Minchan Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/block/zram/zram_drv.c

···
 static ssize_t compact_store(struct device *dev,
 		struct device_attribute *attr, const char *buf, size_t len)
 {
-	unsigned long nr_migrated;
 	struct zram *zram = dev_to_zram(dev);
 	struct zram_meta *meta;
 
···
 	}
 
 	meta = zram->meta;
-	nr_migrated = zs_compact(meta->mem_pool);
-	atomic64_add(nr_migrated, &zram->stats.num_migrated);
+	zs_compact(meta->mem_pool);
 	up_read(&zram->init_lock);
 
 	return len;
···
 		struct device_attribute *attr, char *buf)
 {
 	struct zram *zram = dev_to_zram(dev);
+	struct zs_pool_stats pool_stats;
 	u64 orig_size, mem_used = 0;
 	long max_used;
 	ssize_t ret;
 
+	memset(&pool_stats, 0x00, sizeof(struct zs_pool_stats));
+
 	down_read(&zram->init_lock);
-	if (init_done(zram))
+	if (init_done(zram)) {
 		mem_used = zs_get_total_pages(zram->meta->mem_pool);
+		zs_pool_stats(zram->meta->mem_pool, &pool_stats);
+	}
 
 	orig_size = atomic64_read(&zram->stats.pages_stored);
 	max_used = atomic_long_read(&zram->stats.max_used_pages);
 
 	ret = scnprintf(buf, PAGE_SIZE,
-			"%8llu %8llu %8llu %8lu %8ld %8llu %8llu\n",
+			"%8llu %8llu %8llu %8lu %8ld %8llu %8lu\n",
 			orig_size << PAGE_SHIFT,
 			(u64)atomic64_read(&zram->stats.compr_data_size),
 			mem_used << PAGE_SHIFT,
 			zram->limit_pages << PAGE_SHIFT,
 			max_used << PAGE_SHIFT,
 			(u64)atomic64_read(&zram->stats.zero_pages),
-			(u64)atomic64_read(&zram->stats.num_migrated));
+			pool_stats.num_migrated);
 	up_read(&zram->init_lock);
 
 	return ret;

drivers/block/zram/zram_drv.h

···
 	atomic64_t compr_data_size;	/* compressed size of pages stored */
 	atomic64_t num_reads;	/* failed + successful */
 	atomic64_t num_writes;	/* --do-- */
-	atomic64_t num_migrated;	/* no. of migrated object */
 	atomic64_t failed_reads;	/* can happen when memory is too low */
 	atomic64_t failed_writes;	/* can happen when memory is too low */
 	atomic64_t invalid_io;	/* non-page-aligned I/O requests */

include/linux/zsmalloc.h

···
 	 */
 };
 
+struct zs_pool_stats {
+	/* How many objects were migrated */
+	unsigned long num_migrated;
+};
+
 struct zs_pool;
 
 struct zs_pool *zs_create_pool(char *name, gfp_t flags);
···
 unsigned long zs_get_total_pages(struct zs_pool *pool);
 unsigned long zs_compact(struct zs_pool *pool);
 
+void zs_pool_stats(struct zs_pool *pool, struct zs_pool_stats *stats);
 #endif

mm/zsmalloc.c

···
 	gfp_t flags;	/* allocation flags used when growing pool */
 	atomic_long_t pages_allocated;
 
+	struct zs_pool_stats stats;
 #ifdef CONFIG_ZSMALLOC_STAT
 	struct dentry *stat_dentry;
 #endif
···
 	 /* Starting object index within @s_page which used for live object
 	  * in the subpage. */
 	int index;
-	/* how many of objects are migrated */
+	/* How many of objects were migrated */
 	int nr_migrated;
 };
 
···
 	struct page *s_page = cc->s_page;
 	struct page *d_page = cc->d_page;
 	unsigned long index = cc->index;
-	int nr_migrated = 0;
 	int ret = 0;
 
 	while (1) {
···
 		record_obj(handle, free_obj);
 		unpin_tag(handle);
 		obj_free(pool, class, used_obj);
-		nr_migrated++;
+		cc->nr_migrated++;
 	}
 
 	/* Remember last position in this iteration */
 	cc->s_page = s_page;
 	cc->index = index;
-	cc->nr_migrated = nr_migrated;
 
 	return ret;
 }
···
 	return obj_wasted * get_pages_per_zspage(class->size);
 }
 
-static unsigned long __zs_compact(struct zs_pool *pool,
-				struct size_class *class)
+static void __zs_compact(struct zs_pool *pool, struct size_class *class)
 {
 	struct zs_compact_control cc;
 	struct page *src_page;
 	struct page *dst_page = NULL;
-	unsigned long nr_total_migrated = 0;
 
+	cc.nr_migrated = 0;
 	spin_lock(&class->lock);
 	while ((src_page = isolate_source_page(class))) {
 
···
 				break;
 
 			putback_zspage(pool, class, dst_page);
-			nr_total_migrated += cc.nr_migrated;
 		}
 
 		/* Stop if we couldn't find slot */
···
 		putback_zspage(pool, class, dst_page);
 		putback_zspage(pool, class, src_page);
 		spin_unlock(&class->lock);
-		nr_total_migrated += cc.nr_migrated;
 		cond_resched();
 		spin_lock(&class->lock);
 	}
···
 	if (src_page)
 		putback_zspage(pool, class, src_page);
 
-	spin_unlock(&class->lock);
+	pool->stats.num_migrated += cc.nr_migrated;
 
-	return nr_total_migrated;
+	spin_unlock(&class->lock);
 }
 
 unsigned long zs_compact(struct zs_pool *pool)
 {
 	int i;
-	unsigned long nr_migrated = 0;
 	struct size_class *class;
 
 	for (i = zs_size_classes - 1; i >= 0; i--) {
···
 			continue;
 		if (class->index != i)
 			continue;
-		nr_migrated += __zs_compact(pool, class);
+		__zs_compact(pool, class);
 	}
 
-	return nr_migrated;
+	return pool->stats.num_migrated;
 }
 EXPORT_SYMBOL_GPL(zs_compact);
+
+void zs_pool_stats(struct zs_pool *pool, struct zs_pool_stats *stats)
+{
+	memcpy(stats, &pool->stats, sizeof(struct zs_pool_stats));
+}
+EXPORT_SYMBOL_GPL(zs_pool_stats);
 
 /**
  * zs_create_pool - Creates an allocation pool to work from.