[PATCH v1 9/9] testing/selftests/mm: add soft-dirty merge self-test

Ahmed Elaidy
2026-04-24T21:14:01Z

Assert that we correctly merge VMAs containing VM_SOFTDIRTY flags now that
we correctly handle these as sticky.

In order to do so, we have to account for the fact the pagemap interface
checks soft dirty PTEs and additionally that newly merged VMAs are marked
VM_SOFTDIRTY.

We do this by using use unfaulted anon VMAs, establishing one and clearing
references on that one, before establishing another and merging the two
before checking that soft-dirty is propagated as expected.

We check that this functions correctly with mremap() and mprotect() as
sample cases, because VMA merge of adjacent newly mapped VMAs will
automatically be made soft-dirty due to existing logic which does so.

We are therefore exercising other means of merging VMAs.

Link: https://lkml.kernel.org/r/d5a0f735783fb4f30a604f570ede02ccc5e29be9.1763399675.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Andrey Vagin <avagin@gmail.com>
Cc: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit c7ba92bcfea34f6b4afc744c3b65c8f7420fefe0)
Signed-off-by: Ahmed Elaidy <elaidya225@gmail.com>
---
tools/testing/selftests/mm/soft-dirty.c | 127 +++++++++++++++++++++++-
1 file changed, 126 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/soft-dirty.c b/tools/testing/selftests/mm/soft-dirty.c
index 4ee4db3750c1..c3a9585de98c 100644
--- a/tools/testing/selftests/mm/soft-dirty.c
+++ b/tools/testing/selftests/mm/soft-dirty.c
@@ -184,6 +184,130 @@ static void test_mprotect(int pagemap_fd, int pagesize, bool anon)
close(test_fd);
}

+static void test_merge(int pagemap_fd, int pagesize)
+{
+ char *reserved, *map, *map2;
+
+ /*
+ * Reserve space for tests:
+ *
+ * ---padding to ---
+ * | avoid adj. |
+ * v merge v
+ * |---|---|---|---|---|
+ * | | 1 | 2 | 3 | |
+ * |---|---|---|---|---|
+ */
+ reserved = mmap(NULL, 5 * pagesize, PROT_NONE,
+ MAP_ANON | MAP_PRIVATE, -1, 0);
+ if (reserved == MAP_FAILED)
+ ksft_exit_fail_msg("mmap failed\n");
+ munmap(reserved, 4 * pagesize);
+
+ /*
+ * Establish initial VMA:
+ *
+ * S/D
+ * |---|---|---|---|---|
+ * | | 1 | | | |
+ * |---|---|---|---|---|
+ */
+ map = mmap(&reserved[pagesize], pagesize, PROT_READ | PROT_WRITE,
+ MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
+ if (map == MAP_FAILED)
+ ksft_exit_fail_msg("mmap failed\n");
+
+ /* This will clear VM_SOFTDIRTY too. */
+ clear_softdirty();
+
+ /*
+ * Now place a new mapping which will be marked VM_SOFTDIRTY. Away from
+ * map:
+ *
+ * - S/D
+ * |---|---|---|---|---|
+ * | | 1 | | 2 | |
+ * |---|---|---|---|---|
+ */
+ map2 = mmap(&reserved[3 * pagesize], pagesize, PROT_READ | PROT_WRITE,
+ MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
+ if (map2 == MAP_FAILED)
+ ksft_exit_fail_msg("mmap failed\n");
+
+ /*
+ * Now remap it immediately adjacent to map, if the merge correctly
+ * propagates VM_SOFTDIRTY, we should then observe the VMA as a whole
+ * being marked soft-dirty:
+ *
+ * merge
+ * S/D
+ * |---|-------|---|---|
+ * | | 1 | | |
+ * |---|-------|---|---|
+ */
+ map2 = mremap(map2, pagesize, pagesize, MREMAP_FIXED | MREMAP_MAYMOVE,
+ &reserved[2 * pagesize]);
+ if (map2 == MAP_FAILED)
+ ksft_exit_fail_msg("mremap failed\n");
+ ksft_test_result(pagemap_is_softdirty(pagemap_fd, map) == 1,
+ "Test %s-anon soft-dirty after remap merge 1st pg\n",
+ __func__);
+ ksft_test_result(pagemap_is_softdirty(pagemap_fd, map2) == 1,
+ "Test %s-anon soft-dirty after remap merge 2nd pg\n",
+ __func__);
+
+ munmap(map, 2 * pagesize);
+
+ /*
+ * Now establish another VMA:
+ *
+ * S/D
+ * |---|---|---|---|---|
+ * | | 1 | | | |
+ * |---|---|---|---|---|
+ */
+ map = mmap(&reserved[pagesize], pagesize, PROT_READ | PROT_WRITE,
+ MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
+ if (map == MAP_FAILED)
+ ksft_exit_fail_msg("mmap failed\n");
+
+ /* Clear VM_SOFTDIRTY... */
+ clear_softdirty();
+ /* ...and establish incompatible adjacent VMA:
+ *
+ * - S/D
+ * |---|---|---|---|---|
+ * | | 1 | 2 | | |
+ * |---|---|---|---|---|
+ */
+ map2 = mmap(&reserved[2 * pagesize], pagesize,
+ PROT_READ | PROT_WRITE | PROT_EXEC,
+ MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
+ if (map2 == MAP_FAILED)
+ ksft_exit_fail_msg("mmap failed\n");
+
+ /*
+ * Now mprotect() VMA 1 so it's compatible with 2 and therefore merges:
+ *
+ * merge
+ * S/D
+ * |---|-------|---|---|
+ * | | 1 | | |
+ * |---|-------|---|---|
+ */
+ if (mprotect(map, pagesize, PROT_READ | PROT_WRITE | PROT_EXEC))
+ ksft_exit_fail_msg("mprotect failed\n");
+
+ ksft_test_result(pagemap_is_softdirty(pagemap_fd, map) == 1,
+ "Test %s-anon soft-dirty after mprotect merge 1st pg\n",
+ __func__);
+ ksft_test_result(pagemap_is_softdirty(pagemap_fd, map2) == 1,
+ "Test %s-anon soft-dirty after mprotect merge 2nd pg\n",
+ __func__);
+
+ munmap(map, 2 * pagesize);
+}
+
static void test_mprotect_anon(int pagemap_fd, int pagesize)
{
test_mprotect(pagemap_fd, pagesize, true);
@@ -204,7 +328,7 @@ int main(int argc, char **argv)
if (!softdirty_supported())
ksft_exit_skip("soft-dirty is not support\n");

- ksft_set_plan(15);
+ ksft_set_plan(19);
pagemap_fd = open(PAGEMAP_FILE_PATH, O_RDONLY);
if (pagemap_fd < 0)
ksft_exit_fail_msg("Failed to open %s\n", PAGEMAP_FILE_PATH);
@@ -216,6 +340,7 @@ int main(int argc, char **argv)
test_hugepage(pagemap_fd, pagesize);
test_mprotect_anon(pagemap_fd, pagesize);
test_mprotect_file(pagemap_fd, pagesize);
+ test_merge(pagemap_fd, pagesize);

close(pagemap_fd);

--
2.53.0