Re: [PATCH v4 16/20] tools/testing/selftests/mm: test anonymous page offset merge behaviour

While maintaining anonymous page offsets for VMAs has no impact for most
merge cases, it does impact MAP_PRIVATE-mapped file-backed mappings which
happen to have matching page offset but not matching anonymous page offset.

Assert this behaviour by attempting to map an unfaulted MAP_PRIVATE-memfd
region with a faulted one with compatible file page offsets but
incompatible anonymous page offsets.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 tools/testing/selftests/mm/merge.c | 57 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/tools/testing/selftests/mm/merge.c b/tools/testing/selftests/mm/merge.c
index 519e5ac02db7..cb7cdb6b7ada 100644
--- a/tools/testing/selftests/mm/merge.c
+++ b/tools/testing/selftests/mm/merge.c

@@ -1305,6 +1305,63 @@ TEST_F(merge, merge_vmas_with_mseal)
 	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 2 * page_size);
 }
 
+TEST_F(merge, anon_and_page_offset_mismatch_memfd)
+{
+	struct procmap_fd *procmap = &self->procmap;
+	unsigned int page_size = self->page_size;
+	char *carveout = self->carveout;
+	char *ptr, *ptr2;
+	int fd;
+
+	/* Create a 10 page memfd descriptor. */
+	fd = memfd_create("anon_page_offset_test", MFD_CLOEXEC);
+	ASSERT_NE(fd, -1);
+	ASSERT_EQ(ftruncate(fd, 10 * page_size), 0);
+
+	/* Map a region using the memfd at page offset 0. */
+	ptr = mmap(carveout, 5 * page_size, PROT_READ | PROT_WRITE,
+		   MAP_FIXED | MAP_PRIVATE, fd, 0);
+	ASSERT_NE(ptr, MAP_FAILED);
+
+	/*
+	 * Map another separately, and fault in, at page offset 5:
+	 *