userfaultfd: selftest

This test allocates two virtual areas and bounces the physical memory
across the two virtual areas using only userfaultfd.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Shuah Khan <shuah.kh@samsung.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 650 insertions, 0 deletions

diff --git a/tools/testing/selftests/vm/Makefile b/tools/testing/selftests/vm/Makefile
index 231b9a031f6a..0d6854744b37 100644
--- a/tools/testing/selftests/vm/Makefile
+++ b/tools/testing/selftests/vm/Makefile
@@ -8,10 +8,13 @@ BINARIES += hugetlbfstest
 BINARIES += map_hugetlb
 BINARIES += thuge-gen
 BINARIES += transhuge-stress
+BINARIES += userfaultfd
 
 all: $(BINARIES)
 %: %.c
         $(CC) $(CFLAGS) -o $@ $^ -lrt
+userfaultfd: userfaultfd.c
+        $(CC) $(CFLAGS) -O2 -o $@ $^ -lpthread
 
 TEST_PROGS := run_vmtests
 TEST_FILES := $(BINARIES)
diff --git a/tools/testing/selftests/vm/run_vmtests b/tools/testing/selftests/vm/run_vmtests
index 49ece11ff7fd..831adeb5fc55 100755
--- a/tools/testing/selftests/vm/run_vmtests
+++ b/tools/testing/selftests/vm/run_vmtests
@@ -86,6 +86,17 @@ else
         echo "[PASS]"
 fi
 
+echo "--------------------"
+echo "running userfaultfd"
+echo "--------------------"
+./userfaultfd 128 32
+if [ $? -ne 0 ]; then
+        echo "[FAIL]"
+        exitcode=1
+else
+        echo "[PASS]"
+fi
+
 #cleanup
 umount $mnt
 rm -rf $mnt
diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
new file mode 100644
index 000000000000..0c0b83953352
--- /dev/null
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -0,0 +1,636 @@
+/*
+ * Stress userfaultfd syscall.
+ *
+ *  Copyright (C) 2015  Red Hat, Inc.
+ *
+ *  This work is licensed under the terms of the GNU GPL, version 2. See
+ *  the COPYING file in the top-level directory.
+ *
+ * This test allocates two virtual areas and bounces the physical
+ * memory across the two virtual areas (from area_src to area_dst)
+ * using userfaultfd.
+ *
+ * There are three threads running per CPU:
+ *
+ * 1) one per-CPU thread takes a per-page pthread_mutex in a random
+ *    page of the area_dst (while the physical page may still be in
+ *    area_src), and increments a per-page counter in the same page,
+ *    and checks its value against a verification region.
+ *
+ * 2) another per-CPU thread handles the userfaults generated by
+ *    thread 1 above. userfaultfd blocking reads or poll() modes are
+ *    exercised interleaved.
+ *
+ * 3) one last per-CPU thread transfers the memory in the background
+ *    at maximum bandwidth (if not already transferred by thread
+ *    2). Each cpu thread takes cares of transferring a portion of the
+ *    area.
+ *
+ * When all threads of type 3 completed the transfer, one bounce is
+ * complete. area_src and area_dst are then swapped. All threads are
+ * respawned and so the bounce is immediately restarted in the
+ * opposite direction.
+ *
+ * per-CPU threads 1 by triggering userfaults inside
+ * pthread_mutex_lock will also verify the atomicity of the memory
+ * transfer (UFFDIO_COPY).
+ *
+ * The program takes two parameters: the amounts of physical memory in
+ * megabytes (MiB) of the area and the number of bounces to execute.
