summaryrefslogtreecommitdiff
path: root/tools/testing/selftests/mm/uffd-common.c
blob: 7ad6ba660c7d6f1f5762d0b231b92b05e971a3c5 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
// SPDX-License-Identifier: GPL-2.0-only
/*
 * Userfaultfd tests util functions
 *
 * Copyright (C) 2015-2023  Red Hat, Inc.
 */

#include "uffd-common.h"

#define BASE_PMD_ADDR ((void *)(1UL << 30))

volatile bool test_uffdio_copy_eexist = true;
unsigned long nr_cpus, nr_pages, nr_pages_per_cpu, page_size;
char *area_src, *area_src_alias, *area_dst, *area_dst_alias, *area_remap;
int uffd = -1, uffd_flags, finished, *pipefd, test_type;
bool map_shared;
bool test_uffdio_wp = true;
unsigned long long *count_verify;
uffd_test_ops_t *uffd_test_ops;
uffd_test_case_ops_t *uffd_test_case_ops;
atomic_bool ready_for_fork;

static int uffd_mem_fd_create(off_t mem_size, bool hugetlb)
{
	unsigned int memfd_flags = 0;
	int mem_fd;

	if (hugetlb)
		memfd_flags = MFD_HUGETLB;
	mem_fd = memfd_create("uffd-test", memfd_flags);
	if (mem_fd < 0)
		err("memfd_create");
	if (ftruncate(mem_fd, mem_size))
		err("ftruncate");
	if (fallocate(mem_fd,
		      FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, 0,
		      mem_size))
		err("fallocate");

	return mem_fd;
}

static void anon_release_pages(char *rel_area)
{
	if (madvise(rel_area, nr_pages * page_size, MADV_DONTNEED))
		err("madvise(MADV_DONTNEED) failed");
}

static int anon_allocate_area(void **alloc_area, bool is_src)
{
	*alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
			   MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
	if (*alloc_area == MAP_FAILED) {
		*alloc_area = NULL;
		return -errno;
	}
	return 0;
}

static void noop_alias_mapping(__u64 *start, size_t len, unsigned long offset)
{
}

static void hugetlb_release_pages(char *rel_area)
{
	if (!map_shared) {
		if (madvise(rel_area, nr_pages * page_size, MADV_DONTNEED))
			err("madvise(MADV_DONTNEED) failed");
	} else {
		if (madvise(rel_area, nr_pages * page_size, MADV_REMOVE))
			err("madvise(MADV_REMOVE) failed");
	}
}

static int hugetlb_allocate_area(void **alloc_area, bool is_src)
{
	off_t size = nr_pages * page_size;
	off_t offset = is_src ? 0 : size;
	void *area_alias = NULL;
	char **alloc_area_alias;
	int mem_fd = uffd_mem_fd_create(size * 2, true);

	*alloc_area = mmap(NULL, size, PROT_READ | PROT_WRITE,
			   (map_shared ? MAP_SHARED : MAP_PRIVATE) |
			   (is_src ? 0 : MAP_NORESERVE),
			   mem_fd, offset);
	if (*alloc_area == MAP_FAILED) {
		*alloc_area = NULL;
		return -errno;
	}

	if (map_shared) {
		area_alias = mmap(NULL, size, PROT_READ | PROT_WRITE,
				  MAP_SHARED, mem_fd, offset);
		if (area_alias == MAP_FAILED)
			return -errno;
	}

	if (is_src) {
		alloc_area_alias = &area_src_alias;
	} else {
		alloc_area_alias = &area_dst_alias;
	}
	if (area_alias)
		*alloc_area_alias = area_alias;

	close(mem_fd);
	return 0;
}

static void hugetlb_alias_mapping(__u64 *start, size_t len, unsigned long offset)
{
	if (!map_shared)
		return;

