mm: special mapping nopage

Convert special mapping install from nopage to fault.

Because the "vm_file" is NULL for the special mapping, the generic VM
code has messed up "vm_pgoff" thinking that it's an anonymous mapping
and the offset does't matter.  For that reason, we need to undo the
vm_pgoff offset that got added into vmf->pgoff.

[ We _really_ should clean that up - either by making this whole special
  mapping code just use a real file entry rather than that ugly array of
  "struct page" pointers, or by just making the VM code realize that
  even if vm_file is NULL it may not be a regular anonymous mmap.
							 - Linus ]
Signed-off-by: NNick Piggin <npiggin@suse.de>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -2165,24 +2165,31 @@ int may_expand_vm(struct mm_struct *mm, unsigned long npages)
 }
 
 
-static struct page *special_mapping_nopage(struct vm_area_struct *vma,
-					   unsigned long address, int *type)
+static int special_mapping_fault(struct vm_area_struct *vma,
+				struct vm_fault *vmf)
 {
+	pgoff_t pgoff;
 	struct page **pages;
 
-	BUG_ON(address < vma->vm_start || address >= vma->vm_end);
+	/*
+	 * special mappings have no vm_file, and in that case, the mm
+	 * uses vm_pgoff internally. So we have to subtract it from here.
+	 * We are allowed to do this because we are the mm; do not copy
+	 * this code into drivers!
+	 */
+	pgoff = vmf->pgoff - vma->vm_pgoff;
 
-	address -= vma->vm_start;
-	for (pages = vma->vm_private_data; address > 0 && *pages; ++pages)
-		address -= PAGE_SIZE;
+	for (pages = vma->vm_private_data; pgoff && *pages; ++pages)
+		pgoff--;
 
 	if (*pages) {
 		struct page *page = *pages;
 		get_page(page);
-		return page;
+		vmf->page = page;
+		return 0;
 	}
 
-	return NOPAGE_SIGBUS;
+	return VM_FAULT_SIGBUS;
 }
 
 /*
@@ -2194,7 +2201,7 @@ static void special_mapping_close(struct vm_area_struct *vma)
 
 static struct vm_operations_struct special_mapping_vmops = {
 	.close = special_mapping_close,
-	.nopage	= special_mapping_nopage,
+	.fault = special_mapping_fault,
 };
 
 /*