+ *
+ * # 100MiB 99999 bounces
+ * ./userfaultfd 100 99999
+ *
+ * # 1GiB 99 bounces
+ * ./userfaultfd 1000 99
+ *
+ * # 10MiB-~6GiB 999 bounces, continue forever unless an error triggers
+ * while ./userfaultfd $[RANDOM % 6000 + 10] 999; do true; done
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <errno.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <time.h>
+#include <signal.h>
+#include <poll.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/syscall.h>
+#include <sys/ioctl.h>
+#include <pthread.h>
+#include "../../../../include/uapi/linux/userfaultfd.h"
+
+#ifdef __x86_64__
+#define __NR_userfaultfd 323
+#elif defined(__i386__)
+#define __NR_userfaultfd 359
+#elif defined(__powewrpc__)
+#define __NR_userfaultfd 364
+#else
+#error "missing __NR_userfaultfd definition"
+#endif
+
+static unsigned long nr_cpus, nr_pages, nr_pages_per_cpu, page_size;
+
+#define BOUNCE_RANDOM           (1<<0)
+#define BOUNCE_RACINGFAULTS     (1<<1)
+#define BOUNCE_VERIFY           (1<<2)
+#define BOUNCE_POLL             (1<<3)
+static int bounces;
+
+static unsigned long long *count_verify;
+static int uffd, finished, *pipefd;
+static char *area_src, *area_dst;
+static char *zeropage;
+pthread_attr_t attr;
+
+/* pthread_mutex_t starts at page offset 0 */
+#define area_mutex(___area, ___nr)                                      \
+        ((pthread_mutex_t *) ((___area) + (___nr)*page_size))
+/*
+ * count is placed in the page after pthread_mutex_t naturally aligned
+ * to avoid non alignment faults on non-x86 archs.
+ */
+#define area_count(___area, ___nr)                                      \
+        ((volatile unsigned long long *) ((unsigned long)               \
+                                 ((___area) + (___nr)*page_size +       \
+                                  sizeof(pthread_mutex_t) +             \
+                                  sizeof(unsigned long long) - 1) &     \
+                                 ~(unsigned long)(sizeof(unsigned long long) \
+                                                  -  1)))
+
+static int my_bcmp(char *str1, char *str2, size_t n)
+{
+        unsigned long i;
+        for (i = 0; i < n; i++)
+                if (str1[i] != str2[i])
+                        return 1;
+        return 0;
+}
+
+static void *locking_thread(void *arg)
+{
+        unsigned long cpu = (unsigned long) arg;
+        struct random_data rand;
+        unsigned long page_nr = *(&(page_nr)); /* uninitialized warning */
+        int32_t rand_nr;
+        unsigned long long count;
+        char randstate[64];
+        unsigned int seed;
+        time_t start;
+
+        if (bounces & BOUNCE_RANDOM) {
+                seed = (unsigned int) time(NULL) - bounces;
+                if (!(bounces & BOUNCE_RACINGFAULTS))
+                        seed += cpu;
+                bzero(&rand, sizeof(rand));
+                bzero(&randstate, sizeof(randstate));
+                if (initstate_r(seed, randstate, sizeof(randstate), &rand))
+                        fprintf(stderr, "srandom_r error\n"), exit(1);
+        } else {
+                page_nr = -bounces;
+                if (!(bounces & BOUNCE_RACINGFAULTS))
+                        page_nr += cpu * nr_pages_per_cpu;
+        }
+
+        while (!finished) {
+                if (bounces & BOUNCE_RANDOM) {
+                        if (random_r(&rand, &rand_nr))
+                                fprintf(stderr, "random_r 1 error\n"), exit(1);
+                        page_nr = rand_nr;
+                        if (sizeof(page_nr) > sizeof(rand_nr)) {
+                                if (random_r(&rand, &rand_nr))
+                                        fprintf(stderr, "random_r 2 error\n"), exit(1);
+                                page_nr |= ((unsigned long) rand_nr) << 32;
+                        }
+                } else
+                        page_nr += 1;
+                page_nr %= nr_pages;
+
+                start = time(NULL);
+                if (bounces & BOUNCE_VERIFY) {
+                        count = *area_count(area_dst, page_nr);
+                        if (!count)
+                                fprintf(stderr,
+                                        "page_nr %lu wrong count %Lu %Lu\n",
+                                        page_nr, count,
+                                        count_verify[page_nr]), exit(1);
+
+
+                        /*
+                         * We can't use bcmp (or memcmp) because that
+                         * returns 0 erroneously if the memory is
+                         * changing under it (even if the end of the
+                         * page is never changing and always
+                         * different).
