selftest/mm: ksm_functional_tests: test in mmap_and_merge_range() if anything got merged

Let's extend mmap_and_merge_range() to test if anything in the current
process was merged. range_maps_duplicates() is too unreliable for that
use case, so instead look at KSM stats.

Trigger a complete unmerge first, to cleanup the stable tree and
stabilize accounting of merged pages.

Note that we're using /proc/self/ksm_merging_pages instead of
/proc/self/ksm_stat, because that one is available in more existing
kernels.

If /proc/self/ksm_merging_pages can't be opened, we can't perform any
checks and simply skip them.

We have to special-case the shared zeropage for now. But the only user
-- test_unmerge_zero_pages() -- performs its own merge checks.

Link: https://lkml.kernel.org/r/20230803143208.383663-7-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: liubo <liubo254@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/ksm_functional_tests.c

@@ -30,6 +30,7 @@
 static int ksm_fd;
 static int ksm_full_scans_fd;
 static int proc_self_ksm_stat_fd;
+static int proc_self_ksm_merging_pages_fd;
 static int ksm_use_zero_pages_fd;
 static int pagemap_fd;
 static size_t pagesize;
@@ -88,6 +89,22 @@ static long get_my_ksm_zero_pages(void)
 	return my_ksm_zero_pages;
 }
 
+static long get_my_merging_pages(void)
+{
+	char buf[10];
+	ssize_t ret;
+
+	if (proc_self_ksm_merging_pages_fd < 0)
+		return proc_self_ksm_merging_pages_fd;
+
+	ret = pread(proc_self_ksm_merging_pages_fd, buf, sizeof(buf) - 1, 0);
+	if (ret <= 0)
+		return -errno;
+	buf[ret] = 0;
+
+	return strtol(buf, NULL, 10);
+}
+
 static long ksm_get_full_scans(void)
 {
 	char buf[10];
@@ -120,11 +137,29 @@ static int ksm_merge(void)
 	return 0;
 }
 
+static int ksm_unmerge(void)
+{
+	if (write(ksm_fd, "2", 1) != 1)
+		return -errno;
+	return 0;
+}
+
 static char *mmap_and_merge_range(char val, unsigned long size, bool use_prctl)
 {
 	char *map;
 	int ret;
 
+	/* Stabilize accounting by disabling KSM completely. */
+	if (ksm_unmerge()) {
+		ksft_test_result_fail("Disabling (unmerging) KSM failed\n");
+		goto unmap;
+	}
+
+	if (get_my_merging_pages() > 0) {
+		ksft_test_result_fail("Still pages merged\n");
+		goto unmap;
+	}
+
 	map = mmap(NULL, size, PROT_READ|PROT_WRITE,
 		   MAP_PRIVATE|MAP_ANON, -1, 0);
 	if (map == MAP_FAILED) {
@@ -160,6 +195,16 @@ static char *mmap_and_merge_range(char val, unsigned long size, bool use_prctl)
 		ksft_test_result_fail("Running KSM failed\n");
 		goto unmap;
 	}
+
+	/*
+	 * Check if anything was merged at all. Ignore the zero page that is
+	 * accounted differently (depending on kernel support).
+	 */
+	if (val && !get_my_merging_pages()) {
+		ksft_test_result_fail("No pages got merged\n");
+		goto unmap;
+	}
+
 	return map;
 unmap:
 	munmap(map, size);
@@ -473,6 +518,8 @@ int main(int argc, char **argv)
 	if (pagemap_fd < 0)
 		ksft_exit_skip("open(\"/proc/self/pagemap\") failed\n");
 	proc_self_ksm_stat_fd = open("/proc/self/ksm_stat", O_RDONLY);
+	proc_self_ksm_merging_pages_fd = open("/proc/self/ksm_merging_pages",
+					      O_RDONLY);
 	ksm_use_zero_pages_fd = open("/sys/kernel/mm/ksm/use_zero_pages", O_RDWR);
 
 	test_unmerge();