super.c 39.6 KB
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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553
// SPDX-License-Identifier: GPL-2.0
/*
 *  linux/fs/super.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 *  super.c contains code to handle: - mount structures
 *                                   - super-block tables
 *                                   - filesystem drivers list
 *                                   - mount system call
 *                                   - umount system call
 *                                   - ustat system call
 *
 * GK 2/5/95  -  Changed to support mounting the root fs via NFS
 *
 *  Added kerneld support: Jacques Gelinas and Bjorn Ekwall
 *  Added change_root: Werner Almesberger & Hans Lermen, Feb '96
 *  Added options to /proc/mounts:
 *    Torbjörn Lindh (torbjorn.lindh@gopta.se), April 14, 1996.
 *  Added devfs support: Richard Gooch <rgooch@atnf.csiro.au>, 13-JAN-1998
 *  Heavily rewritten for 'one fs - one tree' dcache architecture. AV, Mar 2000
 */

#include <linux/export.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/mount.h>
#include <linux/security.h>
#include <linux/writeback.h>		/* for the emergency remount stuff */
#include <linux/idr.h>
#include <linux/mutex.h>
#include <linux/backing-dev.h>
#include <linux/rculist_bl.h>
#include <linux/cleancache.h>
#include <linux/fsnotify.h>
#include <linux/lockdep.h>
#include <linux/user_namespace.h>
#include "internal.h"


static LIST_HEAD(super_blocks);
static DEFINE_SPINLOCK(sb_lock);

static char *sb_writers_name[SB_FREEZE_LEVELS] = {
	"sb_writers",
	"sb_pagefaults",
	"sb_internal",
};

/*
 * One thing we have to be careful of with a per-sb shrinker is that we don't
 * drop the last active reference to the superblock from within the shrinker.
 * If that happens we could trigger unregistering the shrinker from within the
 * shrinker path and that leads to deadlock on the shrinker_rwsem. Hence we
 * take a passive reference to the superblock to avoid this from occurring.
 */
static unsigned long super_cache_scan(struct shrinker *shrink,
				      struct shrink_control *sc)
{
	struct super_block *sb;
	long	fs_objects = 0;
	long	total_objects;
	long	freed = 0;
	long	dentries;
	long	inodes;

	sb = container_of(shrink, struct super_block, s_shrink);

	/*
	 * Deadlock avoidance.  We may hold various FS locks, and we don't want
	 * to recurse into the FS that called us in clear_inode() and friends..
	 */
	if (!(sc->gfp_mask & __GFP_FS))
		return SHRINK_STOP;

	if (!trylock_super(sb))
		return SHRINK_STOP;

	if (sb->s_op->nr_cached_objects)
		fs_objects = sb->s_op->nr_cached_objects(sb, sc);

	inodes = list_lru_shrink_count(&sb->s_inode_lru, sc);
	dentries = list_lru_shrink_count(&sb->s_dentry_lru, sc);
	total_objects = dentries + inodes + fs_objects + 1;
	if (!total_objects)
		total_objects = 1;

	/* proportion the scan between the caches */
	dentries = mult_frac(sc->nr_to_scan, dentries, total_objects);
	inodes = mult_frac(sc->nr_to_scan, inodes, total_objects);
	fs_objects = mult_frac(sc->nr_to_scan, fs_objects, total_objects);

	/*
	 * prune the dcache first as the icache is pinned by it, then
	 * prune the icache, followed by the filesystem specific caches
	 *
	 * Ensure that we always scan at least one object - memcg kmem
	 * accounting uses this to fully empty the caches.
	 */
	sc->nr_to_scan = dentries + 1;
	freed = prune_dcache_sb(sb, sc);
	sc->nr_to_scan = inodes + 1;
	freed += prune_icache_sb(sb, sc);

	if (fs_objects) {
		sc->nr_to_scan = fs_objects + 1;
		freed += sb->s_op->free_cached_objects(sb, sc);
	}

	up_read(&sb->s_umount);
	return freed;
}

static unsigned long super_cache_count(struct shrinker *shrink,
				       struct shrink_control *sc)
{
	struct super_block *sb;
	long	total_objects = 0;

	sb = container_of(shrink, struct super_block, s_shrink);

	/*
	 * We don't call trylock_super() here as it is a scalability bottleneck,
	 * so we're exposed to partial setup state. The shrinker rwsem does not
	 * protect filesystem operations backing list_lru_shrink_count() or
	 * s_op->nr_cached_objects(). Counts can change between
	 * super_cache_count and super_cache_scan, so we really don't need locks
	 * here.
	 *
	 * However, if we are currently mounting the superblock, the underlying
	 * filesystem might be in a state of partial construction and hence it
	 * is dangerous to access it.  trylock_super() uses a SB_BORN check to
	 * avoid this situation, so do the same here. The memory barrier is
	 * matched with the one in mount_fs() as we don't hold locks here.
	 */
	if (!(sb->s_flags & SB_BORN))
		return 0;
	smp_rmb();

	if (sb->s_op && sb->s_op->nr_cached_objects)
		total_objects = sb->s_op->nr_cached_objects(sb, sc);

	total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
	total_objects += list_lru_shrink_count(&sb->s_inode_lru, sc);

	total_objects = vfs_pressure_ratio(total_objects);
	return total_objects;
}

static void destroy_super_work(struct work_struct *work)
{
	struct super_block *s = container_of(work, struct super_block,
							destroy_work);
	int i;

	for (i = 0; i < SB_FREEZE_LEVELS; i++)
		percpu_free_rwsem(&s->s_writers.rw_sem[i]);
	kfree(s);
}

static void destroy_super_rcu(struct rcu_head *head)
{
	struct super_block *s = container_of(head, struct super_block, rcu);
	INIT_WORK(&s->destroy_work, destroy_super_work);
	schedule_work(&s->destroy_work);
}

