[PATCH 6.6.y 1/1] selftests/mm: skip soft-dirty tests when CONFIG_MEM_SOFT_DIRTY is disabled

[Lance Yang <lance.yang@linux.dev>]

From: Lance Yang <lance.yang@linux.dev>

The madv_populate and soft-dirty kselftests currently fail on systems
where CONFIG_MEM_SOFT_DIRTY is disabled.

Introduce a new helper softdirty_supported() into vm_util.c/h to ensure
tests are properly skipped when the feature is not enabled.

Link: https://lkml.kernel.org/r/20250917133137.62802-1-lance.yang@linux.dev
Fixes: 9f3265db6ae8 ("selftests: vm: add test for Soft-Dirty PTE bit")
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Acked-by: David Hildenbrand <david@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Gabriel Krisman Bertazi <krisman@collabora.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 0389c305ef56cbadca4cbef44affc0ec3213ed30)
---
 tools/testing/selftests/mm/madv_populate.c | 21 +-----
 tools/testing/selftests/mm/soft-dirty.c    |  5 +-
 tools/testing/selftests/mm/vm_util.c       | 77 ++++++++++++++++++++++
 tools/testing/selftests/mm/vm_util.h       |  1 +
 4 files changed, 84 insertions(+), 20 deletions(-)

diff --git a/tools/testing/selftests/mm/madv_populate.c b/tools/testing/selftests/mm/madv_populate.c
index 17bcb07f19f3..7278623acf8c 100644
--- a/tools/testing/selftests/mm/madv_populate.c
+++ b/tools/testing/selftests/mm/madv_populate.c
@@ -264,23 +264,6 @@ static void test_softdirty(void)
 	munmap(addr, SIZE);
 }
 
-static int system_has_softdirty(void)
-{
-	/*
-	 * There is no way to check if the kernel supports soft-dirty, other
-	 * than by writing to a page and seeing if the bit was set. But the
-	 * tests are intended to check that the bit gets set when it should, so
-	 * doing that check would turn a potentially legitimate fail into a
-	 * skip. Fortunately, we know for sure that arm64 does not support
-	 * soft-dirty. So for now, let's just use the arch as a corse guide.
-	 */
-#if defined(__aarch64__)
-	return 0;
-#else
-	return 1;
-#endif
-}
-
 int main(int argc, char **argv)
 {
 	int nr_tests = 16;
@@ -288,7 +271,7 @@ int main(int argc, char **argv)
 
 	pagesize = getpagesize();
 
-	if (system_has_softdirty())
+	if (softdirty_supported())
 		nr_tests += 5;
 
 	ksft_print_header();
@@ -300,7 +283,7 @@ int main(int argc, char **argv)
 	test_holes();
 	test_populate_read();
 	test_populate_write();
-	if (system_has_softdirty())
+	if (softdirty_supported())
 		test_softdirty();
 
 	err = ksft_get_fail_cnt();
diff --git a/tools/testing/selftests/mm/soft-dirty.c b/tools/testing/selftests/mm/soft-dirty.c
index 7dbfa53d93a0..7095b95d19ae 100644
--- a/tools/testing/selftests/mm/soft-dirty.c
+++ b/tools/testing/selftests/mm/soft-dirty.c
@@ -193,8 +193,11 @@ int main(int argc, char **argv)
 	int pagesize;
 
 	ksft_print_header();
-	ksft_set_plan(15);
 
+	if (!softdirty_supported())
+		ksft_exit_skip("soft-dirty is not support\n");
+
+	ksft_set_plan(15);
 	pagemap_fd = open(PAGEMAP_FILE_PATH, O_RDONLY);
 	if (pagemap_fd < 0)
 		ksft_exit_fail_msg("Failed to open %s\n", PAGEMAP_FILE_PATH);
diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index 558c9cd8901c..b2af70d75711 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -97,6 +97,42 @@ uint64_t read_pmd_pagesize(void)
 	return strtoul(buf, NULL, 10);
 }
 
+char *__get_smap_entry(void *addr, const char *pattern, char *buf, size_t len)
+{
+	int ret;
+	FILE *fp;
+	char *entry = NULL;
+	char addr_pattern[MAX_LINE_LENGTH];
+
+	ret = snprintf(addr_pattern, MAX_LINE_LENGTH, "%08lx-",
+		       (unsigned long)addr);
+	if (ret >= MAX_LINE_LENGTH)
+		ksft_exit_fail_msg("%s: Pattern is too long\n", __func__);
+
+	fp = fopen(SMAP_FILE_PATH, "r");
+	if (!fp)
+		ksft_exit_fail_msg("%s: Failed to open file %s\n", __func__,
+				   SMAP_FILE_PATH);
+
+	if (!check_for_pattern(fp, addr_pattern, buf, len))
+		goto err_out;
+
+	/* Fetch the pattern in the same block */
+	if (!check_for_pattern(fp, pattern, buf, len))
+		goto err_out;
+
+	/* Trim trailing newline */
+	entry = strchr(buf, '\n');
+	if (entry)
+		*entry = '\0';
+
+	entry = buf + strlen(pattern);
+
+err_out:
+	fclose(fp);
+	return entry;
+}
+
 bool __check_huge(void *addr, char *pattern, int nr_hpages,
 		  uint64_t hpage_size)
 {
@@ -269,3 +305,44 @@ int uffd_unregister(int uffd, void *addr, uint64_t len)
 
 	return ret;
 }
+
+static bool check_vmflag(void *addr, const char *flag)
+{
+	char buffer[MAX_LINE_LENGTH];
+	const char *flags;
+	size_t flaglen;
+
+	flags = __get_smap_entry(addr, "VmFlags:", buffer, sizeof(buffer));
+	if (!flags)
+		ksft_exit_fail_msg("%s: No VmFlags for %p\n", __func__, addr);
+
+	while (true) {
+		flags += strspn(flags, " ");
+
+		flaglen = strcspn(flags, " ");
+		if (!flaglen)
+			return false;
+
+		if (flaglen == strlen(flag) && !memcmp(flags, flag, flaglen))
+			return true;
+
+		flags += flaglen;
+	}
+}
+
+bool softdirty_supported(void)
+{
+	char *addr;
+	bool supported = false;
+	const size_t pagesize = getpagesize();
+
+	/* New mappings are expected to be marked with VM_SOFTDIRTY (sd). */
+	addr = mmap(0, pagesize, PROT_READ | PROT_WRITE,
+		    MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
+	if (!addr)
+		ksft_exit_fail_msg("mmap failed\n");
+
+	supported = check_vmflag(addr, "sd");
+	munmap(addr, pagesize);
+	return supported;
+}
diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
index 0c603bec5e20..9816d6e9bce6 100644
--- a/tools/testing/selftests/mm/vm_util.h
+++ b/tools/testing/selftests/mm/vm_util.h
@@ -51,6 +51,7 @@ int uffd_register(int uffd, void *addr, uint64_t len,
 int uffd_unregister(int uffd, void *addr, uint64_t len);
 int uffd_register_with_ioctls(int uffd, void *addr, uint64_t len,
 			      bool miss, bool wp, bool minor, uint64_t *ioctls);
+bool softdirty_supported(void);
 
 /*
  * On ppc64 this will only work with radix 2M hugepage size
-- 
2.49.0