+                         */
+#if 1
+                        if (!my_bcmp(area_dst + page_nr * page_size, zeropage,
+                                     page_size))
+                                fprintf(stderr,
+                                        "my_bcmp page_nr %lu wrong count %Lu %Lu\n",
+                                        page_nr, count,
+                                        count_verify[page_nr]), exit(1);
+#else
+                        unsigned long loops;
+
+                        loops = 0;
+                        /* uncomment the below line to test with mutex */
+                        /* pthread_mutex_lock(area_mutex(area_dst, page_nr)); */
+                        while (!bcmp(area_dst + page_nr * page_size, zeropage,
+                                     page_size)) {
+                                loops += 1;
+                                if (loops > 10)
+                                        break;
+                        }
+                        /* uncomment below line to test with mutex */
+                        /* pthread_mutex_unlock(area_mutex(area_dst, page_nr)); */
+                        if (loops) {
+                                fprintf(stderr,
+                                        "page_nr %lu all zero thread %lu %p %lu\n",
+                                        page_nr, cpu, area_dst + page_nr * page_size,
+                                        loops);
+                                if (loops > 10)
+                                        exit(1);
+                        }
+#endif
+                }
+
+                pthread_mutex_lock(area_mutex(area_dst, page_nr));
+                count = *area_count(area_dst, page_nr);
+                if (count != count_verify[page_nr]) {
+                        fprintf(stderr,
+                                "page_nr %lu memory corruption %Lu %Lu\n",
+                                page_nr, count,
+                                count_verify[page_nr]), exit(1);
+                }
+                count++;
+                *area_count(area_dst, page_nr) = count_verify[page_nr] = count;
+                pthread_mutex_unlock(area_mutex(area_dst, page_nr));
+
+                if (time(NULL) - start > 1)
+                        fprintf(stderr,
+                                "userfault too slow %ld "
+                                "possible false positive with overcommit\n",
+                                time(NULL) - start);
+        }
+
+        return NULL;
+}
+
+static int copy_page(unsigned long offset)
+{
+        struct uffdio_copy uffdio_copy;
+
+        if (offset >= nr_pages * page_size)
+                fprintf(stderr, "unexpected offset %lu\n",
+                        offset), exit(1);
+        uffdio_copy.dst = (unsigned long) area_dst + offset;
+        uffdio_copy.src = (unsigned long) area_src + offset;
+        uffdio_copy.len = page_size;
+        uffdio_copy.mode = 0;
+        uffdio_copy.copy = 0;
+        if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy)) {
+                /* real retval in ufdio_copy.copy */
+                if (uffdio_copy.copy != -EEXIST)
+                        fprintf(stderr, "UFFDIO_COPY error %Ld\n",
+                                uffdio_copy.copy), exit(1);
+        } else if (uffdio_copy.copy != page_size) {
+                fprintf(stderr, "UFFDIO_COPY unexpected copy %Ld\n",
+                        uffdio_copy.copy), exit(1);
+        } else
+                return 1;
+        return 0;
+}
+
+static void *uffd_poll_thread(void *arg)
+{
+        unsigned long cpu = (unsigned long) arg;
+        struct pollfd pollfd[2];
+        struct uffd_msg msg;
+        int ret;
+        unsigned long offset;
+        char tmp_chr;
+        unsigned long userfaults = 0;
+
+        pollfd[0].fd = uffd;
+        pollfd[0].events = POLLIN;
+        pollfd[1].fd = pipefd[cpu*2];
+        pollfd[1].events = POLLIN;
+
+        for (;;) {
+                ret = poll(pollfd, 2, -1);
+                if (!ret)
+                        fprintf(stderr, "poll error %d\n", ret), exit(1);
+                if (ret < 0)
+                        perror("poll"), exit(1);
+                if (pollfd[1].revents & POLLIN) {
+                        if (read(pollfd[1].fd, &tmp_chr, 1) != 1)
+                                fprintf(stderr, "read pipefd error\n"),
+                                        exit(1);
+                        break;
+                }
+                if (!(pollfd[0].revents & POLLIN))
+                        fprintf(stderr, "pollfd[0].revents %d\n",
