Re: [PATCH 6/6] tools/testing/selftests/mm: add MAP_PRIVATE-/dev/zero merge tests

On Mon, Sep 07, 2026 at 07:08:48PM +0200, David Hildenbrand (Arm) wrote:

On 9/2/26 20:00, Lorenzo Stoakes (ARM) wrote:

quoted

Assert that MAP_PRIVATE-mapped /dev/zero mappings behave like they are
anonymous.

We test both unfaulted and faulted/unfaulted merges - each with the regions
having page offset of 0, which would not merge if the mappings were treated
as if they were file-backed.

With the recent change that makes them behave as pure anonymous mappings,
the merges should succeed as their page offsets are equal to their
anonymous page offsets.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 tools/testing/selftests/mm/merge.c | 104 +++++++++++++++++++++++++++++++++++++
 1 file changed, 104 insertions(+)

diff --git a/tools/testing/selftests/mm/merge.c b/tools/testing/selftests/mm/merge.c
index 52b8727b6628..7c528d470404 100644
--- a/tools/testing/selftests/mm/merge.c
+++ b/tools/testing/selftests/mm/merge.c

@@ -1362,6 +1362,110 @@ TEST_F(merge, anon_and_page_offset_mismatch_memfd)
 	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 5 * page_size);
 }

+TEST_F(merge, merge_map_private_dev_zero_unfaulted)
+{
+	struct procmap_fd *procmap = &self->procmap;
+	unsigned int page_size = self->page_size;
+	char *carveout = self->carveout;
+	char *ptr, *ptr2;
+	int fd_zero;
+
+	if (access("/dev/zero", F_OK))
+		SKIP(return, "No /dev/zero.");
+	fd_zero = open("/dev/zero", O_RDWR);
+	ASSERT_NE(fd_zero, -1);
+
+	/*
+	 * Map two MAP_PRIVATE-/dev/zero VMAs next to one another with offset 0
+	 * each.
+	 *
+	 * With these being made truly anonymous upon mapping, they will
+	 * merge. If they were file-backed VMAs the page offsets would prevent
+	 * merge:

Nit: "the" merge? You're the native speaker, so I don't know if what you have is
just correct :)

You're right ;) native speakers are not immune from messing up grammar, as my
copy editor will tell you :P

Will fix up on respin.

quoted

+	 *
+	 * |-----||------|    |-------------|
+	 * | ptr || ptr2 | -> |     ptr     |
+	 * |-----||------|    |-------------|
+	 */
+	ptr = mmap(carveout, 5 * page_size, PROT_READ | PROT_WRITE,
+		   MAP_FIXED | MAP_PRIVATE, fd_zero, 0);
+	if (ptr == MAP_FAILED) {
+		close(fd_zero);
+		ASSERT_TRUE(false);
+	}
+	ptr2 = mmap(&carveout[5 * page_size], 5 * page_size,
+		   PROT_READ | PROT_WRITE, MAP_FIXED | MAP_PRIVATE, fd_zero, 0);
+	if (ptr2 == MAP_FAILED) {
+		close(fd_zero);

Is the close() really required before the ASSERT?  After all, you're also not
munmap'ing, so I wonder to which degree we have to clean up.

So maybe this could just become a

	ASSERT_NE(ptr2, MAP_FAILED);

Same for ptr above.

You could likely also do

ptr = mmap()
ptr2 = mmap()
close(fd_zero);

ASSERT_NE(ptr, MAP_FAILED);
ASSERT_NE(ptr2, MAP_FAILED);

Yeah that's easiest I think! Will fixup on respin.

quoted

+		ASSERT_TRUE(false);
+	}
+	close(fd_zero);
+
+	/* Assert that they merged. */
+	ASSERT_TRUE(find_vma_procmap(procmap, ptr));
+	ASSERT_EQ(procmap->query.vma_start, (unsigned long)ptr);
+	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 10 * page_size);
+}
+
+TEST_F(merge, merge_map_private_dev_zero_faulted_unfaulted)
+{
+	struct procmap_fd *procmap = &self->procmap;
+	unsigned int page_size = self->page_size;
+	char *carveout = self->carveout;
+	char *ptr, *ptr2;
+	int fd_zero;
+
+	if (access("/dev/zero", F_OK))
+		SKIP(return, "No /dev/zero.");
+	fd_zero = open("/dev/zero", O_RDWR);
+	ASSERT_NE(fd_zero, -1);
+
+	/*
+	 * Map a MAP_PRIVATE mapping of /dev/zero with page offset 0, then fault
+	 * it in:
+	 *
+	 * |-------------------------------|
+	 * |           faulted             |
+	 * |-------------------------------|
+	 */
+	ptr = mmap(carveout, 15 * page_size, PROT_READ | PROT_WRITE,
+		   MAP_FIXED | MAP_PRIVATE, fd_zero, 0);
+	if (ptr == MAP_FAILED) {
+		close(fd_zero);
+		ASSERT_TRUE(false);

Same question regarding cleanup requirements. The ASSERT_TRUE(false) looks a bit
odd.

Yeah it does. This case is trickier as you then do a memset(), so maybe just
live with the 'leaked' fd in this case (the tests fail so it aborts the run at
that point anyway and tears down).

Fixed and will send respin!

--
Cheers,

David

--
Cheers, Lorenzo