[PATCH 1/1] selftests/mm: new test that steals pages

[Breno Leitao]

This test stresses the race between of madvise(DONTNEED), a page fault
and a parallel huge page mmap, which should fail due to lack of
available page available for mapping.

This test case must run on a system with one and only one huge page
available.

	# echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

During setup, the test allocates the only available page, and starts
three threads:

  - thread 1:
      * madvise(MADV_DONTNEED) on the allocated huge page
  - thread 2:
      * Write to the allocated huge page
  - thread 3:
      * Tries to allocated (steal) an extra huge page (which is not
        available)

thread 3 should never succeed in the allocation, since the only huge
page was never unmapped, and should be reserved.

Touching the old page after thread3 allocation will raise a SIGBUS.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 tools/testing/selftests/mm/.gitignore         |   1 +
 tools/testing/selftests/mm/Makefile           |   1 +
 .../selftests/mm/hugetlb_madv_vs_map.c        | 124 ++++++++++++++++++
 3 files changed, 126 insertions(+)
 create mode 100644 tools/testing/selftests/mm/hugetlb_madv_vs_map.c

diff --git a/tools/testing/selftests/mm/.gitignore b/tools/testing/selftests/mm/.gitignore
index 4ff10ea61461..d26e962f2ac4 100644
--- a/tools/testing/selftests/mm/.gitignore
+++ b/tools/testing/selftests/mm/.gitignore
@@ -46,3 +46,4 @@ gup_longterm
 mkdirty
 va_high_addr_switch
 hugetlb_fault_after_madv
+hugetlb_madv_vs_map
diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index dede0bcf97a3..f6e42a773e1e 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile
@@ -70,6 +70,7 @@ TEST_GEN_FILES += ksm_tests
 TEST_GEN_FILES += ksm_functional_tests
 TEST_GEN_FILES += mdwe_test
 TEST_GEN_FILES += hugetlb_fault_after_madv
+TEST_GEN_FILES += hugetlb_madv_vs_map
 
 ifneq ($(ARCH),arm64)
 TEST_GEN_FILES += soft-dirty
diff --git a/tools/testing/selftests/mm/hugetlb_madv_vs_map.c b/tools/testing/selftests/mm/hugetlb_madv_vs_map.c
new file mode 100644
index 000000000000..d01e8d4901d0
--- /dev/null
+++ b/tools/testing/selftests/mm/hugetlb_madv_vs_map.c
@@ -0,0 +1,124 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A test case that must run on a system with one and only one huge page available.
+ *	# echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
+ *
+ * During setup, the test allocates the only available page, and starts three threads:
+ *  - thread1:
+ *	* madvise(MADV_DONTNEED) on the allocated huge page
+ *  - thread 2:
+ *	* Write to the allocated huge page
+ *  - thread 3:
+ *	* Try to allocated an extra huge page (which must not available)
+ *
+ *  The test fails if thread3 is able to allocate a page.
+ *
+ *  Touching the first page after thread3's allocation will raise a SIGBUS
+ *
+ *  Author: Breno Leitao <leitao@xxxxxxxxxx>
+ */
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "vm_util.h"
+#include "../kselftest.h"
+
+#define MMAP_SIZE (1 << 21)
+#define INLOOP_ITER 100
+
+char *huge_ptr;
+
+/* Touch the memory while it is being madvised() */
+void *touch(void *unused)
+{
+	for (int i = 0; i < INLOOP_ITER; i++)
+		huge_ptr[0] = '.';
+
+	return NULL;
+}
+
+void *madv(void *unused)
+{
+	for (int i = 0; i < INLOOP_ITER; i++)
+		madvise(huge_ptr, MMAP_SIZE, MADV_DONTNEED);
+
+	return NULL;
+}
+
+/*
+ * We got here, and there must be no huge page available for mapping
+ * The other hugepage should be flipping from used <-> reserved, because
+ * of madvise(DONTNEED).
+ */
+void *map_extra(void *unused)
+{
+	void *ptr;
+
+	for (int i = 0; i < INLOOP_ITER; i++) {
+		ptr = mmap(NULL, MMAP_SIZE, PROT_READ | PROT_WRITE,
+			   MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB,
+			   -1, 0);
+
+		if ((long)ptr != -1) {
+			/* Touching the other page now will cause a SIGBUG
+			 * huge_ptr[0] = '1';
+			 */
+			return ptr;
+		}
+	}
+
+	return NULL;
+}
+
+int main(void)
+{
+	pthread_t thread1, thread2, thread3;
+	unsigned long free_hugepages;
+	void *ret;
+
+	/*
+	 * On kernel 6.7, we are able to reproduce the problem with ~10
+	 * interactions
+	 */
+	int max = 10;
+
+	free_hugepages = get_free_hugepages();
+
+	if (free_hugepages != 1) {
+		ksft_exit_skip("This test needs one and only one page to execute. Got %lu\n",
+			       free_hugepages);
+	}
+
+	while (max--) {
+		huge_ptr = mmap(NULL, MMAP_SIZE, PROT_READ | PROT_WRITE,
+				MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB,
+				-1, 0);
+
+		if ((unsigned long)huge_ptr == -1) {
+			ksft_exit_skip("Failed to allocated huge page\n");
+			return KSFT_SKIP;
+		}
+
+		pthread_create(&thread1, NULL, madv, NULL);
+		pthread_create(&thread2, NULL, touch, NULL);
+		pthread_create(&thread3, NULL, map_extra, NULL);
+
+		pthread_join(thread1, NULL);
+		pthread_join(thread2, NULL);
+		pthread_join(thread3, &ret);
+
+		if (ret) {
+			ksft_test_result_fail("Unexpected huge page allocation\n");
+			return KSFT_FAIL;
+		}
+
+		/* Unmap and restart */
+		munmap(huge_ptr, MMAP_SIZE);
+	}
+
+	return KSFT_PASS;
+}
-- 
2.34.1