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Message-ID: <de6d461b585351e38e2f0ad23d06bdd1d05025bc.1760731772.git.ackerleytng@google.com>
Date: Fri, 17 Oct 2025 13:12:08 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: cgroups@...r.kernel.org, kvm@...r.kernel.org, linux-doc@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
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Subject: [RFC PATCH v1 27/37] KVM: selftests: guest_memfd: 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, for
example via pin_user_pages(), the conversion must be aborted.
This test uses the gup_test debugfs interface (which requires
CONFIG_GUP_TEST) to call pin_user_pages() on a specific page, artificially
increasing its reference count. It then attempts to convert a range of
pages, including the pinned page, from shared to private.
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.
Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
Co-developed-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
.../kvm/guest_memfd_conversions_test.c | 82 +++++++++++++++++++
1 file changed, 82 insertions(+)
diff --git a/tools/testing/selftests/kvm/guest_memfd_conversions_test.c b/tools/testing/selftests/kvm/guest_memfd_conversions_test.c
index e6abf2d30c62d..856166f1b1dfc 100644
--- a/tools/testing/selftests/kvm/guest_memfd_conversions_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_conversions_test.c
@@ -14,6 +14,7 @@
#include "kselftest_harness.h"
#include "test_util.h"
#include "ucall_common.h"
+#include "../../../../mm/gup_test.h"
FIXTURE(gmem_conversions) {
struct kvm_vcpu *vcpu;
@@ -404,6 +405,87 @@ GMEM_CONVERSION_TEST_INIT_SHARED(forked_accesses)
kvm_munmap(test_state, sizeof(*test_state));
}
+static int gup_test_fd;
+
+static void pin_pages(void *vaddr, uint64_t size)
+{
+ const struct pin_longterm_test args = {
+ .addr = (uint64_t)vaddr,
+ .size = size,
+ .flags = PIN_LONGTERM_TEST_FLAG_USE_WRITE,
+ };
+
+ gup_test_fd = open("/sys/kernel/debug/gup_test", O_RDWR);
+ TEST_REQUIRE(gup_test_fd >= 0);
+
+ TEST_ASSERT_EQ(ioctl(gup_test_fd, PIN_LONGTERM_TEST_START, &args), 0);
+}
+
+static void unpin_pages(void)
+{
+ if (gup_test_fd > 0)
+ TEST_ASSERT_EQ(ioctl(gup_test_fd, PIN_LONGTERM_TEST_STOP), 0);
+}
+
+static void test_convert_to_private_fails(test_data_t *t, loff_t pgoff,
+ size_t nr_pages,
+ loff_t expected_error_offset)
+{
+ loff_t offset = pgoff * page_size;
+ loff_t error_offset = -1ul;
+ 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);
+}
+
+/*
+ * This test depends on CONFIG_GUP_TEST to provide a kernel module that exposes
+ * pin_user_pages() to userspace.
+ */
+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));
--
2.51.0.858.gf9c4a03a3a-goog
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