tools/testing/selftests/vm/userfaultfd.c

   1 /*
   2  * Stress userfaultfd syscall.
   3  *
   4  *  Copyright (C) 2015  Red Hat, Inc.
   5  *
   6  *  This work is licensed under the terms of the GNU GPL, version 2. See
   7  *  the COPYING file in the top-level directory.
   8  *
   9  * This test allocates two virtual areas and bounces the physical
  10  * memory across the two virtual areas (from area_src to area_dst)
  11  * using userfaultfd.
  12  *
  13  * There are three threads running per CPU:
  14  *
  15  * 1) one per-CPU thread takes a per-page pthread_mutex in a random
  16  *    page of the area_dst (while the physical page may still be in
  17  *    area_src), and increments a per-page counter in the same page,
  18  *    and checks its value against a verification region.
  19  *
  20  * 2) another per-CPU thread handles the userfaults generated by
  21  *    thread 1 above. userfaultfd blocking reads or poll() modes are
  22  *    exercised interleaved.
  23  *
  24  * 3) one last per-CPU thread transfers the memory in the background
  25  *    at maximum bandwidth (if not already transferred by thread
  26  *    2). Each cpu thread takes cares of transferring a portion of the
  27  *    area.
  28  *
  29  * When all threads of type 3 completed the transfer, one bounce is
  30  * complete. area_src and area_dst are then swapped. All threads are
  31  * respawned and so the bounce is immediately restarted in the
  32  * opposite direction.
  33  *
  34  * per-CPU threads 1 by triggering userfaults inside
  35  * pthread_mutex_lock will also verify the atomicity of the memory
  36  * transfer (UFFDIO_COPY).
  37  *
  38  * The program takes two parameters: the amounts of physical memory in
  39  * megabytes (MiB) of the area and the number of bounces to execute.
  40  *
  41  * # 100MiB 99999 bounces
  42  * ./userfaultfd 100 99999
  43  *
  44  * # 1GiB 99 bounces
  45  * ./userfaultfd 1000 99
  46  *
  47  * # 10MiB-~6GiB 999 bounces, continue forever unless an error triggers
  48  * while ./userfaultfd $[RANDOM % 6000 + 10] 999; do true; done
  49  */
  50
  51 #define _GNU_SOURCE
  52 #include <stdio.h>
  53 #include <errno.h>
  54 #include <unistd.h>
  55 #include <stdlib.h>
  56 #include <sys/types.h>
  57 #include <sys/stat.h>
  58 #include <fcntl.h>
  59 #include <time.h>
  60 #include <signal.h>
  61 #include <poll.h>
  62 #include <string.h>
  63 #include <sys/mman.h>
  64 #include <sys/syscall.h>
  65 #include <sys/ioctl.h>
  66 #include <pthread.h>
  67 #include <linux/userfaultfd.h>
  68
  69 #ifndef __NR_userfaultfd
  70 #error "missing __NR_userfaultfd definition"
  71 #endif
  72
  73 static unsigned long nr_cpus, nr_pages, nr_pages_per_cpu, page_size;
  74
  75 #define BOUNCE_RANDOM           (1<<0)
  76 #define BOUNCE_RACINGFAULTS     (1<<1)
  77 #define BOUNCE_VERIFY           (1<<2)
  78 #define BOUNCE_POLL             (1<<3)
  79 static int bounces;
  80
  81 static unsigned long long *count_verify;
  82 static int uffd, finished, *pipefd;
  83 static char *area_src, *area_dst;
  84 static char *zeropage;
  85 pthread_attr_t attr;
  86
  87 /* pthread_mutex_t starts at page offset 0 */
  88 #define area_mutex(___area, ___nr)                                      \
  89         ((pthread_mutex_t *) ((___area) + (___nr)*page_size))
  90 /*
  91  * count is placed in the page after pthread_mutex_t naturally aligned
  92  * to avoid non alignment faults on non-x86 archs.