	*start = (unsigned long) area_dst_alias + offset;
}

static void shmem_release_pages(char *rel_area)
{
	if (madvise(rel_area, nr_pages * page_size, MADV_REMOVE))
		err("madvise(MADV_REMOVE) failed");
}

static int shmem_allocate_area(void **alloc_area, bool is_src)
{
	void *area_alias = NULL;
	size_t bytes = nr_pages * page_size, hpage_size = read_pmd_pagesize();
	unsigned long offset = is_src ? 0 : bytes;
	char *p = NULL, *p_alias = NULL;
	int mem_fd = uffd_mem_fd_create(bytes * 2, false);

	/* TODO: clean this up.  Use a static addr is ugly */
	p = BASE_PMD_ADDR;
	if (!is_src)
		/* src map + alias + interleaved hpages */
		p += 2 * (bytes + hpage_size);
	p_alias = p;
	p_alias += bytes;
	p_alias += hpage_size;  /* Prevent src/dst VMA merge */

	*alloc_area = mmap(p, bytes, PROT_READ | PROT_WRITE, MAP_SHARED,
			   mem_fd, offset);
	if (*alloc_area == MAP_FAILED) {
		*alloc_area = NULL;
		return -errno;
	}
	if (*alloc_area != p)
		err("mmap of memfd failed at %p", p);

	area_alias = mmap(p_alias, bytes, PROT_READ | PROT_WRITE, MAP_SHARED,
			  mem_fd, offset);
	if (area_alias == MAP_FAILED) {
		munmap(*alloc_area, bytes);
		*alloc_area = NULL;
		return -errno;
	}
	if (area_alias != p_alias)
		err("mmap of anonymous memory failed at %p", p_alias);

	if (is_src)
		area_src_alias = area_alias;
	else
		area_dst_alias = area_alias;

	close(mem_fd);
	return 0;
}

static void shmem_alias_mapping(__u64 *start, size_t len, unsigned long offset)
{
	*start = (unsigned long)area_dst_alias + offset;
}

static void shmem_check_pmd_mapping(void *p, int expect_nr_hpages)
{
	if (!check_huge_shmem(area_dst_alias, expect_nr_hpages,
			      read_pmd_pagesize()))
		err("Did not find expected %d number of hugepages",
		    expect_nr_hpages);
}

struct uffd_test_ops anon_uffd_test_ops = {
	.allocate_area = anon_allocate_area,
	.release_pages = anon_release_pages,
	.alias_mapping = noop_alias_mapping,
	.check_pmd_mapping = NULL,
};

struct uffd_test_ops shmem_uffd_test_ops = {
	.allocate_area = shmem_allocate_area,
	.release_pages = shmem_release_pages,
	.alias_mapping = shmem_alias_mapping,
	.check_pmd_mapping = shmem_check_pmd_mapping,
};

struct uffd_test_ops hugetlb_uffd_test_ops = {
	.allocate_area = hugetlb_allocate_area,
	.release_pages = hugetlb_release_pages,
	.alias_mapping = hugetlb_alias_mapping,
	.check_pmd_mapping = NULL,
};

void uffd_stats_report(struct uffd_args *args, int n_cpus)
{
	int i;
	unsigned long long miss_total = 0, wp_total = 0, minor_total = 0;

	for (i = 0; i < n_cpus; i++) {
		miss_total += args[i].missing_faults;
		wp_total += args[i].wp_faults;
		minor_total += args[i].minor_faults;
	}

	printf("userfaults: ");
	if (miss_total) {
		printf("%llu missing (", miss_total);
		for (i = 0; i < n_cpus; i++)
			printf("%lu+", args[i].missing_faults);
		printf("\b) ");
	}
	if (wp_total) {
		printf("%llu wp (", wp_total);
		for (i = 0; i < n_cpus; i++)
			printf("%lu+", args[i].wp_faults);
		printf("\b) ");
	}
	if (minor_total) {
		printf("%llu minor (", minor_total);
		for (i = 0; i < n_cpus; i++)
			printf("%lu+", args[i].minor_faults);
		printf("\b)");
	}
	printf("\n");
}

