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Message-ID: <CADrL8HXm_UDKvrsNe6Guvo_pPvCN9ZJBKe=p0HM-iYZWufbEfA@mail.gmail.com>
Date: Wed, 2 Apr 2025 14:10:23 -0700
From: James Houghton <jthoughton@...gle.com>
To: Nikita Kalyazin <kalyazin@...zon.com>
Cc: akpm@...ux-foundation.org, pbonzini@...hat.com, shuah@...nel.org,
kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org, lorenzo.stoakes@...cle.com,
david@...hat.com, ryan.roberts@....com, quic_eberman@...cinc.com,
peterx@...hat.com, graf@...zon.de, jgowans@...zon.com, roypat@...zon.co.uk,
derekmn@...zon.com, nsaenz@...zon.es, xmarcalx@...zon.com
Subject: Re: [PATCH v2 5/5] KVM: selftests: test userfaultfd minor for guest_memfd
On Wed, Apr 2, 2025 at 9:08 AM Nikita Kalyazin <kalyazin@...zon.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@...zon.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
>
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