/**
 *	destroy_super	-	frees a superblock
 *	@s: superblock to free
 *
 *	Frees a superblock.
 */
static void destroy_super(struct super_block *s)
{
	list_lru_destroy(&s->s_dentry_lru);
	list_lru_destroy(&s->s_inode_lru);
	security_sb_free(s);
	WARN_ON(!list_empty(&s->s_mounts));
	put_user_ns(s->s_user_ns);
	kfree(s->s_subtype);
	call_rcu(&s->rcu, destroy_super_rcu);
}

/**
 *	alloc_super	-	create new superblock
 *	@type:	filesystem type superblock should belong to
 *	@flags: the mount flags
 *	@user_ns: User namespace for the super_block
 *
 *	Allocates and initializes a new &struct super_block.  alloc_super()
 *	returns a pointer new superblock or %NULL if allocation had failed.
 */
static struct super_block *alloc_super(struct file_system_type *type, int flags,
				       struct user_namespace *user_ns)
{
	struct super_block *s = kzalloc(sizeof(struct super_block),  GFP_USER);
	static const struct super_operations default_op;
	int i;

	if (!s)
		return NULL;

	INIT_LIST_HEAD(&s->s_mounts);
	s->s_user_ns = get_user_ns(user_ns);

	if (security_sb_alloc(s))
		goto fail;

	for (i = 0; i < SB_FREEZE_LEVELS; i++) {
		if (__percpu_init_rwsem(&s->s_writers.rw_sem[i],
					sb_writers_name[i],
					&type->s_writers_key[i]))
			goto fail;
	}
	init_waitqueue_head(&s->s_writers.wait_unfrozen);
	s->s_bdi = &noop_backing_dev_info;
	s->s_flags = flags;
	if (s->s_user_ns != &init_user_ns)
		s->s_iflags |= SB_I_NODEV;
	INIT_HLIST_NODE(&s->s_instances);
	INIT_HLIST_BL_HEAD(&s->s_anon);
	mutex_init(&s->s_sync_lock);
	INIT_LIST_HEAD(&s->s_inodes);
	spin_lock_init(&s->s_inode_list_lock);
	INIT_LIST_HEAD(&s->s_inodes_wb);
	spin_lock_init(&s->s_inode_wblist_lock);

	if (list_lru_init_memcg(&s->s_dentry_lru))
		goto fail;
	if (list_lru_init_memcg(&s->s_inode_lru))
		goto fail;

	init_rwsem(&s->s_umount);
	lockdep_set_class(&s->s_umount, &type->s_umount_key);
	/*
	 * sget() can have s_umount recursion.
	 *
	 * When it cannot find a suitable sb, it allocates a new
	 * one (this one), and tries again to find a suitable old
	 * one.
	 *
	 * In case that succeeds, it will acquire the s_umount
	 * lock of the old one. Since these are clearly distrinct
	 * locks, and this object isn't exposed yet, there's no
	 * risk of deadlocks.
	 *
	 * Annotate this by putting this lock in a different
	 * subclass.
	 */
	down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
	s->s_count = 1;
	atomic_set(&s->s_active, 1);
	mutex_init(&s->s_vfs_rename_mutex);
	lockdep_set_class(&s->s_vfs_rename_mutex, &type->s_vfs_rename_key);
	init_rwsem(&s->s_dquot.dqio_sem);
	s->s_maxbytes = MAX_NON_LFS;
	s->s_op = &default_op;
	s->s_time_gran = 1000000000;
	s->cleancache_poolid = CLEANCACHE_NO_POOL;

	s->s_shrink.seeks = DEFAULT_SEEKS;
	s->s_shrink.scan_objects = super_cache_scan;
	s->s_shrink.count_objects = super_cache_count;
	s->s_shrink.batch = 1024;
	s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
	return s;

fail:
	destroy_super(s);
	return NULL;
}

/* Superblock refcounting  */

/*
 * Drop a superblock's refcount.  The caller must hold sb_lock.
 */
static void __put_super(struct super_block *sb)
{
	if (!--sb->s_count) {
		list_del_init(&sb->s_list);
		destroy_super(sb);
	}
}

/**
 *	put_super	-	drop a temporary reference to superblock
 *	@sb: superblock in question
 *
 *	Drops a temporary reference, frees superblock if there's no
 *	references left.
 */
static void put_super(struct super_block *sb)
{
	spin_lock(&sb_lock);
	__put_super(sb);
	spin_unlock(&sb_lock);
}


/**
 *	deactivate_locked_super	-	drop an active reference to superblock
 *	@s: superblock to deactivate
 *
 *	Drops an active reference to superblock, converting it into a temporary
 *	one if there is no other active references left.  In that case we
 *	tell fs driver to shut it down and drop the temporary reference we
 *	had just acquired.
 *
 *	Caller holds exclusive lock on superblock; that lock is released.
 */
void deactivate_locked_super(struct super_block *s)
{
	struct file_system_type *fs = s->s_type;
	if (atomic_dec_and_test(&s->s_active)) {
		cleancache_invalidate_fs(s);
		unregister_shrinker(&s->s_shrink);
		fs->kill_sb(s);

		/*
		 * Since list_lru_destroy() may sleep, we cannot call it from
		 * put_super(), where we hold the sb_lock. Therefore we destroy
		 * the lru lists right now.
		 */
		list_lru_destroy(&s->s_dentry_lru);
		list_lru_destroy(&s->s_inode_lru);

		put_filesystem(fs);
		put_super(s);
	} else {
		up_write(&s->s_umount);
	}
}

EXPORT_SYMBOL(deactivate_locked_super);

/**
 *	deactivate_super	-	drop an active reference to superblock
 *	@s: superblock to deactivate
 *
 *	Variant of deactivate_locked_super(), except that superblock is *not*
 *	locked by caller.  If we are going to drop the final active reference,
 *	lock will be acquired prior to that.
 */
void deactivate_super(struct super_block *s)
{
        if (!atomic_add_unless(&s->s_active, -1, 1)) {
		down_write(&s->s_umount);
		deactivate_locked_super(s);
	}
}

EXPORT_SYMBOL(deactivate_super);

