mm/mmap/vma_merge: init cleanup, be explicit about the non-mergeable case

Rather than setting err = -1 and only resetting if we hit merge cases,
explicitly check the non-mergeable case to make it abundantly clear that
we only proceed with the rest if something is mergeable, default err to 0
and only update if an error might occur.

Move the merge_prev, merge_next cases closer to the logic determining
curr, next and reorder initial variables so they are more logically
grouped.

This has no functional impact.

Link: https://lkml.kernel.org/r/99259fbc6403e80e270e1cc4612abbc8620b121b.1679516210.git.lstoakes@gmail.comSigned-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vernon Yang <vernon2gm@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -896,18 +896,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
 			struct anon_vma_name *anon_name)
 {
-	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
-	pgoff_t vma_pgoff;
 	struct vm_area_struct *curr, *next, *res;
 	struct vm_area_struct *vma, *adjust, *remove, *remove2;
-	int err = -1;
+	struct vma_prepare vp;
+	pgoff_t vma_pgoff;
+	int err = 0;
 	bool merge_prev = false;
 	bool merge_next = false;
 	bool vma_expanded = false;
-	struct vma_prepare vp;
+	unsigned long vma_start = addr;
 	unsigned long vma_end = end;
+	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
 	long adj_start = 0;
-	unsigned long vma_start = addr;
 
 	validate_mm(mm);
 	/*
@@ -926,24 +926,14 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	else
 		next = NULL;		/* case 5 */
 
-	/*
-	 * By default, we return prev. Cases 3, 4, 8 will instead return next
-	 * and cases 3, 8 will also update vma to point at next.
-	 */
-	res = vma = prev;
-
-	/* Verify some invariant that must be enforced by the caller. */
-	VM_WARN_ON(prev && addr <= prev->vm_start);
-	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
-	VM_WARN_ON(addr >= end);
-
 	if (prev) {
 		vma_start = prev->vm_start;
 		vma_pgoff = prev->vm_pgoff;
+
 		/* Can we merge the predecessor? */
-		if (prev->vm_end == addr && mpol_equal(vma_policy(prev), policy)
+		if (addr == prev->vm_end && mpol_equal(vma_policy(prev), policy)
 		    && can_vma_merge_after(prev, vm_flags, anon_vma, file,
-				   pgoff, vm_userfaultfd_ctx, anon_name)) {
+					   pgoff, vm_userfaultfd_ctx, anon_name)) {
 			merge_prev = true;
 			vma_prev(vmi);
 		}
@@ -951,13 +941,22 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 
 	/* Can we merge the successor? */
 	if (next && mpol_equal(policy, vma_policy(next)) &&
-	    can_vma_merge_before(next, vm_flags,
-				 anon_vma, file, pgoff+pglen,
+	    can_vma_merge_before(next, vm_flags, anon_vma, file, pgoff+pglen,
 				 vm_userfaultfd_ctx, anon_name)) {
 		merge_next = true;
 	}
 
+	if (!merge_prev && !merge_next)
+		return NULL; /* Not mergeable. */
+
+	res = vma = prev;
 	remove = remove2 = adjust = NULL;
+
+	/* Verify some invariant that must be enforced by the caller. */
+	VM_WARN_ON(prev && addr <= prev->vm_start);
+	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
+	VM_WARN_ON(addr >= end);
+
 	/* 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)) {
@@ -970,8 +969,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			if (!next->anon_vma)
 				err = dup_anon_vma(prev, curr);
 		}
-	} else if (merge_prev) {
-		err = 0;				/* case 2 */
+	} else if (merge_prev) {			/* case 2 */
 		if (curr) {
 			err = dup_anon_vma(prev, curr);
 			if (end == curr->vm_end) {	/* case 7 */
@@ -981,7 +979,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 				adj_start = (end - curr->vm_start);
 			}
 		}
-	} else if (merge_next) {
+	} else { /* merge_next */
 		res = next;
 		if (prev && addr < prev->vm_end) {	/* case 4 */
 			vma_end = addr;
@@ -997,7 +995,6 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			vma_start = addr;
 			vma_end = next->vm_end;
 			vma_pgoff = next->vm_pgoff;
-			err = 0;
 			if (curr) {			/* case 8 */
 				vma_pgoff = curr->vm_pgoff;
 				remove = curr;
@@ -1006,7 +1003,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 		}
 	}
 
-	/* Cannot merge or error in anon_vma clone */
+	/* Error in anon_vma clone. */
 	if (err)
 		return NULL;