  93  */
  94 #define area_count(___area, ___nr)                                      \
  95         ((volatile unsigned long long *) ((unsigned long)               \
  96                                  ((___area) + (___nr)*page_size +       \
  97                                   sizeof(pthread_mutex_t) +             \
  98                                   sizeof(unsigned long long) - 1) &     \
  99                                  ~(unsigned long)(sizeof(unsigned long long) \
 100                                                   -  1)))
 101
 102 static int my_bcmp(char *str1, char *str2, size_t n)
 103 {
 104         unsigned long i;
 105         for (i = 0; i < n; i++)
 106                 if (str1[i] != str2[i])
 107                         return 1;
 108         return 0;
 109 }
 110
 111 static void *locking_thread(void *arg)
 112 {
 113         unsigned long cpu = (unsigned long) arg;
 114         struct random_data rand;
 115         unsigned long page_nr = *(&(page_nr)); /* uninitialized warning */
 116         int32_t rand_nr;
 117         unsigned long long count;
 118         char randstate[64];
 119         unsigned int seed;
 120         time_t start;
 121
 122         if (bounces & BOUNCE_RANDOM) {
 123                 seed = (unsigned int) time(NULL) - bounces;
 124                 if (!(bounces & BOUNCE_RACINGFAULTS))
 125                         seed += cpu;
 126                 bzero(&rand, sizeof(rand));
 127                 bzero(&randstate, sizeof(randstate));
 128                 if (initstate_r(seed, randstate, sizeof(randstate), &rand))
 129                         fprintf(stderr, "srandom_r error\n"), exit(1);
 130         } else {
 131                 page_nr = -bounces;
 132                 if (!(bounces & BOUNCE_RACINGFAULTS))
 133                         page_nr += cpu * nr_pages_per_cpu;
 134         }
 135
 136         while (!finished) {
 137                 if (bounces & BOUNCE_RANDOM) {
 138                         if (random_r(&rand, &rand_nr))
 139                                 fprintf(stderr, "random_r 1 error\n"), exit(1);
 140                         page_nr = rand_nr;
 141                         if (sizeof(page_nr) > sizeof(rand_nr)) {
 142                                 if (random_r(&rand, &rand_nr))
 143                                         fprintf(stderr, "random_r 2 error\n"), exit(1);
 144                                 page_nr |= (((unsigned long) rand_nr) << 16) <<
 145                                            16;
 146                         }
 147                 } else
 148                         page_nr += 1;
 149                 page_nr %= nr_pages;
 150
 151                 start = time(NULL);
 152                 if (bounces & BOUNCE_VERIFY) {
 153                         count = *area_count(area_dst, page_nr);
 154                         if (!count)
 155                                 fprintf(stderr,
 156                                         "page_nr %lu wrong count %Lu %Lu\n",
 157                                         page_nr, count,
 158                                         count_verify[page_nr]), exit(1);
 159
 160
 161                         /*
 162                          * We can't use bcmp (or memcmp) because that
 163                          * returns 0 erroneously if the memory is
 164                          * changing under it (even if the end of the
 165                          * page is never changing and always
 166                          * different).
 167                          */
 168 #if 1
 169                         if (!my_bcmp(area_dst + page_nr * page_size, zeropage,
 170                                      page_size))
 171                                 fprintf(stderr,
 172                                         "my_bcmp page_nr %lu wrong count %Lu %Lu\n",
 173                                         page_nr, count,
 174                                         count_verify[page_nr]), exit(1);
 175 #else
 176                         unsigned long loops;
 177
 178                         loops = 0;
 179                         /* uncomment the below line to test with mutex */
 180                         /* pthread_mutex_lock(area_mutex(area_dst, page_nr)); */
 181                         while (!bcmp(area_dst + page_nr * page_size, zeropage,
 182                                      page_size)) {
 183                                 loops += 1;
 184                                 if (loops > 10)
 185                                         break;
