mm/zsmalloc.c: count in handle's size when calculating pages_per_zspage

[Yinghao Xie]

From 159d74b5a8f3d0f07e18ddad74b7025cf17dcc69 Mon Sep 17 00:00:00 2001
From: Yinghao Xie <yinghao.xie@xxxxxxxxxxx>
Date: Thu, 19 Mar 2015 19:32:25 +0800
Subject: [PATCH] mm/zsmalloc.c: count in handle's size when calculating
 size_class's pages_per_zspage

1. Fix wastage calculation;
2. Indirect handle introduced extra ZS_HANDLE_SIZE size for each object,it's transparent
   for upper function, but a size_class's total objects will changed:
   take the 43rd class which class_size = 32 + 43 * 16 = 720 as example:
	4096 * 1 % 720 = 496
	4096 * 2 % 720 = 272
	4096 * 3 % 720 = 48 
	4096 * 4 %720 = 544
   after handle introduced,class_size + ZS_HANDLE_SIZE (4 on 32bit) = 724
	4096 * 1 % 724 = 476
	4096 * 2 % 724 = 228
	4096 * 3 % 724 = 704
	4096 * 4 % 724 = 456
    Clearly, ZS_HANDLE_SIZE should be considered when calculating pages_per_zspage;

3. in get_size_class_index(), min(zs_size_classes - 1, idx) insures a huge class's
   index <= zs_size_classes - 1, so it's no need to check again;

Signed-off-by: Yinghao Xie <yinghao.xie@xxxxxxxxxxx>
---
 mm/zsmalloc.c |   15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 461243e..64c379b 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -760,7 +760,8 @@ out:
  * to form a zspage for each size class. This is important
  * to reduce wastage due to unusable space left at end of
  * each zspage which is given as:
- *	wastage = Zp - Zp % size_class
+ *	wastage = Zp % (class_size + ZS_HANDLE_SIZE)
+ *	usage = Zp - wastage
  * where Zp = zspage size = k * PAGE_SIZE where k = 1, 2, ...
  *
  * For example, for size class of 3/8 * PAGE_SIZE, we should
@@ -773,6 +774,9 @@ static int get_pages_per_zspage(int class_size)
 	/* zspage order which gives maximum used size per KB */
 	int max_usedpc_order = 1;
 
+	if (class_size > ZS_MAX_ALLOC_SIZE)
+		class_size = ZS_MAX_ALLOC_SIZE;
+
 	for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
 		int zspage_size;
 		int waste, usedpc;
@@ -1426,11 +1430,6 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
 	/* extra space in chunk to keep the handle */
 	size += ZS_HANDLE_SIZE;
 	class = pool->size_class[get_size_class_index(size)];
-	/* In huge class size, we store the handle into first_page->private */
-	if (class->huge) {
-		size -= ZS_HANDLE_SIZE;
-		class = pool->size_class[get_size_class_index(size)];
-	}
 
 	spin_lock(&class->lock);
 	first_page = find_get_zspage(class);
@@ -1856,9 +1855,7 @@ struct zs_pool *zs_create_pool(char *name, gfp_t flags)
 		struct size_class *class;
 
 		size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
-		if (size > ZS_MAX_ALLOC_SIZE)
-			size = ZS_MAX_ALLOC_SIZE;
-		pages_per_zspage = get_pages_per_zspage(size);
+		pages_per_zspage = get_pages_per_zspage(size + ZS_HANDLE_SIZE);
 
 		/*
 		 * size_class is used for normal zsmalloc operation such
-- 
1.7.9.5