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

[Nikita Kalyazin <kalyazin@amazon.com>]

On 25/11/2025 18:38, Mike Rapoport wrote:
> From: Nikita Kalyazin <kalyazin@amazon.com>
> 
> 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>
> Co-developed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>   .../testing/selftests/kvm/guest_memfd_test.c  | 103 ++++++++++++++++++
>   1 file changed, 103 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index e7d9aeb418d3..a5d3ed21d7bb 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -10,13 +10,17 @@
>   #include <errno.h>
>   #include <stdio.h>
>   #include <fcntl.h>
> +#include <pthread.h>
> 
>   #include <linux/bitmap.h>
>   #include <linux/falloc.h>
>   #include <linux/sizes.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"
> @@ -254,6 +258,104 @@ static void test_guest_memfd_flags(struct kvm_vm *vm)
>          }
>   }
> 
> +struct fault_args {
> +       char *addr;
> +       volatile char 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_minor(int fd, size_t total_size)
> +{
> +       struct uffdio_api uffdio_api = {
> +               .api = UFFD_API,
> +               .features = UFFD_FEATURE_MINOR_GENERIC,

Should it be UFFD_FEATURE_MINOR_SHMEM instead? 
UFFD_FEATURE_MINOR_GENERIC was removed in the v1.

> +       };
> +       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);
> +
> +       memset(buf, 0xaa, total_size);
> +
> +       uffd = syscall(__NR_userfaultfd, O_CLOEXEC);
> +       TEST_ASSERT(uffd != -1, "userfaultfd creation should succeed");
> +
> +       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");
> +
> +       /*
> +        * wait for fault_thread to finish to make sure fault happened and was
> +        * resolved before we verify the values
> +        */
> +       ret = pthread_join(fault_thread, NULL);
> +       TEST_ASSERT(ret == 0, "pthread_join should succeed");
> +
> +       TEST_ASSERT(args.value == *(char *)(mem_nofault + offset),
> +                   "memory should contain the value that was copied");
> +       TEST_ASSERT(args.value == *(char *)(mem + offset),
> +                   "no further fault is expected");
> +
> +       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);
> +}
> +
>   #define gmem_test(__test, __vm, __flags)                               \
>   do {                                                                   \
>          int fd = vm_create_guest_memfd(__vm, page_size * 4, __flags);   \
> @@ -273,6 +375,7 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)
>                  if (flags & GUEST_MEMFD_FLAG_INIT_SHARED) {
>                          gmem_test(mmap_supported, vm, flags);
>                          gmem_test(fault_overflow, vm, flags);
> +                       gmem_test(uffd_minor, vm, flags);
>                  } else {
>                          gmem_test(fault_private, vm, flags);
>                  }
> --
> 2.50.1
>