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

On Tue, Jun 30, 2026 at 05:36:18PM +0100, Pedro Falcato wrote:

On Mon, Jun 29, 2026 at 01:23:23PM +0100, Lorenzo Stoakes wrote:

quoted

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@kernel.org>
---
 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);

Very nice.

Reviewed-by: Pedro Falcato <pfalcato@suse.de>

Thanks :)

--
Pedro

Cheers, Lorenzo