/**
 *	grab_super - acquire an active reference
 *	@s: reference we are trying to make active
 *
 *	Tries to acquire an active reference.  grab_super() is used when we
 * 	had just found a superblock in super_blocks or fs_type->fs_supers
 *	and want to turn it into a full-blown active reference.  grab_super()
 *	is called with sb_lock held and drops it.  Returns 1 in case of
 *	success, 0 if we had failed (superblock contents was already dead or
 *	dying when grab_super() had been called).  Note that this is only
 *	called for superblocks not in rundown mode (== ones still on ->fs_supers
 *	of their type), so increment of ->s_count is OK here.
 */
static int grab_super(struct super_block *s) __releases(sb_lock)
{
	s->s_count++;
	spin_unlock(&sb_lock);
	down_write(&s->s_umount);
	if ((s->s_flags & SB_BORN) && atomic_inc_not_zero(&s->s_active)) {
		put_super(s);
		return 1;
	}
	up_write(&s->s_umount);
	put_super(s);
	return 0;
}

/*
 *	trylock_super - try to grab ->s_umount shared
 *	@sb: reference we are trying to grab
 *
 *	Try to prevent fs shutdown.  This is used in places where we
 *	cannot take an active reference but we need to ensure that the
 *	filesystem is not shut down while we are working on it. It returns
 *	false if we cannot acquire s_umount or if we lose the race and
 *	filesystem already got into shutdown, and returns true with the s_umount
 *	lock held in read mode in case of success. On successful return,
 *	the caller must drop the s_umount lock when done.
 *
 *	Note that unlike get_super() et.al. this one does *not* bump ->s_count.
 *	The reason why it's safe is that we are OK with doing trylock instead
 *	of down_read().  There's a couple of places that are OK with that, but
 *	it's very much not a general-purpose interface.
 */
bool trylock_super(struct super_block *sb)
{
	if (down_read_trylock(&sb->s_umount)) {
		if (!hlist_unhashed(&sb->s_instances) &&
		    sb->s_root && (sb->s_flags & SB_BORN))
			return true;
		up_read(&sb->s_umount);
	}

	return false;
}

/**
 *	generic_shutdown_super	-	common helper for ->kill_sb()
 *	@sb: superblock to kill
 *
 *	generic_shutdown_super() does all fs-independent work on superblock
 *	shutdown.  Typical ->kill_sb() should pick all fs-specific objects
 *	that need destruction out of superblock, call generic_shutdown_super()
 *	and release aforementioned objects.  Note: dentries and inodes _are_
 *	taken care of and do not need specific handling.
 *
 *	Upon calling this function, the filesystem may no longer alter or
 *	rearrange the set of dentries belonging to this super_block, nor may it
 *	change the attachments of dentries to inodes.
 */
void generic_shutdown_super(struct super_block *sb)
{
	const struct super_operations *sop = sb->s_op;

	if (sb->s_root) {
		shrink_dcache_for_umount(sb);
		sync_filesystem(sb);
		sb->s_flags &= ~SB_ACTIVE;

		fsnotify_unmount_inodes(sb);
		cgroup_writeback_umount();

		evict_inodes(sb);

		if (sb->s_dio_done_wq) {
			destroy_workqueue(sb->s_dio_done_wq);
			sb->s_dio_done_wq = NULL;
		}

		if (sop->put_super)
			sop->put_super(sb);

		if (!list_empty(&sb->s_inodes)) {
			printk("VFS: Busy inodes after unmount of %s. "
			   "Self-destruct in 5 seconds.  Have a nice day...\n",
			   sb->s_id);
		}
	}
	spin_lock(&sb_lock);
	/* should be initialized for __put_super_and_need_restart() */
	hlist_del_init(&sb->s_instances);
	spin_unlock(&sb_lock);
	up_write(&sb->s_umount);
	if (sb->s_bdi != &noop_backing_dev_info) {
		bdi_put(sb->s_bdi);
		sb->s_bdi = &noop_backing_dev_info;
	}
}

EXPORT_SYMBOL(generic_shutdown_super);

/**
 *	sget_userns -	find or create a superblock
 *	@type:	filesystem type superblock should belong to
 *	@test:	comparison callback
 *	@set:	setup callback
 *	@flags:	mount flags
 *	@user_ns: User namespace for the super_block
 *	@data:	argument to each of them
 */
struct super_block *sget_userns(struct file_system_type *type,
			int (*test)(struct super_block *,void *),
			int (*set)(struct super_block *,void *),
			int flags, struct user_namespace *user_ns,
			void *data)
{
	struct super_block *s = NULL;
	struct super_block *old;
	int err;

	if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) &&
	    !(type->fs_flags & FS_USERNS_MOUNT) &&
	    !capable(CAP_SYS_ADMIN))
		return ERR_PTR(-EPERM);
retry:
	spin_lock(&sb_lock);
	if (test) {
		hlist_for_each_entry(old, &type->fs_supers, s_instances) {
			if (!test(old, data))
				continue;
			if (user_ns != old->s_user_ns) {
				spin_unlock(&sb_lock);
				if (s) {
					up_write(&s->s_umount);
					destroy_super(s);
				}
				return ERR_PTR(-EBUSY);
			}
			if (!grab_super(old))
				goto retry;
			if (s) {
				up_write(&s->s_umount);
				destroy_super(s);
				s = NULL;
			}
			return old;
		}
	}
	if (!s) {
		spin_unlock(&sb_lock);
		s = alloc_super(type, (flags & ~SB_SUBMOUNT), user_ns);
		if (!s)
			return ERR_PTR(-ENOMEM);
		goto retry;
	}

	err = set(s, data);
	if (err) {
		spin_unlock(&sb_lock);
		up_write(&s->s_umount);
		destroy_super(s);
		return ERR_PTR(err);
	}
	s->s_type = type;
	strlcpy(s->s_id, type->name, sizeof(s->s_id));
	list_add_tail(&s->s_list, &super_blocks);
	hlist_add_head(&s->s_instances, &type->fs_supers);
	spin_unlock(&sb_lock);
	get_filesystem(type);
	err = register_shrinker(&s->s_shrink);
	if (err) {
		deactivate_locked_super(s);
		s = ERR_PTR(err);
	}
	return s;
}

