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Subject: Re: [PATCH 1/1] dma-mapping: to avoid exception when cpu_addr is NULL
To: Andrew Morton <akpm@linux-foundation.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Will Deacon <will.deacon@arm.com>,
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References: <1457342485-16628-1-git-send-email-thunder.leizhen@huawei.com>
CC: Zefan Li <lizefan@huawei.com>, Xinwei Hu <huxinwei@huawei.com>,
        "Tianhong Ding" <dingtianhong@huawei.com>,
        Hanjun Guo <guohanjun@huawei.com>
From: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
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Date: Mon, 7 Mar 2016 18:43:47 +0800
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Suppose:
CONFIG_SPARSEMEM is opened.
CONFIG_DMA_API_DEBUG or CONFIG_CMA is opened.

Then virt_to_page or phys_to_page will be called. Finally, in __pfn_to_page, __sec = __pfn_to_section(__pfn) is NULL.
So access section->section_mem_map will trigger exception.

---------

#define __pfn_to_page(pfn)				\
({	unsigned long __pfn = (pfn);			\
	struct mem_section *__sec = __pfn_to_section(__pfn);	\
	__section_mem_map_addr(__sec) + __pfn;		\
})

static inline struct page *__section_mem_map_addr(struct mem_section *section)
{
	unsigned long map = section->section_mem_map;
	map &= SECTION_MAP_MASK;
	return (struct page *)map;
}


On 2016/3/7 17:21, Zhen Lei wrote:
> Do this to keep consistent with kfree, which tolerate ptr is NULL.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
>  include/linux/dma-mapping.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 75857cd..fdd4294 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -402,7 +402,10 @@ static inline void *dma_alloc_coherent(struct device *dev, size_t size,
>  static inline void dma_free_coherent(struct device *dev, size_t size,
>  		void *cpu_addr, dma_addr_t dma_handle)
>  {
> -	return dma_free_attrs(dev, size, cpu_addr, dma_handle, NULL);
> +	if (unlikely(!cpu_addr))
> +		return;
> +
> +	dma_free_attrs(dev, size, cpu_addr, dma_handle, NULL);
>  }
> 
>  static inline void *dma_alloc_noncoherent(struct device *dev, size_t size,
> --
> 2.5.0
> 
> 
> 
> .
>