[PATCH] mm: move_pte to remap ZERO_PAGE

Move the ZERO_PAGE remapping complexity to the move_pte macro in
asm-generic, have it conditionally depend on
__HAVE_ARCH_MULTIPLE_ZERO_PAGE, which gets defined for MIPS.

For architectures without __HAVE_ARCH_MULTIPLE_ZERO_PAGE, move_pte becomes
a noop.

From: Hugh Dickins <hugh@veritas.com>

Fix nasty little bug we've missed in Nick's mremap move ZERO_PAGE patch.
The "pte" at that point may be a swap entry or a pte_file entry: we must
check pte_present before perhaps corrupting such an entry.

Patch below against 2.6.14-rc2-mm1, but the same bug is in 2.6.14-rc2's
mm/mremap.c, and more dangerous there since it's affecting all arches: I
think the safest course is to send Nick's patch and Yoichi's build fix and
this fix (build tested) on to Linus - so only MIPS can be affected.
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/asm-generic/pgtable.h

@@ -158,6 +158,19 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
 #define lazy_mmu_prot_update(pte)	do { } while (0)
 #endif
 
+#ifndef __HAVE_ARCH_MULTIPLE_ZERO_PAGE
+#define move_pte(pte, prot, old_addr, new_addr)	(pte)
+#else
+#define move_pte(pte, prot, old_addr, new_addr)				\
+({									\
+ 	pte_t newpte = (pte);						\
+	if (pte_present(pte) && pfn_valid(pte_pfn(pte)) &&		\
+			pte_page(pte) == ZERO_PAGE(old_addr))		\
+		newpte = mk_pte(ZERO_PAGE(new_addr), (prot));		\
+	newpte;								\
+})
+#endif
+
 /*
  * When walking page tables, get the address of the next boundary,
  * or the end address of the range if that comes earlier.  Although no

include/asm-mips/pgtable.h

@@ -68,6 +68,8 @@ extern unsigned long zero_page_mask;
 #define ZERO_PAGE(vaddr) \
 	(virt_to_page(empty_zero_page + (((unsigned long)(vaddr)) & zero_page_mask)))
 
+#define __HAVE_ARCH_MULTIPLE_ZERO_PAGE
+
 extern void paging_init(void);
 
 /*

mm/mremap.c

@@ -141,10 +141,10 @@ move_one_page(struct vm_area_struct *vma, unsigned long old_addr,
 			if (dst) {
 				pte_t pte;
 				pte = ptep_clear_flush(vma, old_addr, src);
+
 				/* ZERO_PAGE can be dependant on virtual addr */
-				if (pfn_valid(pte_pfn(pte)) &&
-					pte_page(pte) == ZERO_PAGE(old_addr))
-					pte = pte_wrprotect(mk_pte(ZERO_PAGE(new_addr), new_vma->vm_page_prot));
+				pte = move_pte(pte, new_vma->vm_page_prot,
+							old_addr, new_addr);
 				set_pte_at(mm, new_addr, dst, pte);
 			} else
 				error = -ENOMEM;