On Wed, Aug 28, 2024 at 06:48:33PM +0100, Lorenzo Stoakes wrote:

On Tue, Aug 27, 2024 at 10:49:22PM GMT, Charlie Jenkins wrote:

quoted

Add a selftest for MAP_BELOW_HINT that maps until it runs out of space
below the hint address.

Signed-off-by: Charlie Jenkins <redacted>
---
 tools/testing/selftests/mm/Makefile         |  1 +
 tools/testing/selftests/mm/map_below_hint.c | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index cfad627e8d94..4e2de85267b5 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile

@@ -50,6 +50,7 @@ TEST_GEN_FILES += hugepage-shm
 TEST_GEN_FILES += hugepage-vmemmap
 TEST_GEN_FILES += khugepaged
 TEST_GEN_FILES += madv_populate
+TEST_GEN_FILES += map_below_hint
 TEST_GEN_FILES += map_fixed_noreplace
 TEST_GEN_FILES += map_hugetlb
 TEST_GEN_FILES += map_populate

diff --git a/tools/testing/selftests/mm/map_below_hint.c b/tools/testing/selftests/mm/map_below_hint.c
new file mode 100644
index 000000000000..305274c5af49
--- /dev/null
+++ b/tools/testing/selftests/mm/map_below_hint.c

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test the MAP_BELOW_HINT mmap flag.
+ */
+#include <sys/mman.h>
+#include "../kselftest.h"
+
+#define ADDR 0x1000000UL
+#define LENGTH (ADDR / 100)
+
+#define MAP_BELOW_HINT	  0x8000000	/* Not defined in all libc */
+
+/*
+ * Map memory with MAP_BELOW_HINT until no memory left. Ensure that all returned
+ * addresses are below the hint.
+ */
+int main(int argc, char **argv)
+{
+	void *addr;
+
+	do {
+		addr = mmap((void *)ADDR, LENGTH, MAP_ANONYMOUS, MAP_BELOW_HINT, -1, 0);

How can this be correct? mmap() has parameters:

       void *mmap(void addr[.length], size_t length, int prot, int flags,
                  int fd, off_t offset);

You'r setting prot = MAP_ANONYMOUS, flags = MAP_BELOW_HINT...

This surely should be:

mmap(..., PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE | MAP_BELOW_HINT, -1, 0);

quoted

+	} while (addr == MAP_FAILED && (unsigned long)addr <= ADDR);

How can addr == MAP_FAILED (i.e. ~0) and addr <= ADDR? This will just loop
through once...

If you want to make sure you're getting mappings only below the hint until
you start getting MAP_FAILED's you'll need to handle this more robustly.

quoted

+
+	if (addr != MAP_FAILED && (unsigned long)addr > ADDR)
+		ksft_exit_fail_msg("mmap returned address above hint with MAP_BELOW_HINT\n");

This is just going to fail because your flags are wrong then wrongly claim
to have passed...

I obviously didn't spend enough time thinking about this test case...
You are correct that I wrote this incorrectly. I will make a proper test case
and send out a new version.

- Charlie

quoted

+
+	ksft_test_result_pass("MAP_BELOW_HINT works\n");
+}

--
2.45.0