EXPORT_SYMBOL(sget_userns);

/**
 *	sget	-	find or create a superblock
 *	@type:	  filesystem type superblock should belong to
 *	@test:	  comparison callback
 *	@set:	  setup callback
 *	@flags:	  mount flags
 *	@data:	  argument to each of them
 */
struct super_block *sget(struct file_system_type *type,
			int (*test)(struct super_block *,void *),
			int (*set)(struct super_block *,void *),
			int flags,
			void *data)
{
	struct user_namespace *user_ns = current_user_ns();

	/* We don't yet pass the user namespace of the parent
	 * mount through to here so always use &init_user_ns
	 * until that changes.
	 */
	if (flags & SB_SUBMOUNT)
		user_ns = &init_user_ns;

	/* Ensure the requestor has permissions over the target filesystem */
	if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) && !ns_capable(user_ns, CAP_SYS_ADMIN))
		return ERR_PTR(-EPERM);

	return sget_userns(type, test, set, flags, user_ns, data);
}

EXPORT_SYMBOL(sget);

void drop_super(struct super_block *sb)
{
	up_read(&sb->s_umount);
	put_super(sb);
}

EXPORT_SYMBOL(drop_super);

void drop_super_exclusive(struct super_block *sb)
{
	up_write(&sb->s_umount);
	put_super(sb);
}
EXPORT_SYMBOL(drop_super_exclusive);

/**
 *	iterate_supers - call function for all active superblocks
 *	@f: function to call
 *	@arg: argument to pass to it
 *
 *	Scans the superblock list and calls given function, passing it
 *	locked superblock and given argument.
 */
void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
{
	struct super_block *sb, *p = NULL;

	spin_lock(&sb_lock);
	list_for_each_entry(sb, &super_blocks, s_list) {
		if (hlist_unhashed(&sb->s_instances))
			continue;
		sb->s_count++;
		spin_unlock(&sb_lock);

		down_read(&sb->s_umount);
		if (sb->s_root && (sb->s_flags & SB_BORN))
			f(sb, arg);
		up_read(&sb->s_umount);

		spin_lock(&sb_lock);
		if (p)
			__put_super(p);
		p = sb;
	}
	if (p)
		__put_super(p);
	spin_unlock(&sb_lock);
}

/**
 *	iterate_supers_type - call function for superblocks of given type
 *	@type: fs type
 *	@f: function to call
 *	@arg: argument to pass to it
 *
 *	Scans the superblock list and calls given function, passing it
 *	locked superblock and given argument.
 */
void iterate_supers_type(struct file_system_type *type,
	void (*f)(struct super_block *, void *), void *arg)
{
	struct super_block *sb, *p = NULL;

	spin_lock(&sb_lock);
	hlist_for_each_entry(sb, &type->fs_supers, s_instances) {
		sb->s_count++;
		spin_unlock(&sb_lock);

		down_read(&sb->s_umount);
		if (sb->s_root && (sb->s_flags & SB_BORN))
			f(sb, arg);
		up_read(&sb->s_umount);

		spin_lock(&sb_lock);
		if (p)
			__put_super(p);
		p = sb;
	}
	if (p)
		__put_super(p);
	spin_unlock(&sb_lock);
}

EXPORT_SYMBOL(iterate_supers_type);

static struct super_block *__get_super(struct block_device *bdev, bool excl)
{
	struct super_block *sb;

	if (!bdev)
		return NULL;

	spin_lock(&sb_lock);
rescan:
	list_for_each_entry(sb, &super_blocks, s_list) {
		if (hlist_unhashed(&sb->s_instances))
			continue;
		if (sb->s_bdev == bdev) {
			sb->s_count++;
			spin_unlock(&sb_lock);
			if (!excl)
				down_read(&sb->s_umount);
			else
				down_write(&sb->s_umount);
			/* still alive? */
			if (sb->s_root && (sb->s_flags & SB_BORN))
				return sb;
			if (!excl)
				up_read(&sb->s_umount);
			else
				up_write(&sb->s_umount);
			/* nope, got unmounted */
			spin_lock(&sb_lock);
			__put_super(sb);
			goto rescan;
		}
	}
	spin_unlock(&sb_lock);
	return NULL;
}

/**
 *	get_super - get the superblock of a device
 *	@bdev: device to get the superblock for
 *
 *	Scans the superblock list and finds the superblock of the file system
 *	mounted on the device given. %NULL is returned if no match is found.
 */
struct super_block *get_super(struct block_device *bdev)
{
	return __get_super(bdev, false);
}
EXPORT_SYMBOL(get_super);

static struct super_block *__get_super_thawed(struct block_device *bdev,
					      bool excl)
{
	while (1) {
		struct super_block *s = __get_super(bdev, excl);
		if (!s || s->s_writers.frozen == SB_UNFROZEN)
			return s;
		if (!excl)
			up_read(&s->s_umount);
		else
			up_write(&s->s_umount);
		wait_event(s->s_writers.wait_unfrozen,
			   s->s_writers.frozen == SB_UNFROZEN);
		put_super(s);
	}
}

/**
 *	get_super_thawed - get thawed superblock of a device
 *	@bdev: device to get the superblock for
 *
 *	Scans the superblock list and finds the superblock of the file system
 *	mounted on the device. The superblock is returned once it is thawed
 *	(or immediately if it was not frozen). %NULL is returned if no match
 *	is found.
 */
struct super_block *get_super_thawed(struct block_device *bdev)
{
	return __get_super_thawed(bdev, false);
}
EXPORT_SYMBOL(get_super_thawed);

/**
 *	get_super_exclusive_thawed - get thawed superblock of a device
 *	@bdev: device to get the superblock for
 *
 *	Scans the superblock list and finds the superblock of the file system
 *	mounted on the device. The superblock is returned once it is thawed
 *	(or immediately if it was not frozen) and s_umount semaphore is held
 *	in exclusive mode. %NULL is returned if no match is found.
 */
struct super_block *get_super_exclusive_thawed(struct block_device *bdev)
{
	return __get_super_thawed(bdev, true);
}
EXPORT_SYMBOL(get_super_exclusive_thawed);

