mmap: change zeroing of maple tree in __vma_adjust()

Only write to the maple tree if we are not inserting or the insert isn't
going to overwrite the area to clear. This avoids spanning writes and
node coealescing when unnecessary.

The change requires a custom search for the linked list addition to find
the correct VMA for the prev link.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Liam R. Howlett <[e-mail]>
Tested-by: Yu Zhao <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Cc: "Matthew Wilcox (Oracle)" <[e-mail]>
Cc: SeongJae Park <[e-mail]>
Cc: Sven Schnelle <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/mmap.c

···
  * mm's list and the mm tree.  It has already been inserted into the interval tree.
  */
 static void __insert_vm_struct(struct mm_struct *mm, struct ma_state *mas,
-			       struct vm_area_struct *vma)
+		struct vm_area_struct *vma, unsigned long location)
 {
 	struct vm_area_struct *prev;
 
-	mas_set(mas, vma->vm_start);
+	mas_set(mas, location);
 	prev = mas_prev(mas, 0);
 	vma_mas_store(vma, mas);
 	__vma_link_list(mm, vma, prev);
···
 	int remove_next = 0;
 	MA_STATE(mas, &mm->mm_mt, 0, 0);
 	struct vm_area_struct *exporter = NULL, *importer = NULL;
+	unsigned long ll_prev = vma->vm_start; /* linked list prev. */
 
 	if (next && !insert) {
 		if (end >= next->vm_end) {
···
 	}
 
 	if (start != vma->vm_start) {
-		if (vma->vm_start < start)
+		if ((vma->vm_start < start) &&
+		    (!insert || (insert->vm_end != start))) {
 			vma_mas_szero(&mas, vma->vm_start, start);
-		vma_changed = true;
+			VM_WARN_ON(insert && insert->vm_start > vma->vm_start);
+		} else {
+			vma_changed = true;
+		}
 		vma->vm_start = start;
 	}
 	if (end != vma->vm_end) {
-		if (vma->vm_end > end)
-			vma_mas_szero(&mas, end, vma->vm_end);
-		vma_changed = true;
+		if (vma->vm_end > end) {
+			if (!insert || (insert->vm_start != end)) {
+				vma_mas_szero(&mas, end, vma->vm_end);
+				VM_WARN_ON(insert &&
+					   insert->vm_end < vma->vm_end);
+			} else if (insert->vm_start == end) {
+				ll_prev = vma->vm_end;
+			}
+		} else {
+			vma_changed = true;
+		}
 		vma->vm_end = end;
 		if (!next)
 			mm->highest_vm_end = vm_end_gap(vma);
···
 		 * us to insert it before dropping the locks
 		 * (it may either follow vma or precede it).
 		 */
-		__insert_vm_struct(mm, &mas, insert);
+		__insert_vm_struct(mm, &mas, insert, ll_prev);
 	}
 
 	if (anon_vma) {
···
 	if (insert && file)
 		uprobe_mmap(insert);
 
+	mas_destroy(&mas);
 	validate_mm(mm);
 	return 0;
 }