selftests: vm: add test for Soft-Dirty PTE bit

This introduces three tests:

1) Sanity check soft dirty basic semantics: allocate area, clean,
dirty, check if the SD bit is flipped.

2) Check VMA reuse: validate the VM_SOFTDIRTY usage

3) Check soft-dirty on huge pages

This was motivated by Will Deacon's fix commit 912efa17e512 ("mm: proc:
Invalidate TLB after clearing soft-dirty page state"). I was tracking the
same issue that he fixed, and this test would have caught it.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Gabriel Krisman Bertazi <[e-mail]>
Signed-off-by: Muhammad Usama Anjum <[e-mail]>
Co-developed-by: Muhammad Usama Anjum <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/vm/.gitignore

···
 hmm-tests
 memfd_secret
 local_config.*
+soft-dirty
 split_huge_page_test
 ksm_tests

tools/testing/selftests/vm/Makefile

···
 TEST_GEN_FILES += thuge-gen
 TEST_GEN_FILES += transhuge-stress
 TEST_GEN_FILES += userfaultfd
+TEST_GEN_PROGS += soft-dirty
 TEST_GEN_PROGS += split_huge_page_test
 TEST_GEN_FILES += ksm_tests
 
···
 include ../lib.mk
 
 $(OUTPUT)/madv_populate: vm_util.c
+$(OUTPUT)/soft-dirty: vm_util.c
 $(OUTPUT)/split_huge_page_test: vm_util.c
 
 ifeq ($(MACHINE),x86_64)

tools/testing/selftests/vm/config

···
 CONFIG_DEVICE_PRIVATE=y
 CONFIG_TEST_HMM=m
 CONFIG_GUP_TEST=y
+CONFIG_TRANSPARENT_HUGEPAGE=y
+CONFIG_MEM_SOFT_DIRTY=y

tools/testing/selftests/vm/soft-dirty.c

···
+// SPDX-License-Identifier: GPL-2.0
+#include <stdio.h>
+#include <string.h>
+#include <stdbool.h>
+#include <fcntl.h>
+#include <stdint.h>
+#include <malloc.h>
+#include <sys/mman.h>
+#include "../kselftest.h"
+#include "vm_util.h"
+
+#define PAGEMAP_FILE_PATH "/proc/self/pagemap"
+#define TEST_ITERATIONS 10000
+
+static void test_simple(int pagemap_fd, int pagesize)
+{
+	int i;
+	char *map;
+
+	map = aligned_alloc(pagesize, pagesize);
+	if (!map)
+		ksft_exit_fail_msg("mmap failed\n");
+
+	clear_softdirty();
+
+	for (i = 0 ; i < TEST_ITERATIONS; i++) {
+		if (pagemap_is_softdirty(pagemap_fd, map) == 1) {
+			ksft_print_msg("dirty bit was 1, but should be 0 (i=%d)\n", i);
+			break;
+		}
+
+		clear_softdirty();
+		// Write something to the page to get the dirty bit enabled on the page
+		map[0]++;
+
+		if (pagemap_is_softdirty(pagemap_fd, map) == 0) {
+			ksft_print_msg("dirty bit was 0, but should be 1 (i=%d)\n", i);
+			break;
+		}
+
+		clear_softdirty();
+	}
+	free(map);
+
+	ksft_test_result(i == TEST_ITERATIONS, "Test %s\n", __func__);
+}
+
+static void test_vma_reuse(int pagemap_fd, int pagesize)
+{
+	char *map, *map2;
+
+	map = mmap(NULL, pagesize, (PROT_READ | PROT_WRITE), (MAP_PRIVATE | MAP_ANON), -1, 0);
+	if (map == MAP_FAILED)
+		ksft_exit_fail_msg("mmap failed");
+
+	// The kernel always marks new regions as soft dirty
+	ksft_test_result(pagemap_is_softdirty(pagemap_fd, map) == 1,
+			 "Test %s dirty bit of allocated page\n", __func__);
+
+	clear_softdirty();
+	munmap(map, pagesize);
+
+	map2 = mmap(NULL, pagesize, (PROT_READ | PROT_WRITE), (MAP_PRIVATE | MAP_ANON), -1, 0);
+	if (map2 == MAP_FAILED)
+		ksft_exit_fail_msg("mmap failed");
+
+	// Dirty bit is set for new regions even if they are reused
+	if (map == map2)
+		ksft_test_result(pagemap_is_softdirty(pagemap_fd, map2) == 1,
+				 "Test %s dirty bit of reused address page\n", __func__);
+	else
+		ksft_test_result_skip("Test %s dirty bit of reused address page\n", __func__);
+
+	munmap(map2, pagesize);
+}
+
+static void test_hugepage(int pagemap_fd, int pagesize)
+{
+	char *map;
+	int i, ret;
+	size_t hpage_len = read_pmd_pagesize();
+
+	map = memalign(hpage_len, hpage_len);
+	if (!map)
+		ksft_exit_fail_msg("memalign failed\n");
+
+	ret = madvise(map, hpage_len, MADV_HUGEPAGE);
+	if (ret)
+		ksft_exit_fail_msg("madvise failed %d\n", ret);
+
+	for (i = 0; i < hpage_len; i++)
+		map[i] = (char)i;
+
+	if (check_huge(map)) {
+		ksft_test_result_pass("Test %s huge page allocation\n", __func__);
+
+		clear_softdirty();
+		for (i = 0 ; i < TEST_ITERATIONS ; i++) {
+			if (pagemap_is_softdirty(pagemap_fd, map) == 1) {
+				ksft_print_msg("dirty bit was 1, but should be 0 (i=%d)\n", i);
+				break;
+			}
+
+			clear_softdirty();
+			// Write something to the page to get the dirty bit enabled on the page
+			map[0]++;
+
+			if (pagemap_is_softdirty(pagemap_fd, map) == 0) {
+				ksft_print_msg("dirty bit was 0, but should be 1 (i=%d)\n", i);
+				break;
+			}
+			clear_softdirty();
+		}
+
+		ksft_test_result(i == TEST_ITERATIONS, "Test %s huge page dirty bit\n", __func__);
+	} else {
+		// hugepage allocation failed. skip these tests
+		ksft_test_result_skip("Test %s huge page allocation\n", __func__);
+		ksft_test_result_skip("Test %s huge page dirty bit\n", __func__);
+	}
+	free(map);
+}
+
+int main(int argc, char **argv)
+{
+	int pagemap_fd;
+	int pagesize;
+
+	ksft_print_header();
+	ksft_set_plan(5);
+
+	pagemap_fd = open(PAGEMAP_FILE_PATH, O_RDONLY);
+	if (pagemap_fd < 0)
+		ksft_exit_fail_msg("Failed to open %s\n", PAGEMAP_FILE_PATH);
+
+	pagesize = getpagesize();
+
+	test_simple(pagemap_fd, pagesize);
+	test_vma_reuse(pagemap_fd, pagesize);
+	test_hugepage(pagemap_fd, pagesize);
+
+	close(pagemap_fd);
+
+	return ksft_exit_pass();
+}