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Message-ID: <57844872.1060806@huawei.com>
Date: Tue, 12 Jul 2016 09:31:30 +0800
From: Xishi Qiu <qiuxishi@huawei.com>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
CC: <alan@lxorguk.ukuu.org.uk>, Linux MM <linux-mm@kvack.org>,
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Subject: Re: a question about protection_map[]
References: <5783710E.3070602@huawei.com> <20160711133015.GA8028@node.shutemov.name>
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On 2016/7/11 21:30, Kirill A. Shutemov wrote:

> On Mon, Jul 11, 2016 at 06:12:30PM +0800, Xishi Qiu wrote:
>> Hi,
>>
>> We can use mprotect to set read only or read/write.
>>
>> mprotect_fixup()
>> 	vma_set_page_prot()
>> 		vm_pgprot_modify()
>> 			vm_get_page_prot()
>> 				protection_map[vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]
>>
>> The following code shows that prots from __P001(PROT_READ) and __P010(PROT_WRITE)
>> are the same, so how does it distinguish read only or read/write from mprotect?
> 
> It doesn't.
> 
> Write protection will be removed by fault handler on next write access to
> the page. Somewhat suboptiomal, but zero page implemenation relies on this
> to work properly.
> 

Hi Kirill,

I know, PAGE_READONLY and PAGE_COPY are both missed _PAGE_RW,
so it will cause page fault, then we will set new prot flag from
vma, right?

Thanks,
Xishi Qiu