[RFC Patch 5/5] anon_vma: add test to assert no double-reuse

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

commit 2555283eb40d ("mm/rmap: Fix anon_vma->degree ambiguity leading to
double-reuse") fixed anon_vma double-reuse issue introduced by commit
7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy").

Add a test case to assert no double-reuse.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Harry Yoo <harry.yoo@oracle.com>
---
 tools/testing/anon_vma/anon_vma.c | 47 ++++++++++++++++++++++++++++++-
 1 file changed, 46 insertions(+), 1 deletion(-)

diff --git a/tools/testing/anon_vma/anon_vma.c b/tools/testing/anon_vma/anon_vma.c
index 495cd02ea661..633a62f5acba 100644
--- a/tools/testing/anon_vma/anon_vma.c
+++ b/tools/testing/anon_vma/anon_vma.c
@@ -558,7 +558,7 @@ static bool test_mergeable_vma(void)
 
 static bool test_reuse_anon_vma(void)
 {
-	struct vm_area_struct *root_vma, *vma, *vma1, *vma2;
+	struct vm_area_struct *root_vma, *vma, *vma1, *vma2, *vma3;
 	struct anon_vma *root_anon_vma, *reused_anon_vma;
 	struct anon_vma_chain *avc;
 
@@ -690,6 +690,51 @@ static bool test_reuse_anon_vma(void)
 		ASSERT_TRUE(avc->vma == vma1 || avc->vma == vma2);
 	}
 
+	/* Fork from vma2 */
+	/*
+	 * When commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma
+	 * hierarchy") introduce anon_vma reuse, it embedded an issue of
+	 * double-reuse.
+	 *
+	 * It happens when vma2 reuse an existing anon_vma and we fork from
+	 * vma2 later. Before commit 2555283eb40d ("mm/rmap: Fix
+	 * anon_vma->degree ambiguity leading to double-reuse"), the forked
+	 * vma3 would reuse vma1->anon_vma, which is already in use.
+	 *
+	 * The following case assert vma1->anon_vma will not double-reuse.
+	 *
+	 *  root_anon_vma
+	 *  +-----------+
+	 *  |           |
+	 *  +-----------+
+	 *                \
+	 *                |
+	 *                \
+	 *                |\   vma1
+	 *                | \  +-----------+
+	 *                |  > |         av| != root_anon_vma
+	 *                |    +-----------+
+	 *                \
+	 *                |\   vma2
+	 *                | \  +-----------+
+	 *                |  > |         av| == reused_anon_vma
+	 *                |    +-----------+
+	 *                \
+	 *                 \   vma3
+	 *                  \  +-----------+
+	 *                   > |         av| != vma1->anon_vma
+	 *                     +-----------+
+	 */
+	/* vma1->anon_vma already has active vma */
+	ASSERT_NE(NULL, vma1->anon_vma);
+	vma3 = alloc_vma(0x3000, 0x5000, 3);
+	anon_vma_fork(vma3, vma2);
+	ASSERT_NE(NULL, vma3->anon_vma);
+	/* Root is root_vma->anon_vma */
+	ASSERT_EQ(vma3->anon_vma->root, root_anon_vma);
+	/* vma1->anon_vma is NOT reused here */
+	ASSERT_NE(vma3->anon_vma, vma1->anon_vma);
+
 	cleanup();
 
 	ASSERT_EQ(0, nr_allocated);
-- 
2.34.1