root/drivers/md/dm-snap-persistent.c

DEFINITIONS

1. alloc_area
2. free_area
3. do_metadata
4. chunk_io
5. area_location
6. skip_metadata
7. area_io
8. zero_memory_area
9. zero_disk_area
10. read_header
11. write_header
12. get_exception
13. read_exception
14. write_exception
15. clear_exception
16. insert_exceptions
17. read_exceptions
18. get_info
19. persistent_usage
20. persistent_dtr
21. persistent_read_metadata
22. persistent_prepare_exception
23. persistent_commit_exception
24. persistent_prepare_merge
25. persistent_commit_merge
26. persistent_drop_snapshot
27. persistent_ctr
28. persistent_status
29. dm_persistent_snapshot_init
30. dm_persistent_snapshot_exit

   1 /*
   2  * Copyright (C) 2001-2002 Sistina Software (UK) Limited.
   3  * Copyright (C) 2006-2008 Red Hat GmbH
   4  *
   5  * This file is released under the GPL.
   6  */
   7 
   8 #include "dm-exception-store.h"
   9 
  10 #include <linux/ctype.h>
  11 #include <linux/mm.h>
  12 #include <linux/pagemap.h>
  13 #include <linux/vmalloc.h>
  14 #include <linux/export.h>
  15 #include <linux/slab.h>
  16 #include <linux/dm-io.h>
  17 #include <linux/dm-bufio.h>
  18 
  19 #define DM_MSG_PREFIX "persistent snapshot"
  20 #define DM_CHUNK_SIZE_DEFAULT_SECTORS 32U       /* 16KB */
  21 
  22 #define DM_PREFETCH_CHUNKS              12
  23 
  24 /*-----------------------------------------------------------------
  25  * Persistent snapshots, by persistent we mean that the snapshot
  26  * will survive a reboot.
  27  *---------------------------------------------------------------*/
  28 
  29 /*
  30  * We need to store a record of which parts of the origin have
  31  * been copied to the snapshot device.  The snapshot code
  32  * requires that we copy exception chunks to chunk aligned areas
  33  * of the COW store.  It makes sense therefore, to store the
  34  * metadata in chunk size blocks.
  35  *
  36  * There is no backward or forward compatibility implemented,
  37  * snapshots with different disk versions than the kernel will
  38  * not be usable.  It is expected that "lvcreate" will blank out
  39  * the start of a fresh COW device before calling the snapshot
  40  * constructor.
  41  *
  42  * The first chunk of the COW device just contains the header.
  43  * After this there is a chunk filled with exception metadata,
  44  * followed by as many exception chunks as can fit in the
  45  * metadata areas.
  46  *
  47  * All on disk structures are in little-endian format.  The end
  48  * of the exceptions info is indicated by an exception with a
  49  * new_chunk of 0, which is invalid since it would point to the
  50  * header chunk.
  51  */
  52 
  53 /*
  54  * Magic for persistent snapshots: "SnAp" - Feeble isn't it.
  55  */
  56 #define SNAP_MAGIC 0x70416e53
  57 
  58 /*
  59  * The on-disk version of the metadata.
  60  */
  61 #define SNAPSHOT_DISK_VERSION 1
  62 
  63 #define NUM_SNAPSHOT_HDR_CHUNKS 1
  64 
  65 struct disk_header {
  66         __le32 magic;
  67 
  68         /*
  69          * Is this snapshot valid.  There is no way of recovering
  70          * an invalid snapshot.
  71          */
  72         __le32 valid;
  73 
  74         /*
  75          * Simple, incrementing version. no backward
  76          * compatibility.
  77          */
  78         __le32 version;
  79 
  80         /* In sectors */
  81         __le32 chunk_size;
  82 } __packed;
  83 
  84 struct disk_exception {
  85         __le64 old_chunk;
  86         __le64 new_chunk;
  87 } __packed;
  88 
  89 struct core_exception {
  90         uint64_t old_chunk;
  91         uint64_t new_chunk;
  92 };
  93 
  94 struct commit_callback {
  95         void (*callback)(void *, int success);
  96         void *context;
  97 };
  98 
  99 /*
 100  * The top level structure for a persistent exception store.
 101  */
 102 struct pstore {
 103         struct dm_exception_store *store;
 104         int version;
 105         int valid;
 106         uint32_t exceptions_per_area;
 107 
 108         /*
