[PATCH] mm: follow_page invalid pte_page

The follow_page write-access case is relying on pte_page before checking
pfn_valid: rearrange that - and we don't need three struct page *pages.

(I notice mempolicy.c's verify_pages is also relying on pte_page, but I'll
leave that to Andi: maybe it ought to be failing on, or skipping over, VM_IO
or VM_RESERVED vmas?)
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/memory.c

@@ -637,15 +637,11 @@ follow_page(struct mm_struct *mm, unsigned long address, int write)
 	if (pte_present(pte)) {
 		if (write && !pte_write(pte))
 			goto out;
-		if (write && !pte_dirty(pte)) {
-			struct page *page = pte_page(pte);
-			if (!PageDirty(page))
-				set_page_dirty(page);
-		}
 		pfn = pte_pfn(pte);
 		if (pfn_valid(pfn)) {
-			struct page *page = pfn_to_page(pfn);
-			
+			page = pfn_to_page(pfn);
+			if (write && !pte_dirty(pte) && !PageDirty(page))
+				set_page_dirty(page);
 			mark_page_accessed(page);
 			return page;
 		}