[RFC][PATCH v3 3/5] mm/zsmalloc: introduce zs_huge_object()

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

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 zs_huge_object() function which tells whether the supplied
object's size belongs to a 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@gmail.com>
---
 include/linux/zsmalloc.h |  2 ++
 mm/zsmalloc.c            | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
index 34eb160..7184ee1 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);
+
+bool zs_huge_object(size_t sz);
 #endif
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 0bb060f..06a7d87 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 size_t huge_class_size_watermark;
+
+/*
  * We assign a page to ZS_ALMOST_EMPTY fullness group when:
  *	n <= N / f, where
  * n = number of allocated objects
@@ -1244,6 +1249,12 @@ unsigned long zs_get_total_pages(struct zs_pool *pool)
 }
 EXPORT_SYMBOL_GPL(zs_get_total_pages);
 
+bool zs_huge_object(size_t sz)
+{
+	return sz > huge_class_size_watermark;
+}
+EXPORT_SYMBOL_GPL(zs_huge_object);
+
 /**
  * zs_map_object - get address of allocated object from handle.
  * @pool: pool from which the object was allocated
@@ -1922,6 +1933,8 @@ struct zs_pool *zs_create_pool(const char *name, gfp_t flags)
 		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;
-- 
2.8.0.rc0

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