Re: [PATCH] thp: define HPAGE_PMD_* constans as BUILD_BUG() if !THP

[Kirill A. Shutemov]

Andrew Morton wrote:
> On Tue, 18 Jun 2013 01:05:40 +0300 "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> 
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > 
> > Currently, HPAGE_PMD_* constans rely on PMD_SHIFT regardless of
> > CONFIG_TRANSPARENT_HUGEPAGE. PMD_SHIFT is not defined everywhere (e.g.
> > arm nommu case).
> > 
> > It means we can't use anything like this in generic code:
> > 
> >         if (PageTransHuge(page))
> >                 zero_huge_user(page, 0, HPAGE_PMD_SIZE);
> >         else
> >                 clear_highpage(page);
> > 
> > For !THP case, PageTransHuge() is 0 and compiler can eliminate
> > zero_huge_user() call. But it still need to be valid C expression, means
> > HPAGE_PMD_SIZE has to expand to something compiler can understand.
> > 
> > Previously, HPAGE_PMD_* were defined to BUILD_BUG() for !THP. Let's come
> > back to it.
> > 
> > ...
> >
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -58,11 +58,12 @@ extern pmd_t *page_check_address_pmd(struct page *page,
> >  
> >  #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
> >  #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >  #define HPAGE_PMD_SHIFT PMD_SHIFT
> >  #define HPAGE_PMD_SIZE	((1UL) << HPAGE_PMD_SHIFT)
> >  #define HPAGE_PMD_MASK	(~(HPAGE_PMD_SIZE - 1))
> >  
> > -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >  extern bool is_vma_temporary_stack(struct vm_area_struct *vma);
> >  
> >  #define transparent_hugepage_enabled(__vma)				\
> > @@ -180,6 +181,9 @@ extern int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vm
> >  				unsigned long addr, pmd_t pmd, pmd_t *pmdp);
> >  
> >  #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> > +#define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
> > +#define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
> > +#define HPAGE_PMD_SIZE ({ BUILD_BUG(); 0; })
> >  
> 
> We've done this sort of thing before and it blew up.  We do want to be
> able to use things like HPAGE_PMD_foo in global-var initialisers and
> definitions, but the problem is that BUILD_BUG() can't be used outside
> functions.

I don't see how it's a blocker. For global variables, we will have to use
#ifdefs, but the approach is still useful for in-function code.

-- 
 Kirill A. Shutemov
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