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

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Date:	Tue, 6 Oct 2015 19:59:03 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Hui Zhu <zhuhui@...omi.com>
Cc:	minchan@...nel.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

On (10/05/15 16:23), 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);
> 	} else
> 		return *(unsigned long *)obj;
> It is used as a pointer.
> 

um...
obj_to_head() is not for obj_malloc(), but for record_obj() that follows.
handle is a `void *' returned from alloc_handle()->kmem_cache_alloc(), and
casted to 'unsigned long'.

we store obj as:

static void record_obj(unsigned long handle, unsigned long obj)
{
	*(unsigned long *)handle = obj;
}

regardless `class->huge'.


and retrieve it as  `*(unsigned long *)foo', which is either
	`*(unsigned long *)page_private(page)'
or
	`*(unsigned long *)obj'

'return p' and `return *p' do slightly different things for pointers.


am I missing something?

	-ss

> So change obj_to_head use page_private(page) as value but not pointer
> in obj_to_head.
> 
> Signed-off-by: Hui Zhu <zhuhui@...omi.com>
> ---
>  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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