[PATCH] tools/testing/selftests/vm/mlock2-tests: fix mlock2 false-negative errors

[Rafael Aquini <aquini@redhat.com>]

Changes for commit 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs")
break this test expectations on the behavior of mlock syscall family immediately
inserting the recently faulted pages into the UNEVICTABLE_LRU, when MCL_ONFAULT is
passed to the syscall as part of its flag-set.

There is no functional error introduced by the aforementioned commit,
but it opens up a time window where the recently faulted and locked pages
might yet not be put back into the UNEVICTABLE_LRU, thus causing a
subsequent and immediate PFN flag check for the UNEVICTABLE bit
to trip on false-negative errors, as it happens with this test.

This patch fix the false negative by forcefully resorting to a code path that
will call a CPU pagevec drain right after the fault but before the PFN flag
check takes place, sorting out the race that way.

Fixes: 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs")
Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 tools/testing/selftests/vm/mlock2-tests.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/tools/testing/selftests/vm/mlock2-tests.c b/tools/testing/selftests/vm/mlock2-tests.c
index 637b6d0ac0d0..26dc320ca3c9 100644
--- a/tools/testing/selftests/vm/mlock2-tests.c
+++ b/tools/testing/selftests/vm/mlock2-tests.c
@@ -7,6 +7,7 @@
 #include <sys/time.h>
 #include <sys/resource.h>
 #include <stdbool.h>
+#include <sched.h>
 #include "mlock2.h"
 
 #include "../kselftest.h"
@@ -328,6 +329,22 @@ static int test_mlock_lock()
 	return ret;
 }
 
+/*
+ * After commit 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs")
+ * changes made by calls to mlock* family might not be immediately reflected
+ * on the LRUs, thus checking the PFN flags might race against pagevec drain.
+ *
+ * In order to sort out that race, and get the after fault checks consistent,
+ * the "quick and dirty" trick below is required in order to force a call to
+ * lru_add_drain_all() to get the recently MLOCK_ONFAULT pages moved to
+ * the unevictable LRU, as expected by the checks in this selftest.
+ */
+static void force_lru_add_drain_all(void)
+{
+	sched_yield();
+	system("echo 1 > /proc/sys/vm/compact_memory");
+}
+
 static int onfault_check(char *map)
 {
 	unsigned long page_size = getpagesize();
@@ -343,6 +360,9 @@ static int onfault_check(char *map)
 	}
 
 	*map = 'a';
+
+	force_lru_add_drain_all();
+
 	page1_flags = get_pageflags((unsigned long)map);
 	page2_flags = get_pageflags((unsigned long)map + page_size);
 
@@ -465,6 +485,8 @@ static int test_lock_onfault_of_present()
 		goto unmap;
 	}
 
+	force_lru_add_drain_all();
+
 	page1_flags = get_pageflags((unsigned long)map);
 	page2_flags = get_pageflags((unsigned long)map + page_size);
 	page1_flags = get_kpageflags(page1_flags & PFN_MASK);
-- 
2.24.1