[PATCH 12/30] mm/vma: clean up anon_vma_compatible()

[Lorenzo Stoakes]

Break up the existing very large conditional, add comments and use
vma_[start/end]_pgoff() to make clearer what we're doing here.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
---
 mm/vma.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index b60375c6c5c3..6296acecf3b7 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -1967,14 +1967,25 @@ static int anon_vma_compatible(struct vm_area_struct *a, struct vm_area_struct *
 {
 	vma_flags_t diff = vma_flags_diff_pair(&a->flags, &b->flags);
 
+	/* Ignore flags that mprotect() can change. */
 	vma_flags_clear_mask(&diff, VMA_ACCESS_FLAGS);
+	/* Ignore flags that do not impact merging. */
 	vma_flags_clear_mask(&diff, VMA_IGNORE_MERGE_FLAGS);
 
-	return a->vm_end == b->vm_start &&
-		mpol_equal(vma_policy(a), vma_policy(b)) &&
-		a->vm_file == b->vm_file &&
-		vma_flags_empty(&diff) &&
-		b->vm_pgoff == a->vm_pgoff + ((b->vm_start - a->vm_start) >> PAGE_SHIFT);
+	/* Must be adjacent. */
+	if (a->vm_end != b->vm_start)
+		return false;
+	/* Must have matching policy. */
+	if (!mpol_equal(vma_policy(a), vma_policy(b)))
+		return false;
+	/* Must both be anon or map the same file (MAP_PRIVATE case). */
+	if (a->vm_file != b->vm_file)
+		return false;
+	/* Flags must be equivalent modulo mprotect(). */
+	if (!vma_flags_empty(&diff))
+		return false;
+	/* Page offset must align. */
+	return vma_end_pgoff(a) == vma_start_pgoff(b);
 }
 
 /*
-- 
2.54.0