+                                pollfd[0].revents), exit(1);
+                ret = read(uffd, &msg, sizeof(msg));
+                if (ret < 0) {
+                        if (errno == EAGAIN)
+                                continue;
+                        perror("nonblocking read error"), exit(1);
+                }
+                if (msg.event != UFFD_EVENT_PAGEFAULT)
+                        fprintf(stderr, "unexpected msg event %u\n",
+                                msg.event), exit(1);
+                if (msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)
+                        fprintf(stderr, "unexpected write fault\n"), exit(1);
+                offset = (char *)msg.arg.pagefault.address - area_dst;
+                offset &= ~(page_size-1);
+                if (copy_page(offset))
+                        userfaults++;
+        }
+        return (void *)userfaults;
+}
+
+pthread_mutex_t uffd_read_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+static void *uffd_read_thread(void *arg)
+{
+        unsigned long *this_cpu_userfaults;
+        struct uffd_msg msg;
+        unsigned long offset;
+        int ret;
+
+        this_cpu_userfaults = (unsigned long *) arg;
+        *this_cpu_userfaults = 0;
+
+        pthread_mutex_unlock(&uffd_read_mutex);
+        /* from here cancellation is ok */
+
+        for (;;) {
+                ret = read(uffd, &msg, sizeof(msg));
+                if (ret != sizeof(msg)) {
+                        if (ret < 0)
+                                perror("blocking read error"), exit(1);
+                        else
+                                fprintf(stderr, "short read\n"), exit(1);
+                }
+                if (msg.event != UFFD_EVENT_PAGEFAULT)
+                        fprintf(stderr, "unexpected msg event %u\n",
+                                msg.event), exit(1);
+                if (bounces & BOUNCE_VERIFY &&
+                    msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)
+                        fprintf(stderr, "unexpected write fault\n"), exit(1);
+                offset = (char *)msg.arg.pagefault.address - area_dst;
+                offset &= ~(page_size-1);
+                if (copy_page(offset))
+                        (*this_cpu_userfaults)++;
+        }
+        return (void *)NULL;
+}
+
+static void *background_thread(void *arg)
+{
+        unsigned long cpu = (unsigned long) arg;
+        unsigned long page_nr;
+
+        for (page_nr = cpu * nr_pages_per_cpu;
+             page_nr < (cpu+1) * nr_pages_per_cpu;
+             page_nr++)
+                copy_page(page_nr * page_size);
+
+        return NULL;
+}
+
+static int stress(unsigned long *userfaults)
+{
+        unsigned long cpu;
+        pthread_t locking_threads[nr_cpus];
+        pthread_t uffd_threads[nr_cpus];
+        pthread_t background_threads[nr_cpus];
+        void **_userfaults = (void **) userfaults;
+
+        finished = 0;
+        for (cpu = 0; cpu < nr_cpus; cpu++) {
+                if (pthread_create(&locking_threads[cpu], &attr,
+                                   locking_thread, (void *)cpu))
+                        return 1;
+                if (bounces & BOUNCE_POLL) {
+                        if (pthread_create(&uffd_threads[cpu], &attr,
+                                           uffd_poll_thread, (void *)cpu))
+                                return 1;
+                } else {
+                        if (pthread_create(&uffd_threads[cpu], &attr,
+                                           uffd_read_thread,
+                                           &_userfaults[cpu]))
+                                return 1;
+                        pthread_mutex_lock(&uffd_read_mutex);
+                }
+                if (pthread_create(&background_threads[cpu], &attr,
+                                   background_thread, (void *)cpu))
+                        return 1;
+        }
+        for (cpu = 0; cpu < nr_cpus; cpu++)
+                if (pthread_join(background_threads[cpu], NULL))
+                        return 1;
+
+        /*
+         * Be strict and immediately zap area_src, the whole area has
+         * been transferred already by the background treads. The
+         * area_src could then be faulted in in a racy way by still
+         * running uffdio_threads reading zeropages after we zapped
+         * area_src (but they're guaranteed to get -EEXIST from
+         * UFFDIO_COPY without writing zero pages into area_dst
+         * because the background threads already completed).