 186                         }
 187                         /* uncomment below line to test with mutex */
 188                         /* pthread_mutex_unlock(area_mutex(area_dst, page_nr)); */
 189                         if (loops) {
 190                                 fprintf(stderr,
 191                                         "page_nr %lu all zero thread %lu %p %lu\n",
 192                                         page_nr, cpu, area_dst + page_nr * page_size,
 193                                         loops);
 194                                 if (loops > 10)
 195                                         exit(1);
 196                         }
 197 #endif
 198                 }
 199
 200                 pthread_mutex_lock(area_mutex(area_dst, page_nr));
 201                 count = *area_count(area_dst, page_nr);
 202                 if (count != count_verify[page_nr]) {
 203                         fprintf(stderr,
 204                                 "page_nr %lu memory corruption %Lu %Lu\n",
 205                                 page_nr, count,
 206                                 count_verify[page_nr]), exit(1);
 207                 }
 208                 count++;
 209                 *area_count(area_dst, page_nr) = count_verify[page_nr] = count;
 210                 pthread_mutex_unlock(area_mutex(area_dst, page_nr));
 211
 212                 if (time(NULL) - start > 1)
 213                         fprintf(stderr,
 214                                 "userfault too slow %ld "
 215                                 "possible false positive with overcommit\n",
 216                                 time(NULL) - start);
 217         }
 218
 219         return NULL;
 220 }
 221
 222 static int copy_page(unsigned long offset)
 223 {
 224         struct uffdio_copy uffdio_copy;
 225
 226         if (offset >= nr_pages * page_size)
 227                 fprintf(stderr, "unexpected offset %lu\n",
 228                         offset), exit(1);
 229         uffdio_copy.dst = (unsigned long) area_dst + offset;
 230         uffdio_copy.src = (unsigned long) area_src + offset;
 231         uffdio_copy.len = page_size;
 232         uffdio_copy.mode = 0;
 233         uffdio_copy.copy = 0;
 234         if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy)) {
 235                 /* real retval in ufdio_copy.copy */
 236                 if (uffdio_copy.copy != -EEXIST)
 237                         fprintf(stderr, "UFFDIO_COPY error %Ld\n",
 238                                 uffdio_copy.copy), exit(1);
 239         } else if (uffdio_copy.copy != page_size) {
 240                 fprintf(stderr, "UFFDIO_COPY unexpected copy %Ld\n",
 241                         uffdio_copy.copy), exit(1);
 242         } else
 243                 return 1;
 244         return 0;
 245 }
 246
 247 static void *uffd_poll_thread(void *arg)
 248 {
 249         unsigned long cpu = (unsigned long) arg;
 250         struct pollfd pollfd[2];
 251         struct uffd_msg msg;
 252         int ret;
 253         unsigned long offset;
 254         char tmp_chr;
 255         unsigned long userfaults = 0;
 256
 257         pollfd[0].fd = uffd;
 258         pollfd[0].events = POLLIN;
 259         pollfd[1].fd = pipefd[cpu*2];
 260         pollfd[1].events = POLLIN;
 261
 262         for (;;) {
 263                 ret = poll(pollfd, 2, -1);
 264                 if (!ret)
 265                         fprintf(stderr, "poll error %d\n", ret), exit(1);
 266                 if (ret < 0)
 267                         perror("poll"), exit(1);
 268                 if (pollfd[1].revents & POLLIN) {
 269                         if (read(pollfd[1].fd, &tmp_chr, 1) != 1)
 270                                 fprintf(stderr, "read pipefd error\n"),
 271                                         exit(1);
 272                         break;
 273                 }
 274                 if (!(pollfd[0].revents & POLLIN))
 275                         fprintf(stderr, "pollfd[0].revents %d\n",
 276                                 pollfd[0].revents), exit(1);
 277                 ret = read(uffd, &msg, sizeof(msg));
 278                 if (ret < 0) {
 279                         if (errno == EAGAIN)
 280                                 continue;
 281                         perror("nonblocking read error"), exit(1);
 282                 }
 283                 if (msg.event != UFFD_EVENT_PAGEFAULT)
 284                         fprintf(stderr, "unexpected msg event %u\n",
 285                                 msg.event), exit(1);
