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

[Nikita Kalyazin <kalyazin@amazon.com>]

On 02/04/2025 22:10, James Houghton wrote:
> 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`.

This was to prevent the compiler from omitting the write to the value, 
because it's never read later on.

> 
>> +};
>> +
>> +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.

Yes, I will introduce UFFD_FEATURE_MINOR_GUEST_MEMFD in the next version.

> 
> 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.

My concern is the exact reason of the failure may not be apparent to the 
caller in that case.

> 
>> +       };
>> +       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);

Good idea, thanks.  I don't need the volatile anymore :)

> 
>> +
>> +       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
>>