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. For example, --- a/fs/open.c +++ b/fs/open.c @@ -34,6 +34,11 @@ #include "internal.h" +#define FOOB ({ BUILD_BUG(); 0; }) + +int x = FOOB; +int y[FOOB]; + int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs, struct file *filp) { fs/open.c:39: error: braced-group within expression allowed only inside a function fs/open.c:40: error: braced-group within expression allowed only inside a function -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>