+         */
+        if (madvise(area_src, nr_pages * page_size, MADV_DONTNEED)) {
+                perror("madvise");
+                return 1;
+        }
+
+        for (cpu = 0; cpu < nr_cpus; cpu++) {
+                char c;
+                if (bounces & BOUNCE_POLL) {
+                        if (write(pipefd[cpu*2+1], &c, 1) != 1) {
+                                fprintf(stderr, "pipefd write error\n");
+                                return 1;
+                        }
+                        if (pthread_join(uffd_threads[cpu], &_userfaults[cpu]))
+                                return 1;
+                } else {
+                        if (pthread_cancel(uffd_threads[cpu]))
+                                return 1;
+                        if (pthread_join(uffd_threads[cpu], NULL))
+                                return 1;
+                }
+        }
+
+        finished = 1;
+        for (cpu = 0; cpu < nr_cpus; cpu++)
+                if (pthread_join(locking_threads[cpu], NULL))
+                        return 1;
+
+        return 0;
+}
+
+static int userfaultfd_stress(void)
+{
+        void *area;
+        char *tmp_area;
+        unsigned long nr;
+        struct uffdio_register uffdio_register;
+        struct uffdio_api uffdio_api;
+        unsigned long cpu;
+        int uffd_flags;
+        unsigned long userfaults[nr_cpus];
+
+        if (posix_memalign(&area, page_size, nr_pages * page_size)) {
+                fprintf(stderr, "out of memory\n");
+                return 1;
+        }
+        area_src = area;
+        if (posix_memalign(&area, page_size, nr_pages * page_size)) {
+                fprintf(stderr, "out of memory\n");
+                return 1;
+        }
+        area_dst = area;
+
+        uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+        if (uffd < 0) {
+                fprintf(stderr,
+                        "userfaultfd syscall not available in this kernel\n");
+                return 1;
+        }
+        uffd_flags = fcntl(uffd, F_GETFD, NULL);
+
+        uffdio_api.api = UFFD_API;
+        uffdio_api.features = 0;
+        if (ioctl(uffd, UFFDIO_API, &uffdio_api)) {
+                fprintf(stderr, "UFFDIO_API\n");
+                return 1;
+        }
+        if (uffdio_api.api != UFFD_API) {
+                fprintf(stderr, "UFFDIO_API error %Lu\n", uffdio_api.api);
+                return 1;
+        }
+
+        count_verify = malloc(nr_pages * sizeof(unsigned long long));
+        if (!count_verify) {
+                perror("count_verify");
+                return 1;
+        }
+
+        for (nr = 0; nr < nr_pages; nr++) {
+                *area_mutex(area_src, nr) = (pthread_mutex_t)
+                        PTHREAD_MUTEX_INITIALIZER;
+                count_verify[nr] = *area_count(area_src, nr) = 1;
+        }
+
+        pipefd = malloc(sizeof(int) * nr_cpus * 2);
+        if (!pipefd) {
+                perror("pipefd");
+                return 1;
+        }
+        for (cpu = 0; cpu < nr_cpus; cpu++) {
+                if (pipe2(&pipefd[cpu*2], O_CLOEXEC | O_NONBLOCK)) {
+                        perror("pipe");
+                        return 1;
+                }
+        }
+
+        if (posix_memalign(&area, page_size, page_size)) {
+                fprintf(stderr, "out of memory\n");
+                return 1;
+        }
+        zeropage = area;
+        bzero(zeropage, page_size);
+
+        pthread_mutex_lock(&uffd_read_mutex);
+
+        pthread_attr_init(&attr);
+        pthread_attr_setstacksize(&attr, 16*1024*1024);
+
+        while (bounces--) {
+                unsigned long expected_ioctls;
+
+                printf("bounces: %d, mode:", bounces);
+                if (bounces & BOUNCE_RANDOM)
+                        printf(" rnd");
+                if (bounces & BOUNCE_RACINGFAULTS)
+                        printf(" racing");
+                if (bounces & BOUNCE_VERIFY)
+                        printf(" ver");
+                if (bounces & BOUNCE_POLL)
+                        printf(" poll");
+                printf(", ");
+                fflush(stdout);
+
+                if (bounces & BOUNCE_POLL)
+                        fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
+                else
+                        fcntl(uffd, F_SETFL, uffd_flags & ~O_NONBLOCK);
+
+                /* register */
+                uffdio_register.range.start = (unsigned long) area_dst;
+                uffdio_register.range.len = nr_pages * page_size;
+                uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
+                if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register)) {
+                        fprintf(stderr, "register failure\n");
+                        return 1;
+                }
+                expected_ioctls = (1 << _UFFDIO_WAKE) |
+                                  (1 << _UFFDIO_COPY) |
+                                  (1 << _UFFDIO_ZEROPAGE);
+                if ((uffdio_register.ioctls & expected_ioctls) !=
+                    expected_ioctls) {
+                        fprintf(stderr,
+                                "unexpected missing ioctl for anon memory\n");
+                        return 1;
+                }
+
+                /*
+                 * The madvise done previously isn't enough: some
+                 * uffd_thread could have read userfaults (one of
+                 * those already resolved by the background thread)
+                 * and it may be in the process of calling
+                 * UFFDIO_COPY. UFFDIO_COPY will read the zapped
+                 * area_src and it would map a zero page in it (of
+                 * course such a UFFDIO_COPY is perfectly safe as it'd
+                 * return -EEXIST). The problem comes at the next
+                 * bounce though: that racing UFFDIO_COPY would
+                 * generate zeropages in the area_src, so invalidating
+                 * the previous MADV_DONTNEED. Without this additional
+                 * MADV_DONTNEED those zeropages leftovers in the
+                 * area_src would lead to -EEXIST failure during the
+                 * next bounce, effectively leaving a zeropage in the
+                 * area_dst.
