KVM: selftests: Test conversion with elevated page refcount

Add a selftest to verify that converting a shared guest_memfd page to a
private page fails if the page has an elevated reference count.

When KVM converts a shared page to a private one, it expects the page to
have a reference count equal to the reference counts taken by the
filemap. If another kernel subsystem holds a reference to the page, the
conversion must be aborted.

The test asserts that both bulk and single-page conversion attempts
correctly fail with EAGAIN for the pinned page. After the page is unpinned,
the test verifies that subsequent conversions succeed.

Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://patch.msgid.link/20260910-gmem-inplace-conversion-v13-37-dd6fbf94f4e1@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 56 insertions, 0 deletions

diff --git a/tools/testing/selftests/kvm/x86/guest_memfd_conversions_test.c b/tools/testing/selftests/kvm/x86/guest_memfd_conversions_test.c
index a972a09c5c6e..e1c84df0f059 100644
--- a/tools/testing/selftests/kvm/x86/guest_memfd_conversions_test.c
+++ b/tools/testing/selftests/kvm/x86/guest_memfd_conversions_test.c
@@ -444,6 +444,62 @@ GMEM_CONVERSION_TEST_INIT_SHARED(forked_accesses)
 #undef TEST_STATE_AWAIT
 }
 
+static void test_convert_to_private_fails(test_data_t *t, u64 pgoff,
+					  size_t nr_pages,
+					  u64 expected_error_offset)
+{
+	/* +1 to make it anything but expected_error_offset. */
+	u64 error_offset = expected_error_offset + 1;
+	u64 offset = pgoff * page_size;
+	int ret;
+
+	do {
+		ret = __gmem_set_private(t->gmem_fd, offset,
+					 nr_pages * page_size, &error_offset);
+	} while (ret == -1 && errno == EINTR);
+	TEST_ASSERT(ret == -1 && errno == EAGAIN,
+		    "Wanted EAGAIN on page %lu, got %d (ret = %d)", pgoff,
+		    errno, ret);
+	TEST_ASSERT_EQ(error_offset, expected_error_offset);
+}
+
+GMEM_CONVERSION_MULTIPAGE_TEST_INIT_SHARED(elevated_refcount, 4)
+{
+	int i;
+
+	pin_pages(t->mem + test_page * page_size, page_size);
+
+	for (i = 0; i < nr_pages; i++)
+		test_shared(t, i, 0, 'A', 'B');
+
+	/*
+	 * Converting in bulk should fail as long any page in the range has
+	 * unexpected refcounts.
+	 */
+	test_convert_to_private_fails(t, 0, nr_pages, test_page * page_size);
+
+	for (i = 0; i < nr_pages; i++) {
+		/*
+		 * Converting page-wise should also fail as long any page in the
+		 * range has unexpected refcounts.
+		 */
+		if (i == test_page)
+			test_convert_to_private_fails(t, i, 1, test_page * page_size);
+		else
+			test_convert_to_private(t, i, 'B', 'C');
+	}
+
+	unpin_pages();
+
+	gmem_set_private(t->gmem_fd, 0, nr_pages * page_size);
+
+	for (i = 0; i < nr_pages; i++) {
+		char expected = i == test_page ? 'B' : 'C';
+
+		test_private(t, i, expected, 'D');
+	}
+}
+
 int main(int argc, char *argv[])
 {
 	TEST_REQUIRE(kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SW_PROTECTED_VM));