Re: [RFC Patch 2/5] anon_vma: add skeleton code for userland testing of anon_vma logic

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Apr 29, 2025 at 09:06:36AM +0000, Wei Yang wrote:
[...]
>+
>+static bool test_fork_grand_child(void)
>+{
>+	struct vm_area_struct *root_vma, *grand_vma, *vma1, *vma2;
>+	struct anon_vma_chain *avc;
>+	struct anon_vma *root_anon_vma;
>+	DECLARE_BITMAP(expected, 10);
>+	DECLARE_BITMAP(found, 10);
>+
>+	bitmap_zero(expected, 10);
>+	bitmap_zero(found, 10);
>+
>+	/*
>+	 *  root_anon_vma      root_vma
>+	 *  +-----------+      +-----------+
>+	 *  |           | ---> |           |
>+	 *  +-----------+      +-----------+
>+	 */
>+
>+	root_vma = alloc_vma(0x3000, 0x5000, 3);
>+	/* First fault on parent anonymous vma. */
>+	__anon_vma_prepare(root_vma);
>+	root_anon_vma = root_vma->anon_vma;
>+	ASSERT_NE(NULL, root_anon_vma);
>+	bitmap_set(expected, root_vma->index, 1);
>+
>+	/* First fork */
>+	/*
>+	 *  root_anon_vma      root_vma
>+	 *  +-----------+      +-----------+
>+	 *  |           | ---> |           |
>+	 *  +-----------+      +-----------+
>+	 *                \
>+	 *                 \   vma1
>+	 *                  \  +-----------+
>+	 *                   > |           |
>+	 *                     +-----------+
>+	 */
>+	vma1 = alloc_vma(0x3000, 0x5000, 3);
>+	anon_vma_fork(vma1, root_vma);
>+	ASSERT_NE(NULL, vma1->anon_vma);
>+	bitmap_set(expected, vma1->index, 1);
>+	/* Parent/Root is root_vma->anon_vma */
>+	ASSERT_EQ(vma1->anon_vma->parent, root_vma->anon_vma);
>+	ASSERT_EQ(vma1->anon_vma->root, root_vma->anon_vma);
>+
>+	/* Second fork */
>+	/*
>+	 *  root_anon_vma      root_vma
>+	 *  +-----------+      +-----------+
>+	 *  |           | ---> |           |
>+	 *  +-----------+      +-----------+
>+	 *               \
>+	 *                \------------------+
>+	 *                 \   vma1           \   vma2
>+	 *                  \  +-----------+   \  +-----------+
>+	 *                   > |           |    > |           |
>+	 *                     +-----------+      +-----------+
>+	 */
>+	vma2 = alloc_vma(0x3000, 0x5000, 3);
>+	anon_vma_fork(vma2, root_vma);
>+	ASSERT_NE(NULL, vma2->anon_vma);
>+	bitmap_set(expected, vma2->index, 1);
>+	/* Parent/Root is root_vma->anon_vma */
>+	ASSERT_EQ(vma2->anon_vma->parent, root_vma->anon_vma);
>+	ASSERT_EQ(vma2->anon_vma->root, root_vma->anon_vma);
>+	dump_anon_vma_interval_tree(root_vma->anon_vma);
>+
>+	/* Fork grand child from second child */
>+	/*
>+	 *  root_anon_vma      root_vma
>+	 *  +-----------+      +-----------+
>+	 *  |           | ---> |           |
>+	 *  +-----------+      +-----------+
>+	 *               \
>+	 *                \------------------+
>+	 *                |\   vma1           \   vma2
>+	 *                | \  +-----------+   \  +-----------+
>+	 *                |  > |           |    > |           |
>+	 *                |    +-----------+      +-----------+
>+	 *                \
>+	 *                 \   grand_vma
>+	 *                  \  +-----------+
>+	 *                   > |           |
>+	 *                     +-----------+
>+	 */
>+	grand_vma = alloc_vma(0x3000, 0x5000, 3);
>+	anon_vma_fork(grand_vma, vma2);
>+	ASSERT_NE(NULL, grand_vma->anon_vma);
>+	bitmap_set(expected, grand_vma->index, 1);
>+	/* Root is root_vma->anon_vma */
>+	ASSERT_EQ(grand_vma->anon_vma->root, root_vma->anon_vma);
>+	/* Parent is vma2->anon_vma */
>+	ASSERT_EQ(grand_vma->anon_vma->parent, vma2->anon_vma);

Hi, Lorenzo

Here is the case I am talking about in another thread[1].

The naming is a little different from that.

  * root_vma  is VMA A
  * vma2      is VMA B
  * grand_vma is VMA C

If you add following debug code here.

```
	printf("root num_children %d\n", root_vma->anon_vma->num_children);
	printf("vma2 num_children %d\n", vma2->anon_vma->num_children);
	printf("grand_vma num_children %d\n", grand_vma->anon_vma->num_children);
```

You would see vma2 num_children is 1, but it has a child.

If I missed something, feel free to correct me.

[1]: https://lkml.kernel.org/r/20250501011845.ktbfgymor4oz5sok@master

>+
>+	/* Expect to find only vmas from second fork */
>+	anon_vma_interval_tree_foreach(avc, &vma2->anon_vma->rb_root, 3, 4) {
>+		ASSERT_TRUE(avc->vma == vma2 || avc->vma == grand_vma);
>+	}
>+
>+	anon_vma_interval_tree_foreach(avc, &root_vma->anon_vma->rb_root, 3, 4) {
>+		bitmap_set(found, avc->vma->index, 1);
>+	}
>+	/* Expect to find all vma including child and grand child. */
>+	ASSERT_TRUE(bitmap_equal(expected, found, 10));
>+
>+	/* Root process exit or unmap root_vma. */
>+	/*
>+	 *  root_anon_vma
>+	 *  +-----------+
>+	 *  |           |
>+	 *  +-----------+
>+	 *               \
>+	 *                \------------------+
>+	 *                |\   vma1           \   vma2
>+	 *                | \  +-----------+   \  +-----------+
>+	 *                |  > |           |    > |           |
>+	 *                |    +-----------+      +-----------+
>+	 *                \
>+	 *                 \   grand_vma
>+	 *                  \  +-----------+
>+	 *                   > |           |
>+	 *                     +-----------+
>+	 */
>+	bitmap_clear(expected, root_vma->index, 1);
>+	unlink_anon_vmas(root_vma);
>+	ASSERT_EQ(0, root_anon_vma->num_active_vmas);
>+
>+	bitmap_zero(found, 10);
>+	anon_vma_interval_tree_foreach(avc, &root_anon_vma->rb_root, 3, 4) {
>+		bitmap_set(found, avc->vma->index, 1);
>+	}
>+	/* Expect to find all vmas even root_vma released. */
>+	ASSERT_TRUE(bitmap_equal(expected, found, 10));
>+
>+	cleanup();
>+
>+	ASSERT_EQ(0, nr_allocated);
>+	return true;
>+}
>+


-- 
Wei Yang
Help you, Help me