linux-kernel - Re: [PATCH 1/2] arm64: Get existing page protections in split

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Message-ID: <563C0131.2060707@redhat.com>
Date:	Thu, 5 Nov 2015 17:24:01 -0800
From:	Laura Abbott <labbott@...hat.com>
To:	Xishi Qiu <qiuxishi@...wei.com>,
	Laura Abbott <labbott@...oraproject.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 1/2] arm64: Get existing page protections in split_pmd

On 11/05/2015 02:15 AM, Xishi Qiu wrote:
> On 2015/11/4 5:48, Laura Abbott wrote:
>
>>
>> Rather than always putting the least restrictived permissions
>> (PAGE_KERNEL_EXEC) when spliting a pmd into pages, use
>> the existing permissions from the pmd for the page.
>>
>> Signed-off-by: Laura Abbott <labbott@...oraproject.org>
>> ---
>>   arch/arm64/mm/mmu.c | 9 ++++-----
>>   1 file changed, 4 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 9211b85..ff41efa 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -75,14 +75,13 @@ static void __init *early_alloc(unsigned long sz)
>>   static void split_pmd(pmd_t *pmd, pte_t *pte)
>>   {
>>   	unsigned long pfn = pmd_pfn(*pmd);
>> +	unsigned long addr = pfn << PAGE_SHIFT;
>> +	pgprot_t prot = __pgprot(pmd_val(*pmd) ^ addr) | PTE_TYPE_PAGE;
>> +
>
> Hi Laura,
>
> I'm not quite understand, I find split_pud() doesn't set the flag
> PMD_TYPE_TABLE. If we clear xx_TABLE_BIT, does that means the page
> is large page?

I'm assuming by large page you mean a block mapping. Yes, without
that entry the kernel treats this as a block mapping

> And what is the different from the flag xx_TYPE_SECT?

That would mark it this as a block mapping which is not what we want
here.

>
> Thanks,
> Xishi Qiu
>



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