+ tools-testing-selftests-mm-add-map_private-dev-zero-merge-tests.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: tools/testing/selftests/mm: add MAP_PRIVATE-/dev/zero merge tests
has been added to the -mm mm-unstable branch.  Its filename is
     tools-testing-selftests-mm-add-map_private-dev-zero-merge-tests.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/tools-testing-selftests-mm-add-map_private-dev-zero-merge-tests.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Subject: tools/testing/selftests/mm: add MAP_PRIVATE-/dev/zero merge tests
Date: Tue, 08 Sep 2026 12:23:43 +0100

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

Test both unfaulted and faulted/unfaulted merges with page offset 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.

Link: https://lore.kernel.org/20260908-map-private-dev-zero-v2-6-acc7b5625305@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 tools/testing/selftests/mm/merge.c |   95 +++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)

--- a/tools/testing/selftests/mm/merge.c~tools-testing-selftests-mm-add-map_private-dev-zero-merge-tests
+++ a/tools/testing/selftests/mm/merge.c
@@ -1362,6 +1362,101 @@ TEST_F(merge, anon_and_page_offset_misma
 	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
+	 * the merge:
+	 *
+	 * |-----||------|    |-------------|
+	 * | ptr || ptr2 | -> |     ptr     |
+	 * |-----||------|    |-------------|
+	 */
+	ptr = mmap(carveout, 5 * page_size, PROT_READ | PROT_WRITE,
+		   MAP_FIXED | MAP_PRIVATE, fd_zero, 0);
+	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(ptr, MAP_FAILED);
+	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 + 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);
+	ASSERT_NE(ptr, MAP_FAILED);
+	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;
_

Patches currently in -mm which might be from ljs@kernel.org are

mm-vma-correctly-unaccount-on-mmap_prepare-failure.patch
mm-vmpressure-remove-window-size-todo.patch
tools-testing-selftests-mm-add-missing-gitignore-entries.patch
mm-move-drivers-char-memc-to-mm-char-memc.patch
mm-implement-file_is_dev_zero-to-uniquely-identify-dev-zero.patch
mm-vma-only-permit-map_private-dev-zero-to-be-mapped-anonymous.patch
mm-vma-make-map_private-mapped-dev-zero-mappings-truly-anonymous.patch
tools-testing-vma-add-test-to-assert-map_private-dev-zero-is-anon.patch
tools-testing-selftests-mm-add-map_private-dev-zero-merge-tests.patch
mm-madvise-swap-in-cowd-map_private-file-mappings-on-madv_willneed.patch
mm-huge_memory-zap-deposited-page-tables-after-an-rcu-grace-period.patch
mm-enable-mmu_gather_rcu_table_free-for-most-2-level-architectures.patch
mm-enable-mmu_gather_rcu_table_free-for-mmu-riscv.patch
mm-enable-mmu_gather_rcu_table_free-for-mmu-arm.patch
mm-enable-mmu_gather_rcu_table_free-for-arc-microblaze-xtensa.patch
mm-enable-mmu_gather_rcu_table_free-for-sparc64.patch
mm-enable-mmu_gather_rcu_table_free-for-m68k-coldfire.patch
mm-enable-mmu_gather_rcu_table_free-for-sh-x2.patch
mm-enable-mmu_gather_rcu_table_free-for-m68k-motorola.patch
mm-enable-mmu_gather_rcu_table_free-for-sparc32.patch
mm-make-userland-page-table-freeing-rcu-safe.patch
mm-change-the-contract-for-free_pgtables-update-docs.patch