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

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Message-ID: <20151006135303.GA31853@blaptop>
Date:	Tue, 6 Oct 2015 22:54:31 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Hui Zhu <zhuhui@...omi.com>
Cc:	ngupta@...are.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, teawater@...il.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] zsmalloc: fix obj_to_head use page_private(page) as
 value but not pointer

Hello,

On Mon, Oct 05, 2015 at 04:23:01PM +0800, Hui Zhu wrote:
> 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 page_private(page);

Typo.
 		return *(unsigned long*)page_private(page);

Please fix the description.

> 	} else
> 		return *(unsigned long *)obj;
> It is used as a pointer.
> 
> So change obj_to_head use page_private(page) as value but not pointer
> in obj_to_head.

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>

With fixing the comment,

Acked-by: Minchan Kim <minchan@...nel.org>

Thanks for the fix, Hui.

-- 
Kind regards,
Minchan Kim
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