/**
 * get_active_super - get an active reference to the superblock of a device
 * @bdev: device to get the superblock for
 *
 * Scans the superblock list and finds the superblock of the file system
 * mounted on the device given.  Returns the superblock with an active
 * reference or %NULL if none was found.
 */
struct super_block *get_active_super(struct block_device *bdev)
{
	struct super_block *sb;

	if (!bdev)
		return NULL;

restart:
	spin_lock(&sb_lock);
	list_for_each_entry(sb, &super_blocks, s_list) {
		if (hlist_unhashed(&sb->s_instances))
			continue;
		if (sb->s_bdev == bdev) {
			if (!grab_super(sb))
				goto restart;
			up_write(&sb->s_umount);
			return sb;
		}
	}
	spin_unlock(&sb_lock);
	return NULL;
}

struct super_block *user_get_super(dev_t dev)
{
	struct super_block *sb;

	spin_lock(&sb_lock);
rescan:
	list_for_each_entry(sb, &super_blocks, s_list) {
		if (hlist_unhashed(&sb->s_instances))
			continue;
		if (sb->s_dev ==  dev) {
			sb->s_count++;
			spin_unlock(&sb_lock);
			down_read(&sb->s_umount);
			/* still alive? */
			if (sb->s_root && (sb->s_flags & SB_BORN))
				return sb;
			up_read(&sb->s_umount);
			/* nope, got unmounted */
			spin_lock(&sb_lock);
			__put_super(sb);
			goto rescan;
		}
	}
	spin_unlock(&sb_lock);
	return NULL;
}

/**
 *	do_remount_sb - asks filesystem to change mount options.
 *	@sb:	superblock in question
 *	@sb_flags: revised superblock flags
 *	@data:	the rest of options
 *      @force: whether or not to force the change
 *
 *	Alters the mount options of a mounted file system.
 */
int do_remount_sb(struct super_block *sb, int sb_flags, void *data, int force)
{
	int retval;
	int remount_ro;

	if (sb->s_writers.frozen != SB_UNFROZEN)
		return -EBUSY;

#ifdef CONFIG_BLOCK
	if (!(sb_flags & SB_RDONLY) && bdev_read_only(sb->s_bdev))
		return -EACCES;
#endif

	remount_ro = (sb_flags & SB_RDONLY) && !sb_rdonly(sb);

	if (remount_ro) {
		if (!hlist_empty(&sb->s_pins)) {
			up_write(&sb->s_umount);
			group_pin_kill(&sb->s_pins);
			down_write(&sb->s_umount);
			if (!sb->s_root)
				return 0;
			if (sb->s_writers.frozen != SB_UNFROZEN)
				return -EBUSY;
			remount_ro = (sb_flags & SB_RDONLY) && !sb_rdonly(sb);
		}
	}
	shrink_dcache_sb(sb);

	/* If we are remounting RDONLY and current sb is read/write,
	   make sure there are no rw files opened */
	if (remount_ro) {
		if (force) {
			sb->s_readonly_remount = 1;
			smp_wmb();
		} else {
			retval = sb_prepare_remount_readonly(sb);
			if (retval)
				return retval;
		}
	}

	if (sb->s_op->remount_fs) {
		retval = sb->s_op->remount_fs(sb, &sb_flags, data);
		if (retval) {
			if (!force)
				goto cancel_readonly;
			/* If forced remount, go ahead despite any errors */
			WARN(1, "forced remount of a %s fs returned %i\n",
			     sb->s_type->name, retval);
		}
	}
	sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (sb_flags & MS_RMT_MASK);
	/* Needs to be ordered wrt mnt_is_readonly() */
	smp_wmb();
	sb->s_readonly_remount = 0;

	/*
	 * Some filesystems modify their metadata via some other path than the
	 * bdev buffer cache (eg. use a private mapping, or directories in
	 * pagecache, etc). Also file data modifications go via their own
	 * mappings. So If we try to mount readonly then copy the filesystem
	 * from bdev, we could get stale data, so invalidate it to give a best
	 * effort at coherency.
	 */
	if (remount_ro && sb->s_bdev)
		invalidate_bdev(sb->s_bdev);
	return 0;

cancel_readonly:
	sb->s_readonly_remount = 0;
	return retval;
}

static void do_emergency_remount(struct work_struct *work)
{
	struct super_block *sb, *p = NULL;

	spin_lock(&sb_lock);
	list_for_each_entry(sb, &super_blocks, s_list) {
		if (hlist_unhashed(&sb->s_instances))
			continue;
		sb->s_count++;
		spin_unlock(&sb_lock);
		down_write(&sb->s_umount);
		if (sb->s_root && sb->s_bdev && (sb->s_flags & SB_BORN) &&
		    !sb_rdonly(sb)) {
			/*
			 * What lock protects sb->s_flags??
			 */
			do_remount_sb(sb, SB_RDONLY, NULL, 1);
		}
		up_write(&sb->s_umount);
		spin_lock(&sb_lock);
		if (p)
			__put_super(p);
		p = sb;
	}
	if (p)
		__put_super(p);
	spin_unlock(&sb_lock);
	kfree(work);
	printk("Emergency Remount complete\n");
}

void emergency_remount(void)
{
	struct work_struct *work;

	work = kmalloc(sizeof(*work), GFP_ATOMIC);
	if (work) {
		INIT_WORK(work, do_emergency_remount);
		schedule_work(work);
	}
}

