[PATCH v3 0/2] fix MAP_DROPPABLE not supported errno

[PATCH v3 0/2] fix MAP_DROPPABLE not supported errno

[Anthony Yznaga]

Mark Brown reported seeing a regression in -next on 32 bit arm with the
mlock selftests. Before exiting and marking the tests failed, the following
message was logged after an attempt to create a MAP_DROPPABLE mapping:

Bail out! mmap error: Unknown error 524

It turns out error 524 is ENOTSUPP which is an error that userspace is not
supposed to see, but it indicates in this instance that MAP_DROPPABLE is
not supported.

The first patch changes the errno returned to EOPNOTSUPP. The second patch
is a second version of a prior patch to introduce selftests to verify
locking behavior with droppable mappings with the additional change to skip
the tests when MAP_DROPPABLE is not supported.

v3:
- Added acked-by's and reviewed-by's to patch 1
- Addressed sashiko AI reviewbot questions:
  - Call munlockall() if test_mlockall_future_droppable() returns early
  - Skip the tests if they are compiled for an older kernel without
    MAP_DROPPABLE.

v2:
- Remove unnecessary mlock() call from selftest (DavidH)
- Change tests to not exit on failure (DavidH)

Anthony Yznaga (2):
  mm: fix mmap errno value when MAP_DROPPABLE is not supported
  selftests/mm: verify droppable mappings cannot be locked

 mm/mmap.c                                 |   2 +-
 tools/testing/selftests/mm/mlock2-tests.c | 100 ++++++++++++++++++++--
 2 files changed, 92 insertions(+), 10 deletions(-)

-- 
2.47.3

[PATCH v3 1/2] mm: fix mmap errno value when MAP_DROPPABLE is not supported

[Anthony Yznaga]

On configs where MAP_DROPPABLE is not supported (currently any 32-bit
config except for PPC32), mmap fails with errno set to ENOTSUPP.
However, ENOTSUPP is not a standard error value that userspace knows
about. The acceptable userspace-visible errno to use is EOPNOTSUPP.
checkpatch.pl has a warning to this effect.

Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
Cc: <stable@vger.kernel.org>
Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
Acked-by: David Hildenbrand <david@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reported-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
---
 mm/mmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 5754d1c36462..2311ae7c2ff4 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -504,7 +504,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 			break;
 		case MAP_DROPPABLE:
 			if (VM_DROPPABLE == VM_NONE)
-				return -ENOTSUPP;
+				return -EOPNOTSUPP;
 			/*
 			 * A locked or stack area makes no sense to be droppable.
 			 *
-- 
2.47.3

[PATCH v3 2/2] selftests/mm: verify droppable mappings cannot be locked

[Anthony Yznaga]

For configs that support MAP_DROPPABLE verify that a mapping created
with MAP_DROPPABLE cannot be locked via mlock(), and that it will not
be locked if it's created after mlockall(MCL_FUTURE).

Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
---
 tools/testing/selftests/mm/mlock2-tests.c | 100 ++++++++++++++++++++--
 1 file changed, 91 insertions(+), 9 deletions(-)

diff --git a/tools/testing/selftests/mm/mlock2-tests.c b/tools/testing/selftests/mm/mlock2-tests.c
index b474f2b20def..47da1a05c243 100644
--- a/tools/testing/selftests/mm/mlock2-tests.c
+++ b/tools/testing/selftests/mm/mlock2-tests.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #define _GNU_SOURCE
 #include <sys/mman.h>
+#include <linux/mman.h>
 #include <stdint.h>
 #include <unistd.h>
 #include <string.h>
@@ -163,14 +164,17 @@ static int lock_check(unsigned long addr)
 	return (vma_rss == vma_size);
 }
 
-static int unlock_lock_check(char *map)
+static int unlock_lock_check(char *map, bool mlock_supported)
 {
-	if (is_vmflag_set((unsigned long)map, LOCKED)) {
+	if (!is_vmflag_set((unsigned long)map, LOCKED))
+		return 0;
+
+	if (mlock_supported)
 		ksft_print_msg("VMA flag %s is present on page 1 after unlock\n", LOCKED);
-		return 1;
-	}
+	else
+		ksft_print_msg("VMA flag %s is present on an unsupported VMA\n", LOCKED);
 
-	return 0;
+	return 1;
 }
 
 static void test_mlock_lock(void)
@@ -196,7 +200,7 @@ static void test_mlock_lock(void)
 		ksft_exit_fail_msg("munlock(): %s\n", strerror(errno));
 	}
 
