tools/testing/selftests/mm: add MAP_PRIVATE-/dev/zero merge tests

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 virtual
page offsets.

Link: https://lore.kernel.org/20260720-b4-scalable-cow-virt-pgoff-v2-15-2d549757a76f@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Gregory Price <gourry@gourry.net>
Cc: Harry Yoo <harry@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: xu xin <xu.xin16@zte.com.cn>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 104 insertions, 0 deletions

diff --git a/tools/testing/selftests/mm/merge.c b/tools/testing/selftests/mm/merge.c
index c093e27d0aea..edfbd39ae58e 100644
--- a/tools/testing/selftests/mm/merge.c
+++ b/tools/testing/selftests/mm/merge.c
@@ -1362,6 +1362,110 @@ TEST_F(merge, virt_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:
+	 *
+	 * |-----||------|    |-------------|
+	 * | 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);
+		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);
+	}
+	memset(ptr, 'x', 15 * page_size);
+
+	/*
+	 * Unmap the middle:
+	 *
+	 * |---------|           |---------|
+	 * | faulted |           | faulted |
+	 * |---------|           |---------|
+	 */
+	ASSERT_EQ(munmap(&ptr[5 * page_size], 5 * page_size), 0);
+
+	/*
+	 * Map in a new unfaulted mapping in the middle with page offset 0 -
+	 * this should merge and would not if it were treated as a file rather
+	 * than pure anon:
+	 *
+	 * |---------|-----------|---------|
+	 * | faulted | unfaulted | faulted |
+	 * |---------|-----------|---------|
+	 */
+	ptr2 = mmap(&carveout[5 * page_size], 5 * page_size,
+		    PROT_READ | PROT_WRITE, MAP_FIXED | MAP_PRIVATE,
+		    fd_zero, 0);
+	close(fd_zero);
+	ASSERT_NE(ptr2, MAP_FAILED);
+
+	/* 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 + 15 * page_size);
+}
+
 TEST_F(merge_with_fork, mremap_faulted_to_unfaulted_prev)
 {
 	struct procmap_fd *procmap = &self->procmap;