/*
 * Unnamed block devices are dummy devices used by virtual
 * filesystems which don't use real block-devices.  -- jrs
 */

static DEFINE_IDA(unnamed_dev_ida);
static DEFINE_SPINLOCK(unnamed_dev_lock);/* protects the above */
/* Many userspace utilities consider an FSID of 0 invalid.
 * Always return at least 1 from get_anon_bdev.
 */
static int unnamed_dev_start = 1;

int get_anon_bdev(dev_t *p)
{
	int dev;
	int error;

 retry:
	if (ida_pre_get(&unnamed_dev_ida, GFP_ATOMIC) == 0)
		return -ENOMEM;
	spin_lock(&unnamed_dev_lock);
	error = ida_get_new_above(&unnamed_dev_ida, unnamed_dev_start, &dev);
	if (!error)
		unnamed_dev_start = dev + 1;
	spin_unlock(&unnamed_dev_lock);
	if (error == -EAGAIN)
		/* We raced and lost with another CPU. */
		goto retry;
	else if (error)
		return -EAGAIN;

	if (dev >= (1 << MINORBITS)) {
		spin_lock(&unnamed_dev_lock);
		ida_remove(&unnamed_dev_ida, dev);
		if (unnamed_dev_start > dev)
			unnamed_dev_start = dev;
		spin_unlock(&unnamed_dev_lock);
		return -EMFILE;
	}
	*p = MKDEV(0, dev & MINORMASK);
	return 0;
}
EXPORT_SYMBOL(get_anon_bdev);

void free_anon_bdev(dev_t dev)
{
	int slot = MINOR(dev);
	spin_lock(&unnamed_dev_lock);
	ida_remove(&unnamed_dev_ida, slot);
	if (slot < unnamed_dev_start)
		unnamed_dev_start = slot;
	spin_unlock(&unnamed_dev_lock);
}
EXPORT_SYMBOL(free_anon_bdev);

int set_anon_super(struct super_block *s, void *data)
{
	return get_anon_bdev(&s->s_dev);
}

EXPORT_SYMBOL(set_anon_super);

void kill_anon_super(struct super_block *sb)
{
	dev_t dev = sb->s_dev;
	generic_shutdown_super(sb);
	free_anon_bdev(dev);
}

EXPORT_SYMBOL(kill_anon_super);

void kill_litter_super(struct super_block *sb)
{
	if (sb->s_root)
		d_genocide(sb->s_root);
	kill_anon_super(sb);
}

EXPORT_SYMBOL(kill_litter_super);

static int ns_test_super(struct super_block *sb, void *data)
{
	return sb->s_fs_info == data;
}

static int ns_set_super(struct super_block *sb, void *data)
{
	sb->s_fs_info = data;
	return set_anon_super(sb, NULL);
}

struct dentry *mount_ns(struct file_system_type *fs_type,
	int flags, void *data, void *ns, struct user_namespace *user_ns,
	int (*fill_super)(struct super_block *, void *, int))
{
	struct super_block *sb;

	/* Don't allow mounting unless the caller has CAP_SYS_ADMIN
	 * over the namespace.
	 */
	if (!(flags & SB_KERNMOUNT) && !ns_capable(user_ns, CAP_SYS_ADMIN))
		return ERR_PTR(-EPERM);

	sb = sget_userns(fs_type, ns_test_super, ns_set_super, flags,
			 user_ns, ns);
	if (IS_ERR(sb))
		return ERR_CAST(sb);

	if (!sb->s_root) {
		int err;
		err = fill_super(sb, data, flags & SB_SILENT ? 1 : 0);
		if (err) {
			deactivate_locked_super(sb);
			return ERR_PTR(err);
		}

		sb->s_flags |= SB_ACTIVE;
	}

	return dget(sb->s_root);
}

EXPORT_SYMBOL(mount_ns);

#ifdef CONFIG_BLOCK
static int set_bdev_super(struct super_block *s, void *data)
{
	s->s_bdev = data;
	s->s_dev = s->s_bdev->bd_dev;
	s->s_bdi = bdi_get(s->s_bdev->bd_bdi);

	return 0;
}

static int test_bdev_super(struct super_block *s, void *data)
{
	return (void *)s->s_bdev == data;
}

struct dentry *mount_bdev(struct file_system_type *fs_type,
	int flags, const char *dev_name, void *data,
	int (*fill_super)(struct super_block *, void *, int))
{
	struct block_device *bdev;
	struct super_block *s;
	fmode_t mode = FMODE_READ | FMODE_EXCL;
	int error = 0;

	if (!(flags & SB_RDONLY))
		mode |= FMODE_WRITE;

	bdev = blkdev_get_by_path(dev_name, mode, fs_type);
	if (IS_ERR(bdev))
		return ERR_CAST(bdev);

	/*
	 * once the super is inserted into the list by sget, s_umount
	 * will protect the lockfs code from trying to start a snapshot
	 * while we are mounting
	 */
	mutex_lock(&bdev->bd_fsfreeze_mutex);
	if (bdev->bd_fsfreeze_count > 0) {
		mutex_unlock(&bdev->bd_fsfreeze_mutex);
		error = -EBUSY;
		goto error_bdev;
	}
	s = sget(fs_type, test_bdev_super, set_bdev_super, flags | SB_NOSEC,
		 bdev);
	mutex_unlock(&bdev->bd_fsfreeze_mutex);
	if (IS_ERR(s))
		goto error_s;

	if (s->s_root) {
		if ((flags ^ s->s_flags) & SB_RDONLY) {
			deactivate_locked_super(s);
			error = -EBUSY;
			goto error_bdev;
		}

		/*
		 * s_umount nests inside bd_mutex during
		 * __invalidate_device().  blkdev_put() acquires
		 * bd_mutex and can't be called under s_umount.  Drop
		 * s_umount temporarily.  This is safe as we're
		 * holding an active reference.
		 */
		up_write(&s->s_umount);
		blkdev_put(bdev, mode);
		down_write(&s->s_umount);
	} else {
		s->s_mode = mode;
		snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
		sb_set_blocksize(s, block_size(bdev));
		error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
		if (error) {
			deactivate_locked_super(s);
			goto error;
		}

		s->s_flags |= SB_ACTIVE;
		bdev->bd_super = s;
	}

	return dget(s->s_root);

error_s:
	error = PTR_ERR(s);
error_bdev:
	blkdev_put(bdev, mode);
error:
	return ERR_PTR(error);
}
EXPORT_SYMBOL(mount_bdev);

void kill_block_super(struct super_block *sb)
{
	struct block_device *bdev = sb->s_bdev;
	fmode_t mode = sb->s_mode;

	bdev->bd_super = NULL;
	generic_shutdown_super(sb);
	sync_blockdev(bdev);
	WARN_ON_ONCE(!(mode & FMODE_EXCL));
	blkdev_put(bdev, mode | FMODE_EXCL);
}

