selftests/mm: reduce uffd-unit-test poison test to minimum

The test will still generate quite some unwanted MCE error messages to
syslog. There was old proposal ratelimiting the MCE messages from kernel,
but that has risk of hiding real useful information on production systems.

We can at least reduce the test to minimum to not over-pollute dmesg,
however trying to not lose its coverage too much.

[[e-mail]: reduce uffd-unit-test poison test to minimum]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Peter Xu <[e-mail]>
Cc: Axel Rasmussen <[e-mail]>
Cc: Brendan Jackman <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/mm/uffd-unit-tests.c

···
 		do_uffdio_poison(uffd, offset);
 }
 
+/* Make sure to cover odd/even, and minimum duplications */
+#define  UFFD_POISON_TEST_NPAGES  4
+
 static void uffd_poison_test(uffd_test_args_t *targs)
 {
 	pthread_t uffd_mon;
···
 	struct uffd_args args = { 0 };
 	struct sigaction act = { 0 };
 	unsigned long nr_sigbus = 0;
-	unsigned long nr;
+	unsigned long nr, poison_pages = UFFD_POISON_TEST_NPAGES;
+
+	if (nr_pages < poison_pages) {
+		uffd_test_skip("Too few pages for POISON test");
+		return;
+	}
 
 	fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
 
-	uffd_register_poison(uffd, area_dst, nr_pages * page_size);
-	memset(area_src, 0, nr_pages * page_size);
+	uffd_register_poison(uffd, area_dst, poison_pages * page_size);
+	memset(area_src, 0, poison_pages * page_size);
 
 	args.handle_fault = uffd_poison_handle_fault;
 	if (pthread_create(&uffd_mon, NULL, uffd_poll_thread, &args))
···
 	if (sigaction(SIGBUS, &act, 0))
 		err("sigaction");
 
-	for (nr = 0; nr < nr_pages; ++nr) {
+	for (nr = 0; nr < poison_pages; ++nr) {
 		unsigned long offset = nr * page_size;
 		const char *bytes = (const char *) area_dst + offset;
 		const char *i;
···
 	if (pthread_join(uffd_mon, NULL))
 		err("pthread_join()");
 
-	if (nr_sigbus != nr_pages / 2)
+	if (nr_sigbus != poison_pages / 2)
 		err("expected to receive %lu SIGBUS, actually received %lu",
-		    nr_pages / 2, nr_sigbus);
+		    poison_pages / 2, nr_sigbus);
 
 	uffd_test_pass();
 }