mm/mmap/vma_merge: use the proper vma pointers in cases 1 and 6

Case 1 is now shown in the comment as next vma being merged with prev, so
use 'next' instead of 'mid'.  In case 1 they both point to the same vma.

As a consequence, in case 6, the dup_anon_vma() is now tried first on
'next' and then on 'mid', before it was the opposite order.  This is not a
functional change, as those two vma's cannnot have a different anon_vma,
as that would have prevented the merging in the first place.

Link: https://lkml.kernel.org/r/20230309111258.24079-4-vbabka@suse.czSigned-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -605,7 +605,7 @@ static inline void vma_complete(struct vma_prepare *vp,
 
 		/*
 		 * In mprotect's case 6 (see comments on vma_merge),
-		 * we must remove the one after next as well.
+		 * we are removing both mid and next vmas
 		 */
 		if (vp->remove2) {
 			vp->remove = vp->remove2;
@@ -948,13 +948,14 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	/* Can we merge both the predecessor and the successor? */
 	if (merge_prev && merge_next &&
 	    is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL)) {
-		remove = mid;				/* case 1 */
+		remove = next;				/* case 1 */
 		vma_end = next->vm_end;
-		err = dup_anon_vma(prev, mid);
+		err = dup_anon_vma(prev, next);
 		if (mid != next) {			/* case 6 */
+			remove = mid;
 			remove2 = next;
-			if (!mid->anon_vma)
-				err = dup_anon_vma(prev, next);
+			if (!next->anon_vma)
+				err = dup_anon_vma(prev, mid);
 		}
 	} else if (merge_prev) {
 		err = 0;				/* case 2 */