EXPORT_SYMBOL(kill_block_super);
#endif

struct dentry *mount_nodev(struct file_system_type *fs_type,
	int flags, void *data,
	int (*fill_super)(struct super_block *, void *, int))
{
	int error;
	struct super_block *s = sget(fs_type, NULL, set_anon_super, flags, NULL);

	if (IS_ERR(s))
		return ERR_CAST(s);

	error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
	if (error) {
		deactivate_locked_super(s);
		return ERR_PTR(error);
	}
	s->s_flags |= SB_ACTIVE;
	return dget(s->s_root);
}
EXPORT_SYMBOL(mount_nodev);

static int compare_single(struct super_block *s, void *p)
{
	return 1;
}

struct dentry *mount_single(struct file_system_type *fs_type,
	int flags, void *data,
	int (*fill_super)(struct super_block *, void *, int))
{
	struct super_block *s;
	int error;

	s = sget(fs_type, compare_single, set_anon_super, flags, NULL);
	if (IS_ERR(s))
		return ERR_CAST(s);
	if (!s->s_root) {
		error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
		if (error) {
			deactivate_locked_super(s);
			return ERR_PTR(error);
		}
		s->s_flags |= SB_ACTIVE;
	} else {
		do_remount_sb(s, flags, data, 0);
	}
	return dget(s->s_root);
}
EXPORT_SYMBOL(mount_single);

struct dentry *
mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
{
	struct dentry *root;
	struct super_block *sb;
	char *secdata = NULL;
	int error = -ENOMEM;

	if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {
		secdata = alloc_secdata();
		if (!secdata)
			goto out;

		error = security_sb_copy_data(data, secdata);
		if (error)
			goto out_free_secdata;
	}

	root = type->mount(type, flags, name, data);
	if (IS_ERR(root)) {
		error = PTR_ERR(root);
		goto out_free_secdata;
	}
	sb = root->d_sb;
	BUG_ON(!sb);
	WARN_ON(!sb->s_bdi);

	/*
	 * Write barrier is for super_cache_count(). We place it before setting
	 * SB_BORN as the data dependency between the two functions is the
	 * superblock structure contents that we just set up, not the SB_BORN
	 * flag.
	 */
	smp_wmb();
	sb->s_flags |= SB_BORN;

	error = security_sb_kern_mount(sb, flags, secdata);
	if (error)
		goto out_sb;

	/*
	 * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
	 * but s_maxbytes was an unsigned long long for many releases. Throw
	 * this warning for a little while to try and catch filesystems that
	 * violate this rule.
	 */
	WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
		"negative value (%lld)\n", type->name, sb->s_maxbytes);

	up_write(&sb->s_umount);
	free_secdata(secdata);
	return root;
out_sb:
	dput(root);
	deactivate_locked_super(sb);
out_free_secdata:
	free_secdata(secdata);
out:
	return ERR_PTR(error);
}

/*
 * Setup private BDI for given superblock. It gets automatically cleaned up
 * in generic_shutdown_super().
 */
int super_setup_bdi_name(struct super_block *sb, char *fmt, ...)
{
	struct backing_dev_info *bdi;
	int err;
	va_list args;

	bdi = bdi_alloc(GFP_KERNEL);
	if (!bdi)
		return -ENOMEM;

	bdi->name = sb->s_type->name;

	va_start(args, fmt);
	err = bdi_register_va(bdi, fmt, args);
	va_end(args);
	if (err) {
		bdi_put(bdi);
		return err;
	}
	WARN_ON(sb->s_bdi != &noop_backing_dev_info);
	sb->s_bdi = bdi;

	return 0;
}
EXPORT_SYMBOL(super_setup_bdi_name);

/*
 * Setup private BDI for given superblock. I gets automatically cleaned up
 * in generic_shutdown_super().
 */
int super_setup_bdi(struct super_block *sb)
{
	static atomic_long_t bdi_seq = ATOMIC_LONG_INIT(0);

	return super_setup_bdi_name(sb, "%.28s-%ld", sb->s_type->name,
				    atomic_long_inc_return(&bdi_seq));
}
EXPORT_SYMBOL(super_setup_bdi);

/*
 * This is an internal function, please use sb_end_{write,pagefault,intwrite}
 * instead.
 */
void __sb_end_write(struct super_block *sb, int level)
{
	percpu_up_read(sb->s_writers.rw_sem + level-1);
}
EXPORT_SYMBOL(__sb_end_write);

/*
 * This is an internal function, please use sb_start_{write,pagefault,intwrite}
 * instead.
 */
int __sb_start_write(struct super_block *sb, int level, bool wait)
{
	bool force_trylock = false;
	int ret = 1;

#ifdef CONFIG_LOCKDEP
	/*
	 * We want lockdep to tell us about possible deadlocks with freezing
	 * but it's it bit tricky to properly instrument it. Getting a freeze
	 * protection works as getting a read lock but there are subtle
	 * problems. XFS for example gets freeze protection on internal level
	 * twice in some cases, which is OK only because we already hold a
	 * freeze protection also on higher level. Due to these cases we have
	 * to use wait == F (trylock mode) which must not fail.
	 */
	if (wait) {
		int i;

		for (i = 0; i < level - 1; i++)
			if (percpu_rwsem_is_held(sb->s_writers.rw_sem + i)) {
				force_trylock = true;
				break;
			}
	}
#endif
	if (wait && !force_trylock)
		percpu_down_read(sb->s_writers.rw_sem + level-1);
	else
		ret = percpu_down_read_trylock(sb->s_writers.rw_sem + level-1);

