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Date: Mon, 7 Mar 2016 14:59:07 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Zefan Li <lizefan@huawei.com>, Xinwei Hu <huxinwei@huawei.com>,
        "Tianhong Ding" <dingtianhong@huawei.com>,
        Hanjun Guo <guohanjun@huawei.com>
Subject: Re: [PATCH 1/1] dma-mapping: to avoid exception when cpu_addr is
 NULL
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On Mon, 7 Mar 2016 18:43:47 +0800 "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com> wrote:

> 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;
> }

I'm having a bit of trouble understanding this.

Perhaps you could explain the bug more carefully (inclusion of an oops
output would help) then we'll be in a better position to understand the
proposed fix(es).

Thanks.