Re: [PATCH v2 5/5] KVM: selftests: test userfaultfd minor for guest_memfd

[James Houghton <jthoughton@google.com>]

On Wed, Apr 2, 2025 at 9:08 AM Nikita Kalyazin <kalyazin@amazon.com> wrote:
>
> The test demonstrates that a minor userfaultfd event in guest_memfd can
> be resolved via a memcpy followed by a UFFDIO_CONTINUE ioctl.
>
> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
> ---
>  .../testing/selftests/kvm/guest_memfd_test.c  | 94 +++++++++++++++++++
>  1 file changed, 94 insertions(+)
>
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 38c501e49e0e..9b47b796f3aa 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -10,12 +10,16 @@
>  #include <errno.h>
>  #include <stdio.h>
>  #include <fcntl.h>
> +#include <pthread.h>
>
>  #include <linux/bitmap.h>
>  #include <linux/falloc.h>
> +#include <linux/userfaultfd.h>
>  #include <sys/mman.h>
>  #include <sys/types.h>
>  #include <sys/stat.h>
> +#include <sys/syscall.h>
> +#include <sys/ioctl.h>
>
>  #include "kvm_util.h"
>  #include "test_util.h"
> @@ -206,6 +210,93 @@ static void test_create_guest_memfd_multiple(struct kvm_vm *vm)
>         close(fd1);
>  }
>
> +struct fault_args {
> +       char *addr;
> +       volatile char value;

I think you should/must put volatile on `addr` and not on `value`.

> +};
> +
> +static void *fault_thread_fn(void *arg)
> +{
> +       struct fault_args *args = arg;
> +
> +       /* Trigger page fault */
> +       args->value = *args->addr;
> +       return NULL;
> +}
> +
> +static void test_uffd_missing(int fd, size_t page_size, size_t total_size)

test_uffd_minor? :)

> +{
> +       struct uffdio_register uffd_reg;
> +       struct uffdio_continue uffd_cont;
> +       struct uffd_msg msg;
> +       struct fault_args args;
> +       pthread_t fault_thread;
> +       void *mem, *mem_nofault, *buf = NULL;
> +       int uffd, ret;
> +       off_t offset = page_size;
> +       void *fault_addr;
> +
> +       ret = posix_memalign(&buf, page_size, total_size);
> +       TEST_ASSERT_EQ(ret, 0);
> +
> +       uffd = syscall(__NR_userfaultfd, O_CLOEXEC);
> +       TEST_ASSERT(uffd != -1, "userfaultfd creation should succeed");
> +
> +       struct uffdio_api uffdio_api = {
> +               .api = UFFD_API,
> +               .features = UFFD_FEATURE_MISSING_SHMEM,

I think you mean UFFD_FEATURE_MINOR_SHMEM...?

And I'm trying to think through what feature we should expose for
guest_memfd; UFFD_FEATURE_MINOR_SHMEM already indicates support for
shmem.

We could have UFFD_FEATURE_MINOR_GUESTMEMFD, perhaps that's enough.

Or we could have UFFD_FEATURE_MINOR_GENERIC (or nothing at all!). Some
VMAs might not support the minor mode, and the user will figure that
out when UFFDIO_REGISTER fails.

> +       };
> +       ret = ioctl(uffd, UFFDIO_API, &uffdio_api);
> +       TEST_ASSERT(ret != -1, "ioctl(UFFDIO_API) should succeed");
> +
> +       mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +       TEST_ASSERT(mem != MAP_FAILED, "mmap should succeed");
> +
> +       mem_nofault = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +       TEST_ASSERT(mem_nofault != MAP_FAILED, "mmap should succeed");
> +
> +       uffd_reg.range.start = (unsigned long)mem;
> +       uffd_reg.range.len = total_size;
> +       uffd_reg.mode = UFFDIO_REGISTER_MODE_MINOR;
> +       ret = ioctl(uffd, UFFDIO_REGISTER, &uffd_reg);
> +       TEST_ASSERT(ret != -1, "ioctl(UFFDIO_REGISTER) should succeed");
> +
> +       ret = fallocate(fd, FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE,
> +                       offset, page_size);
> +       TEST_ASSERT(!ret, "fallocate(PUNCH_HOLE) should succeed");
> +
> +       fault_addr = mem + offset;
> +       args.addr = fault_addr;
> +
> +       ret = pthread_create(&fault_thread, NULL, fault_thread_fn, &args);
> +       TEST_ASSERT(ret == 0, "pthread_create should succeed");
> +
> +       ret = read(uffd, &msg, sizeof(msg));
> +       TEST_ASSERT(ret != -1, "read from userfaultfd should succeed");
> +       TEST_ASSERT(msg.event == UFFD_EVENT_PAGEFAULT, "event type should be pagefault");
> +       TEST_ASSERT((void *)(msg.arg.pagefault.address & ~(page_size - 1)) == fault_addr,
> +                   "pagefault should occur at expected address");
> +
> +       memcpy(mem_nofault + offset, buf + offset, page_size);
> +
> +       uffd_cont.range.start = (unsigned long)fault_addr;
> +       uffd_cont.range.len = page_size;
> +       uffd_cont.mode = 0;
> +       ret = ioctl(uffd, UFFDIO_CONTINUE, &uffd_cont);
> +       TEST_ASSERT(ret != -1, "ioctl(UFFDIO_CONTINUE) should succeed");
> +
> +       ret = pthread_join(fault_thread, NULL);
> +       TEST_ASSERT(ret == 0, "pthread_join should succeed");

And maybe also:

/* Right value? */
TEST_ASSERT(args.value == *(char *)mem_nofault));
/* No second fault? */
TEST_ASSERT(args.value == *(char *)mem);

> +
> +       ret = munmap(mem_nofault, total_size);
> +       TEST_ASSERT(!ret, "munmap should succeed");
> +
> +       ret = munmap(mem, total_size);
> +       TEST_ASSERT(!ret, "munmap should succeed");
> +       free(buf);
> +       close(uffd);
> +}
> +
>  unsigned long get_shared_type(void)
>  {
>  #ifdef __x86_64__
> @@ -244,6 +335,9 @@ void test_vm_type(unsigned long type, bool is_shared)
>         test_fallocate(fd, page_size, total_size);
>         test_invalid_punch_hole(fd, page_size, total_size);
>
> +       if (is_shared)
> +               test_uffd_missing(fd, page_size, total_size);
> +
>         close(fd);
>         kvm_vm_release(vm);
>  }
> --
> 2.47.1
>