int userfaultfd_open(uint64_t *features)
{
	struct uffdio_api uffdio_api;

	uffd = uffd_open(UFFD_FLAGS);
	if (uffd < 0)
		return -1;
	uffd_flags = fcntl(uffd, F_GETFD, NULL);

	uffdio_api.api = UFFD_API;
	uffdio_api.features = *features;
	if (ioctl(uffd, UFFDIO_API, &uffdio_api))
		/* Probably lack of CAP_PTRACE? */
		return -1;
	if (uffdio_api.api != UFFD_API)
		err("UFFDIO_API error: %" PRIu64, (uint64_t)uffdio_api.api);

	*features = uffdio_api.features;
	return 0;
}

static inline void munmap_area(void **area)
{
	if (*area)
		if (munmap(*area, nr_pages * page_size))
			err("munmap");

	*area = NULL;
}

void uffd_test_ctx_clear(void)
{
	size_t i;

	if (pipefd) {
		for (i = 0; i < nr_cpus * 2; ++i) {
			if (close(pipefd[i]))
				err("close pipefd");
		}
		free(pipefd);
		pipefd = NULL;
	}

	if (count_verify) {
		free(count_verify);
		count_verify = NULL;
	}

	if (uffd != -1) {
		if (close(uffd))
			err("close uffd");
		uffd = -1;
	}

	munmap_area((void **)&area_src);
	munmap_area((void **)&area_src_alias);
	munmap_area((void **)&area_dst);
	munmap_area((void **)&area_dst_alias);
	munmap_area((void **)&area_remap);
}

int uffd_test_ctx_init(uint64_t features, const char **errmsg)
{
	unsigned long nr, cpu;
	int ret;

	if (uffd_test_case_ops && uffd_test_case_ops->pre_alloc) {
		ret = uffd_test_case_ops->pre_alloc(errmsg);
		if (ret)
			return ret;
	}

	ret = uffd_test_ops->allocate_area((void **)&area_src, true);
	ret |= uffd_test_ops->allocate_area((void **)&area_dst, false);
	if (ret) {
		if (errmsg)
			*errmsg = "memory allocation failed";
		return ret;
	}

	if (uffd_test_case_ops && uffd_test_case_ops->post_alloc) {
		ret = uffd_test_case_ops->post_alloc(errmsg);
		if (ret)
			return ret;
	}

	ret = userfaultfd_open(&features);
	if (ret) {
		if (errmsg)
			*errmsg = "possible lack of priviledge";
		return ret;
	}

	count_verify = malloc(nr_pages * sizeof(unsigned long long));
	if (!count_verify)
		err("count_verify");

	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;
		/*
		 * In the transition between 255 to 256, powerpc will
		 * read out of order in my_bcmp and see both bytes as
		 * zero, so leave a placeholder below always non-zero
		 * after the count, to avoid my_bcmp to trigger false
		 * positives.
		 */
		*(area_count(area_src, nr) + 1) = 1;
	}

	/*
	 * After initialization of area_src, we must explicitly release pages
	 * for area_dst to make sure it's fully empty.  Otherwise we could have
	 * some area_dst pages be errornously initialized with zero pages,
	 * hence we could hit memory corruption later in the test.
	 *
	 * One example is when THP is globally enabled, above allocate_area()
	 * calls could have the two areas merged into a single VMA (as they
	 * will have the same VMA flags so they're mergeable).  When we
	 * initialize the area_src above, it's possible that some part of
	 * area_dst could have been faulted in via one huge THP that will be
	 * shared between area_src and area_dst.  It could cause some of the
	 * area_dst won't be trapped by missing userfaults.
	 *
	 * This release_pages() will guarantee even if that happened, we'll
	 * proactively split the thp and drop any accidentally initialized
	 * pages within area_dst.
	 */
	uffd_test_ops->release_pages(area_dst);

	pipefd = malloc(sizeof(int) * nr_cpus * 2);
	if (!pipefd)
		err("pipefd");
	for (cpu = 0; cpu < nr_cpus; cpu++)
		if (pipe2(&pipefd[cpu * 2], O_CLOEXEC | O_NONBLOCK))
			err("pipe");

	return 0;
}

void wp_range(int ufd, __u64 start, __u64 len, bool wp)
{
	struct uffdio_writeprotect prms;