	WARN_ON(force_trylock && !ret);
	return ret;
}
EXPORT_SYMBOL(__sb_start_write);

/**
 * sb_wait_write - wait until all writers to given file system finish
 * @sb: the super for which we wait
 * @level: type of writers we wait for (normal vs page fault)
 *
 * This function waits until there are no writers of given type to given file
 * system.
 */
static void sb_wait_write(struct super_block *sb, int level)
{
	percpu_down_write(sb->s_writers.rw_sem + level-1);
}

/*
 * We are going to return to userspace and forget about these locks, the
 * ownership goes to the caller of thaw_super() which does unlock().
 */
static void lockdep_sb_freeze_release(struct super_block *sb)
{
	int level;

	for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
		percpu_rwsem_release(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
}

/*
 * Tell lockdep we are holding these locks before we call ->unfreeze_fs(sb).
 */
static void lockdep_sb_freeze_acquire(struct super_block *sb)
{
	int level;

	for (level = 0; level < SB_FREEZE_LEVELS; ++level)
		percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
}

static void sb_freeze_unlock(struct super_block *sb)
{
	int level;

	for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
		percpu_up_write(sb->s_writers.rw_sem + level);
}

/**
 * freeze_super - lock the filesystem and force it into a consistent state
 * @sb: the super to lock
 *
 * Syncs the super to make sure the filesystem is consistent and calls the fs's
 * freeze_fs.  Subsequent calls to this without first thawing the fs will return
 * -EBUSY.
 *
 * During this function, sb->s_writers.frozen goes through these values:
 *
 * SB_UNFROZEN: File system is normal, all writes progress as usual.
 *
 * SB_FREEZE_WRITE: The file system is in the process of being frozen.  New
 * writes should be blocked, though page faults are still allowed. We wait for
 * all writes to complete and then proceed to the next stage.
 *
 * SB_FREEZE_PAGEFAULT: Freezing continues. Now also page faults are blocked
 * but internal fs threads can still modify the filesystem (although they
 * should not dirty new pages or inodes), writeback can run etc. After waiting
 * for all running page faults we sync the filesystem which will clean all
 * dirty pages and inodes (no new dirty pages or inodes can be created when
 * sync is running).
 *
 * SB_FREEZE_FS: The file system is frozen. Now all internal sources of fs
 * modification are blocked (e.g. XFS preallocation truncation on inode
 * reclaim). This is usually implemented by blocking new transactions for
 * filesystems that have them and need this additional guard. After all
 * internal writers are finished we call ->freeze_fs() to finish filesystem
 * freezing. Then we transition to SB_FREEZE_COMPLETE state. This state is
 * mostly auxiliary for filesystems to verify they do not modify frozen fs.
 *
 * sb->s_writers.frozen is protected by sb->s_umount.
 */
int freeze_super(struct super_block *sb)
{
	int ret;

	atomic_inc(&sb->s_active);
	down_write(&sb->s_umount);
	if (sb->s_writers.frozen != SB_UNFROZEN) {
		deactivate_locked_super(sb);
		return -EBUSY;
	}

	if (!(sb->s_flags & SB_BORN)) {
		up_write(&sb->s_umount);
		return 0;	/* sic - it's "nothing to do" */
	}

	if (sb_rdonly(sb)) {
		/* Nothing to do really... */
		sb->s_writers.frozen = SB_FREEZE_COMPLETE;
		up_write(&sb->s_umount);
		return 0;
	}

	sb->s_writers.frozen = SB_FREEZE_WRITE;
	/* Release s_umount to preserve sb_start_write -> s_umount ordering */
	up_write(&sb->s_umount);
	sb_wait_write(sb, SB_FREEZE_WRITE);
	down_write(&sb->s_umount);

	/* Now we go and block page faults... */
	sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
	sb_wait_write(sb, SB_FREEZE_PAGEFAULT);

	/* All writers are done so after syncing there won't be dirty data */
	sync_filesystem(sb);

	/* Now wait for internal filesystem counter */
	sb->s_writers.frozen = SB_FREEZE_FS;
	sb_wait_write(sb, SB_FREEZE_FS);

	if (sb->s_op->freeze_fs) {
		ret = sb->s_op->freeze_fs(sb);
		if (ret) {
			printk(KERN_ERR
				"VFS:Filesystem freeze failed\n");
			sb->s_writers.frozen = SB_UNFROZEN;
			sb_freeze_unlock(sb);
			wake_up(&sb->s_writers.wait_unfrozen);
			deactivate_locked_super(sb);
			return ret;
		}
	}
	/*
	 * For debugging purposes so that fs can warn if it sees write activity
	 * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
	 */
	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
	lockdep_sb_freeze_release(sb);
	up_write(&sb->s_umount);
	return 0;
}
EXPORT_SYMBOL(freeze_super);

/**
 * thaw_super -- unlock filesystem
 * @sb: the super to thaw
 *
 * Unlocks the filesystem and marks it writeable again after freeze_super().
 */
int thaw_super(struct super_block *sb)
{
	int error;

	down_write(&sb->s_umount);
	if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {
		up_write(&sb->s_umount);
		return -EINVAL;
	}

	if (sb_rdonly(sb)) {
		sb->s_writers.frozen = SB_UNFROZEN;
		goto out;
	}

	lockdep_sb_freeze_acquire(sb);

	if (sb->s_op->unfreeze_fs) {
		error = sb->s_op->unfreeze_fs(sb);
		if (error) {
			printk(KERN_ERR
				"VFS:Filesystem thaw failed\n");
			lockdep_sb_freeze_release(sb);
			up_write(&sb->s_umount);
			return error;
		}
	}

	sb->s_writers.frozen = SB_UNFROZEN;
	sb_freeze_unlock(sb);
out:
	wake_up(&sb->s_writers.wait_unfrozen);
	deactivate_locked_super(sb);
	return 0;
}
EXPORT_SYMBOL(thaw_super);