linux-kernel - Re: [PATCH]mm/vmalloc: interchage the implementation of vmalloc_to

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Date:	Thu, 28 Nov 2013 18:41:57 +0100
From:	Vladimir Murzin <murzin.v@...il.com>
To:	Jianyu Zhan <nasa4836@...il.com>
Cc:	linux-mm@...ck.org, akpm@...ux-foundation.org,
	iamjoonsoo.kim@....com, zhangyanfei@...fujitsu.com,
	liwanp@...ux.vnet.ibm.com, rientjes@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH]mm/vmalloc: interchage the implementation of
 vmalloc_to_{pfn,page}

On Fri, Nov 29, 2013 at 12:29:13AM +0800, Jianyu Zhan wrote:
> Currently we are implementing vmalloc_to_pfn() as a wrapper of 
> vmalloc_to_page(), which is implemented as follow: 
> 
>  1. walks the page talbes to generates the corresponding pfn,
>  2. then wraps the pfn to struct page,
>  3. returns it.
> 
> And vmalloc_to_pfn() re-wraps the vmalloc_to_page() to get the pfn.
> 
> This seems too circuitous, so this patch reverses the way:
> implementing the vmalloc_to_page() as a wrapper of vmalloc_to_pfn().
> This makes vmalloc_to_pfn() and vmalloc_to_page() slightly effective.

Any numbers for efficiency?

> 

> No functional change. 
> 
> Signed-off-by: Jianyu Zhan <nasa4836@...il.com>
> ---
> mm/vmalloc.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 0fdf968..a335e21 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -220,12 +220,12 @@ int is_vmalloc_or_module_addr(const void *x)
>  }
>  
>  /*
> - * Walk a vmap address to the struct page it maps.
> + * Walk a vmap address to the physical pfn it maps to.
>   */
> -struct page *vmalloc_to_page(const void *vmalloc_addr)
> +unsigned long vmalloc_to_pfn(const void *vmalloc_addr)
>  {
>  	unsigned long addr = (unsigned long) vmalloc_addr;
> -	struct page *page = NULL;
> +	unsigned long pfn;

uninitialized pfn will lead to a bug.

>  	pgd_t *pgd = pgd_offset_k(addr);
>  
>  	/*
> @@ -244,23 +244,23 @@ struct page *vmalloc_to_page(const void *vmalloc_addr)
>  				ptep = pte_offset_map(pmd, addr);
>  				pte = *ptep;
>  				if (pte_present(pte))
> -					page = pte_page(pte);
> +					pfn = pte_page(pte);

page_to_pfn is missed here.

Have you ever tested there is no functional changes?

Vladimir

>  				pte_unmap(ptep);
>  			}
>  		}
>  	}
> -	return page;
> +	return pfn;
>  }
> -EXPORT_SYMBOL(vmalloc_to_page);
> +EXPORT_SYMBOL(vmalloc_to_pfn);
>  
>  /*
> - * Map a vmalloc()-space virtual address to the physical page frame number.
> + * Map a vmalloc()-space virtual address to the struct page.
>   */
> -unsigned long vmalloc_to_pfn(const void *vmalloc_addr)
> +struct page *vmalloc_to_page(const void *vmalloc_addr)
>  {
> -	return page_to_pfn(vmalloc_to_page(vmalloc_addr));
> +	return pfn_to_page(vmalloc_to_pfn(vmalloc_addr));
>  }
> -EXPORT_SYMBOL(vmalloc_to_pfn);
> +EXPORT_SYMBOL(vmalloc_to_page);
>  
>  
>  /*** Global kva allocator ***/
> 
> --
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