mm/mmap/vma_merge: always check invariants

We may still have inconsistent input parameters even if we choose not to
merge and the vma_merge() invariant checks are useful for checking this
with no production runtime cost (these are only relevant when
CONFIG_DEBUG_VM is specified).

Therefore, perform these checks regardless of whether we merge.

This is relevant, as a recent issue (addressed in commit "mm/mempolicy:
Correctly update prev when policy is equal on mbind") in the mbind logic
was only picked up in the 6.2.y stable branch where these assertions are
performed prior to determining mergeability.

Had this remained the same in mainline this issue may have been picked up
faster, so moving forward let's always check them.

Link: https://lkml.kernel.org/r/df548a6ae3fa135eec3b446eb3dae8eb4227da97.1682885809.git.lstoakes@gmail.comSigned-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -960,17 +960,17 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 		merge_next = true;
 	}
 
+	/* 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 (!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)) {