linux-kernel - Re: [PATCH] hugetlb: Provide a default HPAGE_SHIFT if !CONFIG_HUGETLB

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Date:	Thu, 17 Nov 2011 09:21:14 -0800
From:	David Daney <ddaney.cavm@...il.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	"ralf@...ux-mips.org" <ralf@...ux-mips.org>,
	"linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH] hugetlb: Provide a default HPAGE_SHIFT if !CONFIG_HUGETLB_PAGE

On 11/16/2011 08:56 PM, David Rientjes wrote:
> On Wed, 16 Nov 2011, David Daney wrote:
>
>>>> It is a well established kernel idiom to supply dummy values for symbols
>>>> that
>>>> are required to be defined in order to form a syntactically correct C
>>>> program,
>>>> but that are known by the programmer to be used only on dead code paths.
>>>>
>>>> This is exactly what we are doing here.
>>>>
>>>> To do otherwise requires that code be cluttered with #ifdefery.
>>>>
>>>
>>> You're wrong,
>>
>> Ok, then please tell me why:
>>
>> 1) the dummy version of is_vm_hugetlb_page() exists in hugetlb_inline.h?
>> 2) the dummy version of PageHuge() exists in hugetlb.h
[...]
>
> To avoid the #ifdef's,

Good, now we are on the same page.

The answer is not, as first suggested, to sprinkle #ifdefs all over the 
place, but ...

> but you'll notice that they return either NULL, 0,
> or are a no-op.  We don't substitute real values in dummy functions where
> they could be used in generic code as though they were valid.  That's the
> problem with defining HPAGE_SHIFT to be PAGE_SHIFT and, yes, defining
> HPAGE_MASK and HPAGE_SIZE as well shouldn't be done and should be removed.
>
> We expect HPAGE_* to represent hugepages, not the native page size of the
> bare metal.  This is why we do things like
>
> 	#define HPAGE_PMD_SHIFT ({ BUG(); 0; })
> 	#define HPAGE_PMD_MASK ({ BUG(); 0; })
> 	#define HPAGE_PMD_SIZE ({ BUG(); 0; })

... rather to fix HPAGE_SHIFT, HPAGE_MASK, and HPAGE_SIZE to be more 
like these.

I will generate a version of the patch that does this, noting that it 
the addition of a dummy definition of HPAGE_SHIFT is added to facilitate 
cleaner architecture specific code for MIPS patches in linux-next.

Thanks,
David Daney


>
> for CONFIG_TRANSPARENT_HUGEPAGE=n.  We don't want to pretend that they're
> valid outside the context of hugepages.
>
> We also never use HPAGE_SHIFT, HPAGE_MASK, or HPAGE_SIZE in generic code
> that isn't dependent on CONFIG_HUGETLB_PAGE.  If you'd like to submit a
> patch to fix this in the mips tree, then that would be good, and if you'd
> like to submit a patch to remove these dummy definitions all together by
> auditing other arch code, then that would be great.
>

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