	/* Write protection page faults */
	prms.range.start = start;
	prms.range.len = len;
	/* Undo write-protect, do wakeup after that */
	prms.mode = wp ? UFFDIO_WRITEPROTECT_MODE_WP : 0;

	if (ioctl(ufd, UFFDIO_WRITEPROTECT, &prms))
		err("clear WP failed: address=0x%"PRIx64, (uint64_t)start);
}

static void continue_range(int ufd, __u64 start, __u64 len, bool wp)
{
	struct uffdio_continue req;
	int ret;

	req.range.start = start;
	req.range.len = len;
	req.mode = 0;
	if (wp)
		req.mode |= UFFDIO_CONTINUE_MODE_WP;

	if (ioctl(ufd, UFFDIO_CONTINUE, &req))
		err("UFFDIO_CONTINUE failed for address 0x%" PRIx64,
		    (uint64_t)start);

	/*
	 * Error handling within the kernel for continue is subtly different
	 * from copy or zeropage, so it may be a source of bugs. Trigger an
	 * error (-EEXIST) on purpose, to verify doing so doesn't cause a BUG.
	 */
	req.mapped = 0;
	ret = ioctl(ufd, UFFDIO_CONTINUE, &req);
	if (ret >= 0 || req.mapped != -EEXIST)
		err("failed to exercise UFFDIO_CONTINUE error handling, ret=%d, mapped=%" PRId64,
		    ret, (int64_t) req.mapped);
}

int uffd_read_msg(int ufd, struct uffd_msg *msg)
{
	int ret = read(uffd, msg, sizeof(*msg));

	if (ret != sizeof(*msg)) {
		if (ret < 0) {
			if (errno == EAGAIN || errno == EINTR)
				return 1;
			err("blocking read error");
		} else {
			err("short read");
		}
	}

	return 0;
}

void uffd_handle_page_fault(struct uffd_msg *msg, struct uffd_args *args)
{
	unsigned long offset;

	if (msg->event != UFFD_EVENT_PAGEFAULT)
		err("unexpected msg event %u", msg->event);

	if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP) {
		/* Write protect page faults */
		wp_range(uffd, msg->arg.pagefault.address, page_size, false);
		args->wp_faults++;
	} else if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_MINOR) {
		uint8_t *area;
		int b;

		/*
		 * Minor page faults
		 *
		 * To prove we can modify the original range for testing
		 * purposes, we're going to bit flip this range before
		 * continuing.
		 *
		 * Note that this requires all minor page fault tests operate on
		 * area_dst (non-UFFD-registered) and area_dst_alias
		 * (UFFD-registered).
		 */

		area = (uint8_t *)(area_dst +
				   ((char *)msg->arg.pagefault.address -
				    area_dst_alias));
		for (b = 0; b < page_size; ++b)
			area[b] = ~area[b];
		continue_range(uffd, msg->arg.pagefault.address, page_size,
			       args->apply_wp);
		args->minor_faults++;
	} else {
		/*
		 * Missing page faults.
		 *
		 * Here we force a write check for each of the missing mode
		 * faults.  It's guaranteed because the only threads that
		 * will trigger uffd faults are the locking threads, and
		 * their first instruction to touch the missing page will
		 * always be pthread_mutex_lock().
		 *
		 * Note that here we relied on an NPTL glibc impl detail to
		 * always read the lock type at the entry of the lock op
		 * (pthread_mutex_t.__data.__type, offset 0x10) before
		 * doing any locking operations to guarantee that.  It's
		 * actually not good to rely on this impl detail because
		 * logically a pthread-compatible lib can implement the
		 * locks without types and we can fail when linking with
		 * them.  However since we used to find bugs with this
		 * strict check we still keep it around.  Hopefully this
		 * could be a good hint when it fails again.  If one day
		 * it'll break on some other impl of glibc we'll revisit.
		 */
		if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)
			err("unexpected write fault");

