[PATCH v2 2/2] selftests/mm/uffd-stress: stricten constraint on free hugepages needed before the test

[Dev Jain <dev.jain@arm.com>]

The test requires at least 2 * (bytes/page_size) hugetlb memory, since
we require identical number of hugepages for src and dst location. Fix
this.

Along with the above, as explained in patch "selftests/mm/uffd-stress:
Make test operate on less hugetlb memory", the racy nature of the test
requires that we have some extra number of hugepages left beyond what is
required. Therefore, stricten this constraint.

Fixes: 5a6aa60d1823 ("selftests/mm: skip uffd hugetlb tests with insufficient hugepages")
Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 tools/testing/selftests/mm/uffd-stress.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/mm/uffd-stress.c b/tools/testing/selftests/mm/uffd-stress.c
index 40af7f67c407..3f6213ae0dd5 100644
--- a/tools/testing/selftests/mm/uffd-stress.c
+++ b/tools/testing/selftests/mm/uffd-stress.c
@@ -448,12 +448,6 @@ int main(int argc, char **argv)
 	parse_test_type_arg(argv[1]);
 	bytes = atol(argv[2]) * 1024 * 1024;
 
-	if (test_type == TEST_HUGETLB &&
-	   get_free_hugepages() < bytes / page_size) {
-		printf("skip: Skipping userfaultfd... not enough hugepages\n");
-		return KSFT_SKIP;
-	}
-
 	nr_cpus = sysconf(_SC_NPROCESSORS_ONLN);
 	if (nr_cpus > 32) {
 		/* Don't let calculation below go to zero. */
@@ -464,6 +458,17 @@ int main(int argc, char **argv)
 		nr_parallel = nr_cpus;
 	}
 
+	/*
+	 * src and dst each require bytes / page_size number of hugepages.
+	 * Ensure nr_parallel - 1 hugepages on top of that to account
+	 * for racy extra reservation of hugepages.
+	 */
+	if (test_type == TEST_HUGETLB &&
+	   get_free_hugepages() < 2 * (bytes / page_size) + nr_parallel - 1) {
+		printf("skip: Skipping userfaultfd... not enough hugepages\n");
+		return KSFT_SKIP;
+	}
+
 	nr_pages_per_cpu = bytes / page_size / nr_parallel;
 	if (!nr_pages_per_cpu) {
 		_err("pages_per_cpu = 0, cannot test (%lu / %lu / %lu)",
-- 
2.30.2