-	ksft_test_result(!unlock_lock_check(map), "%s: Unlocked\n", __func__);
+	ksft_test_result(!unlock_lock_check(map, true), "%s: Unlocked\n", __func__);
 	munmap(map, 2 * page_size);
 }
 
@@ -296,7 +300,7 @@ static void test_munlockall0(void)
 		ksft_exit_fail_msg("munlockall(): %s\n", strerror(errno));
 	}
 
-	ksft_test_result(!unlock_lock_check(map), "%s: No locked memory\n", __func__);
+	ksft_test_result(!unlock_lock_check(map, true), "%s: No locked memory\n", __func__);
 	munmap(map, 2 * page_size);
 }
 
@@ -336,9 +340,85 @@ static void test_munlockall1(void)
 		ksft_exit_fail_msg("munlockall() %s\n", strerror(errno));
 	}
 
-	ksft_test_result(!unlock_lock_check(map), "%s: No locked memory\n", __func__);
+	ksft_test_result(!unlock_lock_check(map, true), "%s: No locked memory\n", __func__);
+	munmap(map, 2 * page_size);
+}
+
+#ifdef MAP_DROPPABLE
+/*
+ * Droppable memory should not be lockable.
+ */
+static void test_mlock_droppable(void)
+{
+	char *map;
+	unsigned long page_size = getpagesize();
+
+	/*
+	 * Ensure MCL_FUTURE is not set.
+	 */
+	if (munlockall()) {
+		ksft_test_result_fail("munlockall() %s\n", strerror(errno));
+		return;
+	}
+
+	map = mmap(NULL, 2 * page_size, PROT_READ | PROT_WRITE,
+		   MAP_ANONYMOUS | MAP_DROPPABLE, -1, 0);
+	if (map == MAP_FAILED) {
+		if (errno == EOPNOTSUPP)
+			ksft_test_result_skip("%s: MAP_DROPPABLE not supported\n", __func__);
+		else
+			ksft_test_result_fail("mmap error: %s\n", strerror(errno));
+		return;
+	}
+
+	if (mlock2_(map, 2 * page_size, 0))
+		ksft_test_result_fail("mlock2(0): %s\n", strerror(errno));
+	else
+		ksft_test_result(!unlock_lock_check(map, false),
+				"%s: droppable memory not locked\n", __func__);
+
+	munmap(map, 2 * page_size);
+}
+
+static void test_mlockall_future_droppable(void)
+{
+	char *map;
+	unsigned long page_size = getpagesize();
+
+	if (mlockall(MCL_CURRENT | MCL_FUTURE)) {
+		ksft_test_result_fail("mlockall(MCL_CURRENT | MCL_FUTURE): %s\n", strerror(errno));
+		return;
+	}
+
+	map = mmap(NULL, 2 * page_size, PROT_READ | PROT_WRITE,
+		   MAP_ANONYMOUS | MAP_DROPPABLE, -1, 0);
+
+	if (map == MAP_FAILED) {
+		if (errno == EOPNOTSUPP)
+			ksft_test_result_skip("%s: MAP_DROPPABLE not supported\n", __func__);
+		else
+			ksft_test_result_fail("mmap error: %s\n", strerror(errno));
+		munlockall();
+		return;
+	}
+
+	ksft_test_result(!unlock_lock_check(map, false), "%s: droppable memory not locked\n",
+			__func__);
+
+	munlockall();
 	munmap(map, 2 * page_size);
 }
+#else
+static void test_mlock_droppable(void)
+{
+	ksft_test_result_skip("%s: MAP_DROPPABLE not supported\n", __func__);
+}
+
+static void test_mlockall_future_droppable(void)
+{
+	ksft_test_result_skip("%s: MAP_DROPPABLE not supported\n", __func__);
+}
+#endif /* MAP_DROPPABLE */
 
 static void test_vma_management(bool call_mlock)
 {
@@ -442,7 +522,7 @@ int main(int argc, char **argv)
 
 	munmap(map, size);
 
-	ksft_set_plan(13);
+	ksft_set_plan(15);
 
 	test_mlock_lock();
 	test_mlock_onfault();
@@ -451,6 +531,8 @@ int main(int argc, char **argv)
 	test_lock_onfault_of_present();
 	test_vma_management(true);
 	test_mlockall();
+	test_mlock_droppable();
+	test_mlockall_future_droppable();
 
 	ksft_finished();
 }
-- 
2.47.3

Re: [PATCH v3 2/2] selftests/mm: verify droppable mappings cannot be locked

[David Hildenbrand (Arm)]

On 4/10/26 01:49, Anthony Yznaga wrote:
> For configs that support MAP_DROPPABLE verify that a mapping created
> with MAP_DROPPABLE cannot be locked via mlock(), and that it will not
> be locked if it's created after mlockall(MCL_FUTURE).
> 
> Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
> ---


[...]

