Re: [PATCH] hugetlb: Provide a default HPAGE_SHIFT if !CONFIG_HUGETLB_PAGE

[David Daney]

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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