mm: hugetlb: add arch hook for clearing page flags before entering pool

The core page allocator ensures that page flags are zeroed when freeing
pages via free_pages_check. A number of architectures (ARM, PPC, MIPS)
rely on this property to treat new pages as dirty with respect to the data
cache and perform the appropriate flushing before mapping the pages into
userspace.

This can lead to cache synchronisation problems when using hugepages,
since the allocator keeps its own pool of pages above the usual page
allocator and does not reset the page flags when freeing a page into the
pool.

This patch adds a new architecture hook, arch_clear_hugepage_flags, so
that architectures which rely on the page flags being in a particular
state for fresh allocations can adjust the flags accordingly when a page
is freed into the pool.

Signed-off-by: Will Deacon <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Reviewed-by: Michal Hocko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/ia64/include/asm/hugetlb.h

···
 {
 }
 
+static inline void arch_clear_hugepage_flags(struct page *page)
+{
+}
+
 #endif /* _ASM_IA64_HUGETLB_H */

arch/mips/include/asm/hugetlb.h

···
 {
 }
 
+static inline void arch_clear_hugepage_flags(struct page *page)
+{
+}
+
 #endif /* __ASM_HUGETLB_H */

arch/powerpc/include/asm/hugetlb.h

···
 {
 }
 
+static inline void arch_clear_hugepage_flags(struct page *page)
+{
+}
+
 #else /* ! CONFIG_HUGETLB_PAGE */
 static inline void flush_hugetlb_page(struct vm_area_struct *vma,
 				      unsigned long vmaddr)

arch/s390/include/asm/hugetlb.h

···
 }
 
 #define hugetlb_prefault_arch_hook(mm)		do { } while (0)
+#define arch_clear_hugepage_flags(page)		do { } while (0)
 
 int arch_prepare_hugepage(struct page *page);
 void arch_release_hugepage(struct page *page);

arch/sh/include/asm/hugetlb.h

···
 #ifndef _ASM_SH_HUGETLB_H
 #define _ASM_SH_HUGETLB_H
 
+#include <asm/cacheflush.h>
 #include <asm/page.h>
 
 
···
 
 static inline void arch_release_hugepage(struct page *page)
 {
+}
+
+static inline void arch_clear_hugepage_flags(struct page *page)
+{
+	clear_bit(PG_dcache_clean, &page->flags);
 }
 
 #endif /* _ASM_SH_HUGETLB_H */

arch/sparc/include/asm/hugetlb.h

···
 {
 }
 
+static inline void arch_clear_hugepage_flags(struct page *page)
+{
+}
+
 #endif /* _ASM_SPARC64_HUGETLB_H */

arch/tile/include/asm/hugetlb.h

···
 {
 }
 
+static inline void arch_clear_hugepage_flags(struct page *page)
+{
+}
+
 #ifdef CONFIG_HUGETLB_SUPER_PAGES
 static inline pte_t arch_make_huge_pte(pte_t entry, struct vm_area_struct *vma,
 				       struct page *page, int writable)

arch/x86/include/asm/hugetlb.h

···
 {
 }
 
+static inline void arch_clear_hugepage_flags(struct page *page)
+{
+}
+
 #endif /* _ASM_X86_HUGETLB_H */

mm/hugetlb.c

···
 		h->surplus_huge_pages--;
 		h->surplus_huge_pages_node[nid]--;
 	} else {
+		arch_clear_hugepage_flags(page);
 		enqueue_huge_page(h, page);
 	}
 	spin_unlock(&hugetlb_lock);