		offset = (char *)(unsigned long)msg->arg.pagefault.address - area_dst;
		offset &= ~(page_size-1);

		if (copy_page(uffd, offset, args->apply_wp))
			args->missing_faults++;
	}
}

void *uffd_poll_thread(void *arg)
{
	struct uffd_args *args = (struct uffd_args *)arg;
	unsigned long cpu = args->cpu;
	struct pollfd pollfd[2];
	struct uffd_msg msg;
	struct uffdio_register uffd_reg;
	int ret;
	char tmp_chr;

	if (!args->handle_fault)
		args->handle_fault = uffd_handle_page_fault;

	pollfd[0].fd = uffd;
	pollfd[0].events = POLLIN;
	pollfd[1].fd = pipefd[cpu*2];
	pollfd[1].events = POLLIN;

	ready_for_fork = true;

	for (;;) {
		ret = poll(pollfd, 2, -1);
		if (ret <= 0) {
			if (errno == EINTR || errno == EAGAIN)
				continue;
			err("poll error: %d", ret);
		}
		if (pollfd[1].revents) {
			if (!(pollfd[1].revents & POLLIN))
				err("pollfd[1].revents %d", pollfd[1].revents);
			if (read(pollfd[1].fd, &tmp_chr, 1) != 1)
				err("read pipefd error");
			break;
		}
		if (!(pollfd[0].revents & POLLIN))
			err("pollfd[0].revents %d", pollfd[0].revents);
		if (uffd_read_msg(uffd, &msg))
			continue;
		switch (msg.event) {
		default:
			err("unexpected msg event %u\n", msg.event);
			break;
		case UFFD_EVENT_PAGEFAULT:
			args->handle_fault(&msg, args);
			break;
		case UFFD_EVENT_FORK:
			close(uffd);
			uffd = msg.arg.fork.ufd;
			pollfd[0].fd = uffd;
			break;
		case UFFD_EVENT_REMOVE:
			uffd_reg.range.start = msg.arg.remove.start;
			uffd_reg.range.len = msg.arg.remove.end -
				msg.arg.remove.start;
			if (ioctl(uffd, UFFDIO_UNREGISTER, &uffd_reg.range))
				err("remove failure");
			break;
		case UFFD_EVENT_REMAP:
			area_remap = area_dst;  /* save for later unmap */
			area_dst = (char *)(unsigned long)msg.arg.remap.to;
			break;
		}
	}

	return NULL;
}

static void retry_copy_page(int ufd, struct uffdio_copy *uffdio_copy,
			    unsigned long offset)
{
	uffd_test_ops->alias_mapping(&uffdio_copy->dst,
				     uffdio_copy->len,
				     offset);
	if (ioctl(ufd, UFFDIO_COPY, uffdio_copy)) {
		/* real retval in ufdio_copy.copy */
		if (uffdio_copy->copy != -EEXIST)
			err("UFFDIO_COPY retry error: %"PRId64,
			    (int64_t)uffdio_copy->copy);
	} else {
		err("UFFDIO_COPY retry unexpected: %"PRId64,
		    (int64_t)uffdio_copy->copy);
	}
}

static void wake_range(int ufd, unsigned long addr, unsigned long len)
{
	struct uffdio_range uffdio_wake;

	uffdio_wake.start = addr;
	uffdio_wake.len = len;

	if (ioctl(ufd, UFFDIO_WAKE, &uffdio_wake))
		fprintf(stderr, "error waking %lu\n",
			addr), exit(1);
}