 286                 if (msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)
 287                         fprintf(stderr, "unexpected write fault\n"), exit(1);
 288                 offset = (char *)(unsigned long)msg.arg.pagefault.address -
 289                          area_dst;
 290                 offset &= ~(page_size-1);
 291                 if (copy_page(offset))
 292                         userfaults++;
 293         }
 294         return (void *)userfaults;
 295 }
 296
 297 pthread_mutex_t uffd_read_mutex = PTHREAD_MUTEX_INITIALIZER;
 298
 299 static void *uffd_read_thread(void *arg)
 300 {
 301         unsigned long *this_cpu_userfaults;
 302         struct uffd_msg msg;
 303         unsigned long offset;
 304         int ret;
 305
 306         this_cpu_userfaults = (unsigned long *) arg;
 307         *this_cpu_userfaults = 0;
 308
 309         pthread_mutex_unlock(&uffd_read_mutex);
 310         /* from here cancellation is ok */
 311
 312         for (;;) {
 313                 ret = read(uffd, &msg, sizeof(msg));
 314                 if (ret != sizeof(msg)) {
 315                         if (ret < 0)
 316                                 perror("blocking read error"), exit(1);
 317                         else
 318                                 fprintf(stderr, "short read\n"), exit(1);
 319                 }
 320                 if (msg.event != UFFD_EVENT_PAGEFAULT)
 321                         fprintf(stderr, "unexpected msg event %u\n",
 322                                 msg.event), exit(1);
 323                 if (bounces & BOUNCE_VERIFY &&
 324                     msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)
 325                         fprintf(stderr, "unexpected write fault\n"), exit(1);
 326                 offset = (char *)(unsigned long)msg.arg.pagefault.address -
 327                          area_dst;
 328                 offset &= ~(page_size-1);
 329                 if (copy_page(offset))
 330                         (*this_cpu_userfaults)++;
 331         }
 332         return (void *)NULL;
 333 }
 334
 335 static void *background_thread(void *arg)
 336 {
 337         unsigned long cpu = (unsigned long) arg;
 338         unsigned long page_nr;
 339
 340         for (page_nr = cpu * nr_pages_per_cpu;
 341              page_nr < (cpu+1) * nr_pages_per_cpu;
 342              page_nr++)
 343                 copy_page(page_nr * page_size);
 344
 345         return NULL;
 346 }
 347
 348 static int stress(unsigned long *userfaults)
 349 {
 350         unsigned long cpu;
 351         pthread_t locking_threads[nr_cpus];
 352         pthread_t uffd_threads[nr_cpus];
 353         pthread_t background_threads[nr_cpus];
 354         void **_userfaults = (void **) userfaults;
 355
 356         finished = 0;
 357         for (cpu = 0; cpu < nr_cpus; cpu++) {
 358                 if (pthread_create(&locking_threads[cpu], &attr,
 359                                    locking_thread, (void *)cpu))
 360                         return 1;
 361                 if (bounces & BOUNCE_POLL) {
 362                         if (pthread_create(&uffd_threads[cpu], &attr,
 363                                            uffd_poll_thread, (void *)cpu))
 364                                 return 1;
 365                 } else {
 366                         if (pthread_create(&uffd_threads[cpu], &attr,
 367                                            uffd_read_thread,
 368                                            &_userfaults[cpu]))
 369                                 return 1;
 370                         pthread_mutex_lock(&uffd_read_mutex);
 371                 }
 372                 if (pthread_create(&background_threads[cpu], &attr,
 373                                    background_thread, (void *)cpu))
 374                         return 1;
 375         }
 376         for (cpu = 0; cpu < nr_cpus; cpu++)
 377                 if (pthread_join(background_threads[cpu], NULL))
 378                         return 1;
 379
 380         /*
 381          * Be strict and immediately zap area_src, the whole area has
 382          * been transferred already by the background treads. The
 383          * area_src could then be faulted in in a racy way by still
 384          * running uffdio_threads reading zeropages after we zapped
 385          * area_src (but they're guaranteed to get -EEXIST from
 386          * UFFDIO_COPY without writing zero pages into area_dst
 387          * because the background threads already completed).
