[RFC][PATCH v2 1/3] mm/zsmalloc: introduce zs_get_huge_class_size_watermark()

[Sergey Senozhatsky]

From: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>

zsmalloc knows the watermark after which classes are considered
to be ->huge -- every object stored consumes the entire zspage (which
consist of a single order-0 page). On x86_64, PAGE_SHIFT 12 box, the
first non-huge class size is 3264, so starting down from size 3264,
objects share page(-s) and thus minimize memory wastage.

zram, however, has its own statically defined watermark for `bad'
compression "3 * PAGE_SIZE / 4 = 3072", and stores every object
larger than this watermark (3072) as a PAGE_SIZE, object, IOW,
to a ->huge class, this results in increased memory consumption and
memory wastage. (With a small exception: 3264 bytes class. zs_malloc()
adds ZS_HANDLE_SIZE to the object's size, so some objects can pass
3072 bytes and get_size_class_index(size) will return 3264 bytes size
class).

Introduce a zs_get_huge_class_size_watermark() function which tells
the size of a first non-huge class; so zram now can store objects
to ->huge clases only when those objects have sizes greater than
huge_class_size_watermark.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
 include/linux/zsmalloc.h |  2 ++
 mm/zsmalloc.c            | 14 ++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
index 34eb160..45dcb51 100644
--- a/include/linux/zsmalloc.h
+++ b/include/linux/zsmalloc.h
@@ -55,4 +55,6 @@ 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);
+
+int zs_get_huge_class_size_watermark(void);
 #endif
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 43e4cbc..e7f10bd 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -188,6 +188,11 @@ static struct dentry *zs_stat_root;
 static int zs_size_classes;
 
 /*
+ * All classes above this class_size are huge classes
+ */
+static int huge_class_size_watermark;
+
+/*
  * We assign a page to ZS_ALMOST_EMPTY fullness group when:
  *	n <= N / f, where
  * n = number of allocated objects
@@ -1241,6 +1246,12 @@ unsigned long zs_get_total_pages(struct zs_pool *pool)
 }
 EXPORT_SYMBOL_GPL(zs_get_total_pages);
 
+int zs_get_huge_class_size_watermark(void)
+{
+	return huge_class_size_watermark;
+}
+EXPORT_SYMBOL_GPL(zs_get_huge_class_size_watermark);
+
 /**
  * zs_map_object - get address of allocated object from handle.
  * @pool: pool from which the object was allocated
@@ -1942,10 +1953,13 @@ struct zs_pool *zs_create_pool(const char *name, gfp_t flags)
 		if (pages_per_zspage == 1 &&
 			get_maxobj_per_zspage(size, pages_per_zspage) == 1)
 			class->huge = true;
+
 		spin_lock_init(&class->lock);
 		pool->size_class[i] = class;
 
 		prev_class = class;
+		if (!class->huge && !huge_class_size_watermark)
+			huge_class_size_watermark = size - ZS_HANDLE_SIZE;
 	}
 
 	pool->flags = flags;
-- 
2.7.1