[RFC PATCH 6/7] iommufd/selftest: Add observability test for iommu-nr-pages

[Pranjal Shrivastava <praan@google.com>]

Add a test case to validate the iommu-nr-pages fdinfo metric. The test
maps sparse IOVAs to force page table directory allocations, verifies
that the metric increments accordingly, and asserts that the count
remains stranded after unmapping, verifying the current behaviour
required for future automated reclamation logic.

Signed-off-by: Pranjal Shrivastava <praan@google.com>
---
 tools/testing/selftests/iommu/iommufd.c | 48 +++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/tools/testing/selftests/iommu/iommufd.c b/tools/testing/selftests/iommu/iommufd.c
index d44b34b05757..f5f4ef01ea4f 100644
--- a/tools/testing/selftests/iommu/iommufd.c
+++ b/tools/testing/selftests/iommu/iommufd.c
@@ -3613,4 +3613,52 @@ TEST_F(iommufd_device_pasid, pasid_attach)
 	test_cmd_mock_domain_replace(self->stdev_id, self->ioas_id);
 }
 
+TEST_F(iommufd_ioas, iommufd_nr_pages)
+{
+	char path[256];
+	char line[128];
+	FILE *f;
+	long nr_pages_initial = -1;
+	long nr_pages_mapped = -1;
+	long nr_pages_final = -1;
+
+	if (!self->stdev_id)
+		SKIP(return, "Needs a device attached for HWPT");
+
+	snprintf(path, sizeof(path), "/proc/self/fdinfo/%d", self->fd);
+	f = fopen(path, "r");
+	ASSERT_NE(NULL, f);
+	while (fgets(line, sizeof(line), f)) {
+		if (sscanf(line, "iommu-nr-pages:\t%ld", &nr_pages_initial) == 1)
+			break;
+	}
+	fclose(f);
+	ASSERT_GE(nr_pages_initial, 0);
+
+	/* Map a page at start and one far away */
+	test_ioctl_ioas_map_fixed(buffer, PAGE_SIZE, self->base_iova);
+	test_ioctl_ioas_map_fixed(buffer, PAGE_SIZE, self->base_iova + (1UL << 30));
+
+	f = fopen(path, "r");
+	ASSERT_NE(NULL, f);
+	while (fgets(line, sizeof(line), f)) {
+		if (sscanf(line, "iommu-nr-pages:\t%ld", &nr_pages_mapped) == 1)
+			break;
+	}
+	fclose(f);
+	ASSERT_GT(nr_pages_mapped, nr_pages_initial);
+
+	test_ioctl_ioas_unmap(self->base_iova, PAGE_SIZE);
+	test_ioctl_ioas_unmap(self->base_iova + (1UL << 30), PAGE_SIZE);
+
+	f = fopen(path, "r");
+	ASSERT_NE(NULL, f);
+	while (fgets(line, sizeof(line), f)) {
+		if (sscanf(line, "iommu-nr-pages:\t%ld", &nr_pages_final) == 1)
+			break;
+	}
+	fclose(f);
+	ASSERT_EQ(nr_pages_final, nr_pages_mapped);
+}
+
 TEST_HARNESS_MAIN
-- 
2.56.0.rc1.315.gc6ed9934b7-goog