> +
> +#ifdef MAP_DROPPABLE
> +/*
> + * Droppable memory should not be lockable.
> + */

Single-line comment.

> +static void test_mlock_droppable(void)
> +{
> +	char *map;
> +	unsigned long page_size = getpagesize();

We could store that in a static global and query it once during main().
Feel free to keep it as is.

> +
> +	/*
> +	 * Ensure MCL_FUTURE is not set.
> +	 */

Dito.

> +	if (munlockall()) {
> +		ksft_test_result_fail("munlockall() %s\n", strerror(errno));
> +		return;
> +	}
> +
> +	map = mmap(NULL, 2 * page_size, PROT_READ | PROT_WRITE,
> +		   MAP_ANONYMOUS | MAP_DROPPABLE, -1, 0);
> +	if (map == MAP_FAILED) {
> +		if (errno == EOPNOTSUPP)
> +			ksft_test_result_skip("%s: MAP_DROPPABLE not supported\n", __func__);
> +		else
> +			ksft_test_result_fail("mmap error: %s\n", strerror(errno));
> +		return;
> +	}
> +
> +	if (mlock2_(map, 2 * page_size, 0))

Weird, is "mlock2_" actually correct? (not "mlock2") ?

> +		ksft_test_result_fail("mlock2(0): %s\n", strerror(errno));
> +	else
> +		ksft_test_result(!unlock_lock_check(map, false),
> +				"%s: droppable memory not locked\n", __func__);
> +
> +	munmap(map, 2 * page_size);
> +}
> +
> +static void test_mlockall_future_droppable(void)
> +{
> +	char *map;
> +	unsigned long page_size = getpagesize();
> +
> +	if (mlockall(MCL_CURRENT | MCL_FUTURE)) {
> +		ksft_test_result_fail("mlockall(MCL_CURRENT | MCL_FUTURE): %s\n", strerror(errno));
> +		return;
> +	}
> +
> +	map = mmap(NULL, 2 * page_size, PROT_READ | PROT_WRITE,
> +		   MAP_ANONYMOUS | MAP_DROPPABLE, -1, 0);
> +
> +	if (map == MAP_FAILED) {
> +		if (errno == EOPNOTSUPP)
> +			ksft_test_result_skip("%s: MAP_DROPPABLE not supported\n", __func__);
> +		else
> +			ksft_test_result_fail("mmap error: %s\n", strerror(errno));
> +		munlockall();
> +		return;
> +	}
> +
> +	ksft_test_result(!unlock_lock_check(map, false), "%s: droppable memory not locked\n",
> +			__func__);
> +
> +	munlockall();
>  	munmap(map, 2 * page_size);
>  }
> +#else
> +static void test_mlock_droppable(void)
> +{
> +	ksft_test_result_skip("%s: MAP_DROPPABLE not supported\n", __func__);
> +}
> +
> +static void test_mlockall_future_droppable(void)
> +{
> +	ksft_test_result_skip("%s: MAP_DROPPABLE not supported\n", __func__);
> +}
> +#endif /* MAP_DROPPABLE */
>  

Why not a above a

#ifndef MAP_DROPPABLE
#define MAP_DROPPABLE	0x08
#endif

instead?

>  static void test_vma_management(bool call_mlock)
>  {
> @@ -442,7 +522,7 @@ int main(int argc, char **argv)
>  
>  	munmap(map, size);
>  
> -	ksft_set_plan(13);
> +	ksft_set_plan(15);
>  
>  	test_mlock_lock();
>  	test_mlock_onfault();
> @@ -451,6 +531,8 @@ int main(int argc, char **argv)
>  	test_lock_onfault_of_present();
>  	test_vma_management(true);
>  	test_mlockall();
> +	test_mlock_droppable();
> +	test_mlockall_future_droppable();
>  
>  	ksft_finished();
>  }

Feel free to add

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David