+                 *
+                 * Try to comment this out madvise to see the memory
+                 * corruption being caught pretty quick.
+                 *
+                 * khugepaged is also inhibited to collapse THP after
+                 * MADV_DONTNEED only after the UFFDIO_REGISTER, so it's
+                 * required to MADV_DONTNEED here.
+                 */
+                if (madvise(area_dst, nr_pages * page_size, MADV_DONTNEED)) {
+                        perror("madvise 2");
+                        return 1;
+                }
+
+                /* bounce pass */
+                if (stress(userfaults))
+                        return 1;
+
+                /* unregister */
+                if (ioctl(uffd, UFFDIO_UNREGISTER, &uffdio_register.range)) {
+                        fprintf(stderr, "register failure\n");
+                        return 1;
+                }
+
+                /* verification */
+                if (bounces & BOUNCE_VERIFY) {
+                        for (nr = 0; nr < nr_pages; nr++) {
+                                if (my_bcmp(area_dst,
+                                            area_dst + nr * page_size,
+                                            sizeof(pthread_mutex_t))) {
+                                        fprintf(stderr,
+                                                "error mutex 2 %lu\n",
+                                                nr);
+                                        bounces = 0;
+                                }
+                                if (*area_count(area_dst, nr) != count_verify[nr]) {
+                                        fprintf(stderr,
+                                                "error area_count %Lu %Lu %lu\n",
+                                                *area_count(area_src, nr),
+                                                count_verify[nr],
+                                                nr);
+                                        bounces = 0;
+                                }
+                        }
+                }
+
+                /* prepare next bounce */
+                tmp_area = area_src;
+                area_src = area_dst;
+                area_dst = tmp_area;
+
+                printf("userfaults:");
+                for (cpu = 0; cpu < nr_cpus; cpu++)
+                        printf(" %lu", userfaults[cpu]);
+                printf("\n");
+        }
+
+        return 0;
+}
+
+int main(int argc, char **argv)
+{
+        if (argc < 3)
+                fprintf(stderr, "Usage: <MiB> <bounces>\n"), exit(1);
+        nr_cpus = sysconf(_SC_NPROCESSORS_ONLN);
+        page_size = sysconf(_SC_PAGE_SIZE);
+        if ((unsigned long) area_count(NULL, 0) + sizeof(unsigned long long) >
+            page_size)
+                fprintf(stderr, "Impossible to run this test\n"), exit(2);
+        nr_pages_per_cpu = atol(argv[1]) * 1024*1024 / page_size /
+                nr_cpus;
+        if (!nr_pages_per_cpu) {
+                fprintf(stderr, "invalid MiB\n");
+                fprintf(stderr, "Usage: <MiB> <bounces>\n"), exit(1);
+        }
+        bounces = atoi(argv[2]);
+        if (bounces <= 0) {
+                fprintf(stderr, "invalid bounces\n");
+                fprintf(stderr, "Usage: <MiB> <bounces>\n"), exit(1);
+        }
+        nr_pages = nr_pages_per_cpu * nr_cpus;
+        printf("nr_pages: %lu, nr_pages_per_cpu: %lu\n",
+               nr_pages, nr_pages_per_cpu);
+        return userfaultfd_stress();
+}