[PATCH v3 3/4] mm/mmap/vma_merge: explicitly assign res, vma, extend invariants

[Lorenzo Stoakes]

Previously, vma was an uninitialised variable which was only definitely
assigned as a result of the logic covering all possible input cases - for
it to have remained uninitialised, prev would have to be NULL, and next
would _have_ to be mergeable.

The value of res defaults to NULL, so we can neatly eliminate the
assignment to res and vma in the if (prev) block and ensure that both res
and vma are both explicitly assigned, by just setting both to prev.

In addition we add an explanation as to under what circumstances both might
change, and since we absolutely do rely on addr == curr->vm_start should
curr exist, assert that this is the case.

Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
---
 mm/mmap.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index dbdbb92493b2..2a4f63716231 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -911,7 +911,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 {
 	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
 	pgoff_t vma_pgoff;
-	struct vm_area_struct *curr, *next, *res = NULL;
+	struct vm_area_struct *curr, *next, *res;
 	struct vm_area_struct *vma, *adjust, *remove, *remove2;
 	int err = -1;
 	bool merge_prev = false;
@@ -939,14 +939,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	else
 		next = NULL;		/* case 5 */
 
-	/* verify some invariant that must be enforced by the caller */
+	/*
+	 * 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 && end > curr->vm_end);
+	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
 	VM_WARN_ON(addr >= end);
 
 	if (prev) {
-		res = prev;
-		vma = prev;
 		vma_start = prev->vm_start;
 		vma_pgoff = prev->vm_pgoff;
 		/* Can we merge the predecessor? */
@@ -957,6 +961,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			vma_prev(vmi);
 		}
 	}
+
 	/* Can we merge the successor? */
 	if (next && mpol_equal(policy, vma_policy(next)) &&
 	    can_vma_merge_before(next, vm_flags,
@@ -997,6 +1002,10 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			adj_start = -(prev->vm_end - addr);
 			err = dup_anon_vma(next, prev);
 		} else {
+			/*
+			 * Note that cases 3 and 8 are the ONLY ones where prev
+			 * is permitted to be (but is not necessarily) NULL.
+			 */
 			vma = next;			/* case 3 */
 			vma_start = addr;
 			vma_end = next->vm_end;
-- 
2.39.2