linux-kernel - [PATCH v2] zsmalloc: fix obj_to_head use page

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Date:	Wed, 7 Oct 2015 12:45:52 +0800
From:	Hui Zhu <zhuhui@...omi.com>
To:	<minchan@...nel.org>, <ngupta@...are.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
CC:	<teawater@...il.com>, <akpm@...ux-foundation.org>,
	<sergey.senozhatsky@...il.com>, Hui Zhu <zhuhui@...omi.com>
Subject: [PATCH v2] zsmalloc: fix obj_to_head use page_private(page) as value but not pointer

In function obj_malloc:
	if (!class->huge)
		/* record handle in the header of allocated chunk */
		link->handle = handle;
	else
		/* record handle in first_page->private */
		set_page_private(first_page, handle);
The huge's page save handle to private directly.

But in obj_to_head:
	if (class->huge) {
		VM_BUG_ON(!is_first_page(page));
		return *(unsigned long *)page_private(page);
	} else
		return *(unsigned long *)obj;
It is used as a pointer.

The reason why there is no problem until now is huge-class page is
born with ZS_FULL so it couldn't be migrated.
Therefore, it shouldn't be real bug in practice.
However, we need this patch for future-work "VM-aware zsmalloced
page migration" to reduce external fragmentation.

Signed-off-by: Hui Zhu <zhuhui@...omi.com>
Acked-by: Minchan Kim <minchan@...nel.org>
---
 mm/zsmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index f135b1b..e881d4f 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -824,7 +824,7 @@ static unsigned long obj_to_head(struct size_class *class, struct page *page,
 {
 	if (class->huge) {
 		VM_BUG_ON(!is_first_page(page));
-		return *(unsigned long *)page_private(page);
+		return page_private(page);
 	} else
 		return *(unsigned long *)obj;
 }
-- 
1.9.1

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