 388          */
 389         if (madvise(area_src, nr_pages * page_size, MADV_DONTNEED)) {
 390                 perror("madvise");
 391                 return 1;
 392         }
 393
 394         for (cpu = 0; cpu < nr_cpus; cpu++) {
 395                 char c;
 396                 if (bounces & BOUNCE_POLL) {
 397                         if (write(pipefd[cpu*2+1], &c, 1) != 1) {
 398                                 fprintf(stderr, "pipefd write error\n");
 399                                 return 1;
 400                         }
 401                         if (pthread_join(uffd_threads[cpu], &_userfaults[cpu]))
 402                                 return 1;
 403                 } else {
 404                         if (pthread_cancel(uffd_threads[cpu]))
 405                                 return 1;
 406                         if (pthread_join(uffd_threads[cpu], NULL))
 407                                 return 1;
 408                 }
 409         }
 410
 411         finished = 1;
 412         for (cpu = 0; cpu < nr_cpus; cpu++)
 413                 if (pthread_join(locking_threads[cpu], NULL))
 414                         return 1;
 415
 416         return 0;
 417 }
 418
 419 static int userfaultfd_stress(void)
 420 {
 421         void *area;
 422         char *tmp_area;
 423         unsigned long nr;
 424         struct uffdio_register uffdio_register;
 425         struct uffdio_api uffdio_api;
 426         unsigned long cpu;
 427         int uffd_flags;
 428         unsigned long userfaults[nr_cpus];
 429
 430         if (posix_memalign(&area, page_size, nr_pages * page_size)) {
 431                 fprintf(stderr, "out of memory\n");
 432                 return 1;
 433         }
 434         area_src = area;
 435         if (posix_memalign(&area, page_size, nr_pages * page_size)) {
 436                 fprintf(stderr, "out of memory\n");
 437                 return 1;
 438         }
 439         area_dst = area;
 440
 441         uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
 442         if (uffd < 0) {
 443                 fprintf(stderr,
 444                         "userfaultfd syscall not available in this kernel\n");
 445                 return 1;
 446         }
 447         uffd_flags = fcntl(uffd, F_GETFD, NULL);
 448
 449         uffdio_api.api = UFFD_API;
 450         uffdio_api.features = 0;
 451         if (ioctl(uffd, UFFDIO_API, &uffdio_api)) {
 452                 fprintf(stderr, "UFFDIO_API\n");
 453                 return 1;
 454         }
 455         if (uffdio_api.api != UFFD_API) {
 456                 fprintf(stderr, "UFFDIO_API error %Lu\n", uffdio_api.api);
 457                 return 1;
 458         }
 459
 460         count_verify = malloc(nr_pages * sizeof(unsigned long long));
 461         if (!count_verify) {
 462                 perror("count_verify");
 463                 return 1;
 464         }
 465
 466         for (nr = 0; nr < nr_pages; nr++) {
 467                 *area_mutex(area_src, nr) = (pthread_mutex_t)
 468                         PTHREAD_MUTEX_INITIALIZER;
 469                 count_verify[nr] = *area_count(area_src, nr) = 1;
 470         }
 471
 472         pipefd = malloc(sizeof(int) * nr_cpus * 2);
 473         if (!pipefd) {
 474                 perror("pipefd");
 475                 return 1;
 476         }
 477         for (cpu = 0; cpu < nr_cpus; cpu++) {
 478                 if (pipe2(&pipefd[cpu*2], O_CLOEXEC | O_NONBLOCK)) {
 479                         perror("pipe");
 480                         return 1;
 481                 }
 482         }
 483
 484         if (posix_memalign(&area, page_size, page_size)) {
 485                 fprintf(stderr, "out of memory\n");
 486                 return 1;
 487         }
 488         zeropage = area;
 489         bzero(zeropage, page_size);
 490
 491         pthread_mutex_lock(&uffd_read_mutex);
 492
 493         pthread_attr_init(&attr);
 494         pthread_attr_setstacksize(&attr, 16*1024*1024);
 495
 496         while (bounces--) {
 497                 unsigned long expected_ioctls;
 498
 499                 printf("bounces: %d, mode:", bounces);
 500                 if (bounces & BOUNCE_RANDOM)
 501                         printf(" rnd");
 502                 if (bounces & BOUNCE_RACINGFAULTS)
 503                         printf(" racing");
 504                 if (bounces & BOUNCE_VERIFY)
 505                         printf(" ver");
 506                 if (bounces & BOUNCE_POLL)
 507                         printf(" poll");
 508                 printf(", ");
 509                 fflush(stdout);
 510
 511                 if (bounces & BOUNCE_POLL)
 512                         fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
 513                 else
 514                         fcntl(uffd, F_SETFL, uffd_flags & ~O_NONBLOCK);
 515
 516                 /* register */
 517                 uffdio_register.range.start = (unsigned long) area_dst;
 518                 uffdio_register.range.len = nr_pages * page_size;
 519                 uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
 520                 if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register)) {
 521                         fprintf(stderr, "register failure\n");
 522                         return 1;
 523                 }
 524                 expected_ioctls = (1 << _UFFDIO_WAKE) |
 525                                   (1 << _UFFDIO_COPY) |
 526                                   (1 << _UFFDIO_ZEROPAGE);
 527                 if ((uffdio_register.ioctls & expected_ioctls) !=
 528                     expected_ioctls) {
 529                         fprintf(stderr,
 530                                 "unexpected missing ioctl for anon memory\n");
 531                         return 1;
 532                 }
 533
 534                 /*
 535                  * The madvise done previously isn't enough: some
 536                  * uffd_thread could have read userfaults (one of
 537                  * those already resolved by the background thread)
 538                  * and it may be in the process of calling
 539                  * UFFDIO_COPY. UFFDIO_COPY will read the zapped
 540                  * area_src and it would map a zero page in it (of
 541                  * course such a UFFDIO_COPY is perfectly safe as it'd
 542                  * return -EEXIST). The problem comes at the next
 543                  * bounce though: that racing UFFDIO_COPY would
 544                  * generate zeropages in the area_src, so invalidating
 545                  * the previous MADV_DONTNEED. Without this additional
 546                  * MADV_DONTNEED those zeropages leftovers in the
 547                  * area_src would lead to -EEXIST failure during the
 548                  * next bounce, effectively leaving a zeropage in the
 549                  * area_dst.