int __copy_page(int ufd, unsigned long offset, bool retry, bool wp)
{
	struct uffdio_copy uffdio_copy;

	if (offset >= nr_pages * page_size)
		err("unexpected offset %lu\n", offset);
	uffdio_copy.dst = (unsigned long) area_dst + offset;
	uffdio_copy.src = (unsigned long) area_src + offset;
	uffdio_copy.len = page_size;
	if (wp)
		uffdio_copy.mode = UFFDIO_COPY_MODE_WP;
	else
		uffdio_copy.mode = 0;
	uffdio_copy.copy = 0;
	if (ioctl(ufd, UFFDIO_COPY, &uffdio_copy)) {
		/* real retval in ufdio_copy.copy */
		if (uffdio_copy.copy != -EEXIST)
			err("UFFDIO_COPY error: %"PRId64,
			    (int64_t)uffdio_copy.copy);
		wake_range(ufd, uffdio_copy.dst, page_size);
	} else if (uffdio_copy.copy != page_size) {
		err("UFFDIO_COPY error: %"PRId64, (int64_t)uffdio_copy.copy);
	} else {
		if (test_uffdio_copy_eexist && retry) {
			test_uffdio_copy_eexist = false;
			retry_copy_page(ufd, &uffdio_copy, offset);
		}
		return 1;
	}
	return 0;
}

int copy_page(int ufd, unsigned long offset, bool wp)
{
	return __copy_page(ufd, offset, false, wp);
}

int move_page(int ufd, unsigned long offset, unsigned long len)
{
	struct uffdio_move uffdio_move;

	if (offset + len > nr_pages * page_size)
		err("unexpected offset %lu and length %lu\n", offset, len);
	uffdio_move.dst = (unsigned long) area_dst + offset;
	uffdio_move.src = (unsigned long) area_src + offset;
	uffdio_move.len = len;
	uffdio_move.mode = UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES;
	uffdio_move.move = 0;
	if (ioctl(ufd, UFFDIO_MOVE, &uffdio_move)) {
		/* real retval in uffdio_move.move */
		if (uffdio_move.move != -EEXIST)
			err("UFFDIO_MOVE error: %"PRId64,
			    (int64_t)uffdio_move.move);
		wake_range(ufd, uffdio_move.dst, len);
	} else if (uffdio_move.move != len) {
		err("UFFDIO_MOVE error: %"PRId64, (int64_t)uffdio_move.move);
	} else
		return 1;
	return 0;
}

int uffd_open_dev(unsigned int flags)
{
	int fd, uffd;

	fd = open("/dev/userfaultfd", O_RDWR | O_CLOEXEC);
	if (fd < 0)
		return fd;
	uffd = ioctl(fd, USERFAULTFD_IOC_NEW, flags);
	close(fd);

	return uffd;
}

int uffd_open_sys(unsigned int flags)
{
#ifdef __NR_userfaultfd
	return syscall(__NR_userfaultfd, flags);
#else
	return -1;
#endif
}

int uffd_open(unsigned int flags)
{
	int uffd = uffd_open_sys(flags);

	if (uffd < 0)
		uffd = uffd_open_dev(flags);

	return uffd;
}

int uffd_get_features(uint64_t *features)
{
	struct uffdio_api uffdio_api = { .api = UFFD_API, .features = 0 };
	/*
	 * This should by default work in most kernels; the feature list
	 * will be the same no matter what we pass in here.
	 */
	int fd = uffd_open(UFFD_USER_MODE_ONLY);

	if (fd < 0)
		/* Maybe the kernel is older than user-only mode? */
		fd = uffd_open(0);

	if (fd < 0)
		return fd;

	if (ioctl(fd, UFFDIO_API, &uffdio_api)) {
		close(fd);
		return -errno;
	}

	*features = uffdio_api.features;
	close(fd);

	return 0;
}