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();
+}