[PATCH] hmm-tests: Fix migrate_dirty_page test

[Alistair Popple <apopple@nvidia.com>]

As noted by John Hubbard the original test relied on side effects of the
implementation of migrate_vma_setup() to detect if pages had been
swapped to disk or not. This is subject to change in future so
explicitly check for swap entries via pagemap instead. Fix a spelling
mistake while we're at it.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
Fixes: 5cc88e844e87 ("selftests/hmm-tests: add test for dirty bits")
---
 tools/testing/selftests/vm/hmm-tests.c | 50 +++++++++++++++++++++++---
 1 file changed, 46 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/vm/hmm-tests.c b/tools/testing/selftests/vm/hmm-tests.c
index 70fdb49b59ed..b5f6a7dc1f12 100644
--- a/tools/testing/selftests/vm/hmm-tests.c
+++ b/tools/testing/selftests/vm/hmm-tests.c
@@ -1261,9 +1261,47 @@ static int destroy_cgroup(void)
 	return 0;
 }
 
+static uint64_t get_pfn(int fd, uint64_t ptr)
+{
+	uint64_t pfn;
+	int ret;
+
+	ret = pread(fd, &pfn, sizeof(ptr),
+		(uint64_t) ptr / getpagesize() * sizeof(ptr));
+	if (ret != sizeof(ptr))
+		return 0;
+
+	return pfn;
+}
+
+#define PAGEMAP_SWAPPED (1ULL << 62)
+
+/* Returns true if at least one page in the range is on swap */
+static bool pages_swapped(void *ptr, unsigned long pages)
+{
+	uint64_t pfn;
+	int fd = open("/proc/self/pagemap", O_RDONLY);
+	unsigned long i;
+
+	if (fd < 0)
+		return false;
+
+	for (i = 0; i < pages; i++) {
+		pfn = get_pfn(fd, (uint64_t) ptr + i * getpagesize());
+
+		if (pfn & PAGEMAP_SWAPPED) {
+			close(fd);
+			return true;
+		}
+	}
+
+	close(fd);
+	return false;
+}
+
 /*
  * Try and migrate a dirty page that has previously been swapped to disk. This
- * checks that we don't loose dirty bits.
+ * checks that we don't lose dirty bits.
  */
 TEST_F(hmm, migrate_dirty_page)
 {
@@ -1300,6 +1338,10 @@ TEST_F(hmm, migrate_dirty_page)
 
 	ASSERT_FALSE(write_cgroup_param(cgroup, "memory.reclaim", 1UL<<30));
 
+	/* Make sure at least some pages got paged to disk. */
+	if (!pages_swapped(buffer->ptr, npages))
+		SKIP(return, "Pages weren't swapped when they should have been");
+
 	/* Fault pages back in from swap as clean pages */
 	for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
 		tmp += ptr[i];
@@ -1309,10 +1351,10 @@ TEST_F(hmm, migrate_dirty_page)
 		ptr[i] = i;
 
 	/*
-	 * Attempt to migrate memory to device, which should fail because
-	 * hopefully some pages are backed by swap storage.
+	 * Attempt to migrate memory to device. This might fail if some pages
+	 * are/were backed by swap but that's ok.
 	 */
-	ASSERT_TRUE(hmm_migrate_sys_to_dev(self->fd, buffer, npages));
+	hmm_migrate_sys_to_dev(self->fd, buffer, npages);
 
 	ASSERT_FALSE(write_cgroup_param(cgroup, "memory.reclaim", 1UL<<30));
 
-- 
2.35.1