mremap: don't do uneccesary checks if new_len == old_len

The "new_len > old_len" branch in vma_to_resize() looks very confusing.
It only covers the VM_DONTEXPAND/pgoff checks but everything below is
equally unneeded if new_len == old_len.

Change this code to return if "new_len == old_len", new_len < old_len is
not possible, otherwise the code below is wrong anyway.

Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: Benjamin LaHaise <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jeff Moyer <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Laurent Dufour <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/mremap.c

@@ -346,6 +346,7 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma = find_vma(mm, addr);
+	unsigned long pgoff;
 
 	if (!vma || vma->vm_start > addr)
 		return ERR_PTR(-EFAULT);
@@ -357,17 +358,17 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
 	if (old_len > vma->vm_end - addr)
 		return ERR_PTR(-EFAULT);
 
+	if (new_len == old_len)
+		return vma;
+
 	/* Need to be careful about a growing mapping */
-	if (new_len > old_len) {
-		unsigned long pgoff;
-
-		if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
-			return ERR_PTR(-EFAULT);
-		pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
-		pgoff += vma->vm_pgoff;
-		if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
-			return ERR_PTR(-EINVAL);
-	}
+	pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
+	pgoff += vma->vm_pgoff;
+	if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
+		return ERR_PTR(-EINVAL);
+
+	if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
+		return ERR_PTR(-EFAULT);
 
 	if (vma->vm_flags & VM_LOCKED) {
 		unsigned long locked, lock_limit;