mm: re-architect the VM_UNPAGED logic

This replaces the (in my opinion horrible) VM_UNMAPPED logic with very
explicit support for a "remapped page range" aka VM_PFNMAP.  It allows a
VM area to contain an arbitrary range of page table entries that the VM
never touches, and never considers to be normal pages.

Any user of "remap_pfn_range()" automatically gets this new
functionality, and doesn't even have to mark the pages reserved or
indeed mark them any other way.  It just works.  As a side effect, doing
mmap() on /dev/mem works for arbitrary ranges.

Sparc update from David in the next commit.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/fremap.c

@@ -27,24 +27,20 @@ static int zap_pte(struct mm_struct *mm, struct vm_area_struct *vma,
 	struct page *page = NULL;
 
 	if (pte_present(pte)) {
-		unsigned long pfn = pte_pfn(pte);
-		flush_cache_page(vma, addr, pfn);
+		flush_cache_page(vma, addr, pte_pfn(pte));
 		pte = ptep_clear_flush(vma, addr, ptep);
-		if (unlikely(!pfn_valid(pfn))) {
-			print_bad_pte(vma, pte, addr);
-			goto out;
+		page = vm_normal_page(vma, addr, pte);
+		if (page) {
+			if (pte_dirty(pte))
+				set_page_dirty(page);
+			page_remove_rmap(page);
+			page_cache_release(page);
 		}
-		page = pfn_to_page(pfn);
-		if (pte_dirty(pte))
-			set_page_dirty(page);
-		page_remove_rmap(page);
-		page_cache_release(page);
 	} else {
 		if (!pte_file(pte))
 			free_swap_and_cache(pte_to_swp_entry(pte));
 		pte_clear(mm, addr, ptep);
 	}
-out:
 	return !!page;
 }
 
@@ -65,8 +61,6 @@ int install_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	pte_t pte_val;
 	spinlock_t *ptl;
 
-	BUG_ON(vma->vm_flags & VM_UNPAGED);
-
 	pgd = pgd_offset(mm, addr);
 	pud = pud_alloc(mm, pgd, addr);
 	if (!pud)
@@ -122,8 +116,6 @@ int install_file_pte(struct mm_struct *mm, struct vm_area_struct *vma,
 	pte_t pte_val;
 	spinlock_t *ptl;
 
-	BUG_ON(vma->vm_flags & VM_UNPAGED);
-
 	pgd = pgd_offset(mm, addr);
 	pud = pud_alloc(mm, pgd, addr);
 	if (!pud)