mm: vma_adjust: remove superfluous confusing update in remove_next ==…

… 1 case

mm->highest_vm_end doesn't need any update.

After finally removing the oddness from vma_merge case 8 that was
causing:

1) constant risk of trouble whenever anybody would check vma fields
   from rmap_walks, like it happened when page migration was
   introduced and it read the vma->vm_page_prot from a rmap_walk

2) the callers of vma_merge to re-initialize any value different from
   the current vma, instead of vma_merge() more reliably returning a
   vma that already matches all fields passed as parameter

.. it is also worth to take the opportunity of cleaning up superfluous
code in vma_adjust(), that if not removed adds up to the hard
readability of the function.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Andrea Arcangeli <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Jan Vorlicek <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/mmap.c

@@ -817,8 +817,28 @@ int vma_adjust(struct vm_area_struct *vma, unsigned long start,
 		}
 		else if (next)
 			vma_gap_update(next);
-		else
-			mm->highest_vm_end = end;
+		else {
+			/*
+			 * If remove_next == 2 we obviously can't
+			 * reach this path.
+			 *
+			 * If remove_next == 3 we can't reach this
+			 * path because pre-swap() next is always not
+			 * NULL. pre-swap() "next" is not being
+			 * removed and its next->vm_end is not altered
+			 * (and furthermore "end" already matches
+			 * next->vm_end in remove_next == 3).
+			 *
+			 * We reach this only in the remove_next == 1
+			 * case if the "next" vma that was removed was
+			 * the highest vma of the mm. However in such
+			 * case next->vm_end == "end" and the extended
+			 * "vma" has vma->vm_end == next->vm_end so
+			 * mm->highest_vm_end doesn't need any update
+			 * in remove_next == 1 case.
+			 */
+			VM_WARN_ON(mm->highest_vm_end != end);
+		}
 	}
 	if (insert && file)
 		uprobe_mmap(insert);