 550                  *
 551                  * Try to comment this out madvise to see the memory
 552                  * corruption being caught pretty quick.
 553                  *
 554                  * khugepaged is also inhibited to collapse THP after
 555                  * MADV_DONTNEED only after the UFFDIO_REGISTER, so it's
 556                  * required to MADV_DONTNEED here.
 557                  */
 558                 if (madvise(area_dst, nr_pages * page_size, MADV_DONTNEED)) {
 559                         perror("madvise 2");
 560                         return 1;
 561                 }
 562
 563                 /* bounce pass */
 564                 if (stress(userfaults))
 565                         return 1;
 566
 567                 /* unregister */
 568                 if (ioctl(uffd, UFFDIO_UNREGISTER, &uffdio_register.range)) {
 569                         fprintf(stderr, "register failure\n");
 570                         return 1;
 571                 }
 572
 573                 /* verification */
 574                 if (bounces & BOUNCE_VERIFY) {
 575                         for (nr = 0; nr < nr_pages; nr++) {
 576                                 if (my_bcmp(area_dst,
 577                                             area_dst + nr * page_size,
 578                                             sizeof(pthread_mutex_t))) {
 579                                         fprintf(stderr,
 580                                                 "error mutex 2 %lu\n",
 581                                                 nr);
 582                                         bounces = 0;
 583                                 }
 584                                 if (*area_count(area_dst, nr) != count_verify[nr]) {
 585                                         fprintf(stderr,
 586                                                 "error area_count %Lu %Lu %lu\n",
 587                                                 *area_count(area_src, nr),
 588                                                 count_verify[nr],
 589                                                 nr);
 590                                         bounces = 0;
 591                                 }
 592                         }
 593                 }
 594
 595                 /* prepare next bounce */
 596                 tmp_area = area_src;
 597                 area_src = area_dst;
 598                 area_dst = tmp_area;
 599
 600                 printf("userfaults:");
 601                 for (cpu = 0; cpu < nr_cpus; cpu++)
 602                         printf(" %lu", userfaults[cpu]);
 603                 printf("\n");
 604         }
 605
 606         return 0;
 607 }
 608
 609 int main(int argc, char **argv)
 610 {
 611         if (argc < 3)
 612                 fprintf(stderr, "Usage: <MiB> <bounces>\n"), exit(1);
 613         nr_cpus = sysconf(_SC_NPROCESSORS_ONLN);
 614         page_size = sysconf(_SC_PAGE_SIZE);
 615         if ((unsigned long) area_count(NULL, 0) + sizeof(unsigned long long) >
 616             page_size)
 617                 fprintf(stderr, "Impossible to run this test\n"), exit(2);
 618         nr_pages_per_cpu = atol(argv[1]) * 1024*1024 / page_size /
 619                 nr_cpus;
 620         if (!nr_pages_per_cpu) {
 621                 fprintf(stderr, "invalid MiB\n");
 622                 fprintf(stderr, "Usage: <MiB> <bounces>\n"), exit(1);
 623         }
 624         bounces = atoi(argv[2]);
 625         if (bounces <= 0) {
 626                 fprintf(stderr, "invalid bounces\n");
 627                 fprintf(stderr, "Usage: <MiB> <bounces>\n"), exit(1);
 628         }
 629         nr_pages = nr_pages_per_cpu * nr_cpus;
 630         printf("nr_pages: %lu, nr_pages_per_cpu: %lu\n",
 631                nr_pages, nr_pages_per_cpu);
 632         return userfaultfd_stress();
 633 }