[RFC][PATCHv3 7/7] zsmalloc: register a shrinker to trigger auto-compaction

[Sergey Senozhatsky]

Perform automatic pool compaction by a shrinker when system
is getting tight on memory.

User-space has a very little knowledge regarding zsmalloc fragmentation
and basically has no mechanism to tell whether compaction will result
in any memory gain. Another issue is that user space is not always
aware of the fact that system is getting tight on memory. Which leads
to very uncomfortable scenarios when user space may start issuing
compaction 'randomly' or from crontab (for example). Fragmentation
is not always necessarily bad, allocated and unused objects, after all,
may be filled with the data later, w/o the need of allocating a new
zspage. On the other hand, we obviously don't want to waste memory
when systems needs it.

Compaction now has a relatively quick pool scan so we are able to
estimate the number of pages that will be freed easily, which makes it
possible to call this function from a shrinker->count_objects() callback.
We also abort compaction as soon as we detect that we can't free any
pages any more, preventing wasteful objects migrations.

Minchan Kim proposed to use the shrinker (the original patch was too
aggressive and was attempting to perform compaction for every
ALMOST_EMPTY zspage).

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Suggested-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/zsmalloc.c | 78 +++++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 71 insertions(+), 7 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index c9aea0a..692b7dc 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -247,7 +247,9 @@ struct zs_pool {
 	atomic_long_t		pages_allocated;
 	/* How many objects were migrated */
 	unsigned long		num_migrated;
-
+	/* Compact classes */
+	struct shrinker		shrinker;
+	bool			shrinker_enabled;
 #ifdef CONFIG_ZSMALLOC_STAT
 	struct dentry		*stat_dentry;
 #endif
@@ -1730,12 +1732,9 @@ static void __zs_compact(struct zs_pool *pool, struct size_class *class)
 
 		while ((dst_page = isolate_target_page(class))) {
 			cc.d_page = dst_page;
-			/*
-			 * If there is no more space in dst_page, resched
-			 * and see if anyone had allocated another zspage.
-			 */
+
 			if (!migrate_zspage(pool, class, &cc))
-				break;
+				goto out;
 
 			putback_zspage(pool, class, dst_page);
 		}
@@ -1750,7 +1749,9 @@ static void __zs_compact(struct zs_pool *pool, struct size_class *class)
 		cond_resched();
 		spin_lock(&class->lock);
 	}
-
+out:
+	if (dst_page)
+		putback_zspage(pool, class, dst_page);
 	if (src_page)
 		putback_zspage(pool, class, src_page);
 
@@ -1774,6 +1775,65 @@ unsigned long zs_compact(struct zs_pool *pool)
 }
 EXPORT_SYMBOL_GPL(zs_compact);
 
+static unsigned long zs_shrinker_scan(struct shrinker *shrinker,
+		struct shrink_control *sc)
+{
+	unsigned long freed;
+	struct zs_pool *pool = container_of(shrinker, struct zs_pool,
+			shrinker);
+
+	freed = pool->num_migrated;
+	/* Compact classes and calculate compaction delta */
+	freed = zs_compact(pool) - freed;
+
+	return freed ? freed : SHRINK_STOP;
+}
+
+static unsigned long zs_shrinker_count(struct shrinker *shrinker,
+		struct shrink_control *sc)
+{
+	int i;
+	struct size_class *class;
+	unsigned long to_free = 0;
+	struct zs_pool *pool = container_of(shrinker, struct zs_pool,
+			shrinker);
+
+	if (!pool->shrinker_enabled)
+		return 0;
+
+	for (i = zs_size_classes - 1; i >= 0; i--) {
+		class = pool->size_class[i];
+		if (!class)
+			continue;
+		if (class->index != i)
+			continue;
+
+		spin_lock(&class->lock);
+		to_free += zs_can_compact(class);
+		spin_unlock(&class->lock);
+	}
+
+	return to_free;
+}
+
+static void zs_unregister_shrinker(struct zs_pool *pool)
+{
+	if (pool->shrinker_enabled) {
+		unregister_shrinker(&pool->shrinker);
+		pool->shrinker_enabled = false;
+	}
+}
+
+static int zs_register_shrinker(struct zs_pool *pool)
+{
+	pool->shrinker.scan_objects = zs_shrinker_scan;
+	pool->shrinker.count_objects = zs_shrinker_count;
+	pool->shrinker.batch = 0;
+	pool->shrinker.seeks = DEFAULT_SEEKS;
+
+	return register_shrinker(&pool->shrinker);
+}
+
 /**
  * zs_create_pool - Creates an allocation pool to work from.
  * @flags: allocation flags used to allocate pool metadata
@@ -1859,6 +1919,9 @@ struct zs_pool *zs_create_pool(char *name, gfp_t flags)
 	if (zs_pool_stat_create(name, pool))
 		goto err;
 
+	/* Not critical, we still can use the pool */
+	if (zs_register_shrinker(pool) == 0)
+		pool->shrinker_enabled = true;
 	return pool;
 
 err:
@@ -1871,6 +1934,7 @@ void zs_destroy_pool(struct zs_pool *pool)
 {
 	int i;
 
+	zs_unregister_shrinker(pool);
 	zs_pool_stat_destroy(pool);
 
 	for (i = 0; i < zs_size_classes; i++) {
-- 
2.4.4

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