linux-kernel - Re: [PATCH] x86/mm: fix regression with huge pages on PAE

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Date:	Tue, 24 Nov 2015 09:59:27 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Ingo Molnar <mingo@...nel.org>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>, hpa@...or.com,
	tglx@...utronix.de, mingo@...hat.com, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, x86@...nel.org,
	jgross@...e.com, konrad.wilk@...cle.com, elliott@....com,
	Toshi Kani <toshi.kani@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/mm: fix regression with huge pages on PAE

On 11/12/2015 04:00 AM, Kirill A. Shutemov wrote:
> On Thu, Nov 12, 2015 at 09:54:18AM +0100, Ingo Molnar wrote:
>> * Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:
>>
>>> diff --git a/arch/x86/include/asm/page_types.h b/arch/x86/include/asm/page_types.h
>>> index c5b7fb2774d0..cc071c6f7d4d 100644
>>> --- a/arch/x86/include/asm/page_types.h
>>> +++ b/arch/x86/include/asm/page_types.h


Kirill, where are we with this patch?

-boris

>>> @@ -9,19 +9,21 @@
>>>   #define PAGE_SIZE	(_AC(1,UL) << PAGE_SHIFT)
>>>   #define PAGE_MASK	(~(PAGE_SIZE-1))
>>>   
>>> +#define PMD_PAGE_SIZE		(_AC(1, UL) << PMD_SHIFT)
>>> +#define PMD_PAGE_MASK		(~(PMD_PAGE_SIZE-1))
>>> +
>>> +#define PUD_PAGE_SIZE		(_AC(1, UL) << PUD_SHIFT)
>>> +#define PUD_PAGE_MASK		(~(PUD_PAGE_SIZE-1))
>>> +
>>>   #define __PHYSICAL_MASK		((phys_addr_t)((1ULL << __PHYSICAL_MASK_SHIFT) - 1))
>>>   #define __VIRTUAL_MASK		((1UL << __VIRTUAL_MASK_SHIFT) - 1)
>>>   
>>> -/* Cast PAGE_MASK to a signed type so that it is sign-extended if
>>> +/* Cast *PAGE_MASK to a signed type so that it is sign-extended if
>>>      virtual addresses are 32-bits but physical addresses are larger
>>>      (ie, 32-bit PAE). */
>>>   #define PHYSICAL_PAGE_MASK	(((signed long)PAGE_MASK) & __PHYSICAL_MASK)
>>> -
>>> -#define PMD_PAGE_SIZE		(_AC(1, UL) << PMD_SHIFT)
>>> -#define PMD_PAGE_MASK		(~(PMD_PAGE_SIZE-1))
>>> -
>>> -#define PUD_PAGE_SIZE		(_AC(1, UL) << PUD_SHIFT)
>>> -#define PUD_PAGE_MASK		(~(PUD_PAGE_SIZE-1))
>>> +#define PHYSICAL_PMD_PAGE_MASK	(((signed long)PMD_PAGE_MASK) & __PHYSICAL_MASK)
>>> +#define PHYSICAL_PUD_PAGE_MASK	(((signed long)PUD_PAGE_MASK) & __PHYSICAL_MASK)
>> that's a really odd way of writing it, 'long' is signed by default ...
> See the comment above (it was there before the patch). 'signed' can be
> considered as documentation -- we want sign-extension here.
>
>> There seems to be 150+ such cases in the kernel source though - weird.
>>
>> More importantly, how does this improve things on 32-bit PAE kernels? If I follow
>> the values correctly then PMD_PAGE_MASK is 'UL' i.e. 32-bit:
>>
>>> +#define PMD_PAGE_SIZE                (_AC(1, UL) << PMD_SHIFT)
>>> +#define PMD_PAGE_MASK                (~(PMD_PAGE_SIZE-1))
>> thus PHYSICAL_PMD_PAGE_MASK is 32-bit too:
>>
>>> +#define PHYSICAL_PMD_PAGE_MASK       (((signed long)PMD_PAGE_MASK) & __PHYSICAL_MASK)
>> so how is the bug fixed?
> Again, see the comment.
> I've checked that it generates correct value (using kernel/bounds.c).
>

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