 109          * Now that we have an asynchronous kcopyd there is no
 110          * need for large chunk sizes, so it wont hurt to have a
 111          * whole chunks worth of metadata in memory at once.
 112          */
 113         void *area;
 114 
 115         /*
 116          * An area of zeros used to clear the next area.
 117          */
 118         void *zero_area;
 119 
 120         /*
 121          * An area used for header. The header can be written
 122          * concurrently with metadata (when invalidating the snapshot),
 123          * so it needs a separate buffer.
 124          */
 125         void *header_area;
 126 
 127         /*
 128          * Used to keep track of which metadata area the data in
 129          * 'chunk' refers to.
 130          */
 131         chunk_t current_area;
 132 
 133         /*
 134          * The next free chunk for an exception.
 135          *
 136          * When creating exceptions, all the chunks here and above are
 137          * free.  It holds the next chunk to be allocated.  On rare
 138          * occasions (e.g. after a system crash) holes can be left in
 139          * the exception store because chunks can be committed out of
 140          * order.
 141          *
 142          * When merging exceptions, it does not necessarily mean all the
 143          * chunks here and above are free.  It holds the value it would
 144          * have held if all chunks had been committed in order of
 145          * allocation.  Consequently the value may occasionally be
 146          * slightly too low, but since it's only used for 'status' and
 147          * it can never reach its minimum value too early this doesn't
 148          * matter.
 149          */
 150 
 151         chunk_t next_free;
 152 
 153         /*
 154          * The index of next free exception in the current
 155          * metadata area.
 156          */
 157         uint32_t current_committed;
 158 
 159         atomic_t pending_count;
 160         uint32_t callback_count;
 161         struct commit_callback *callbacks;
 162         struct dm_io_client *io_client;
 163 
 164         struct workqueue_struct *metadata_wq;
 165 };
 166 
 167 static int alloc_area(struct pstore *ps)
 168 {
 169         int r = -ENOMEM;
 170         size_t len;
 171 
 172         len = ps->store->chunk_size << SECTOR_SHIFT;
 173 
 174         /*
 175          * Allocate the chunk_size block of memory that will hold
 176          * a single metadata area.
 177          */
 178         ps->area = vmalloc(len);
 179         if (!ps->area)
 180                 goto err_area;
 181 
 182         ps->zero_area = vzalloc(len);
 183         if (!ps->zero_area)
 184                 goto err_zero_area;
 185 
 186         ps->header_area = vmalloc(len);
 187         if (!ps->header_area)
 188                 goto err_header_area;
 189 
 190         return 0;
 191 
 192 err_header_area:
 193         vfree(ps->zero_area);
 194 
 195 err_zero_area:
 196         vfree(ps->area);
 197 
 198 err_area:
 199         return r;
 200 }
 201 
 202 static void free_area(struct pstore *ps)
 203 {
 204         vfree(ps->area);
 205         ps->area = NULL;
 206         vfree(ps->zero_area);
 207         ps->zero_area = NULL;
 208         vfree(ps->header_area);
 209         ps->header_area = NULL;
 210 }
 211 
 212 struct mdata_req {
 213         struct dm_io_region *where;
 214         struct dm_io_request *io_req;
 215         struct work_struct work;
 216         int result;
 217 };
 218 
 219 static void do_metadata(struct work_struct *work)
 220 {
 221         struct mdata_req *req = container_of(work, struct mdata_req, work);
 222 
 223         req->result = dm_io(req->io_req, 1, req->where, NULL);
 224 }
 225 
 226 /*
 227  * Read or write a chunk aligned and sized block of data from a device.
 228  */
 229 static int chunk_io(struct pstore *ps, void *area, chunk_t chunk, int op,
 230                     int op_flags, int metadata)
 231 {
 232         struct dm_io_region where = {
 233                 .bdev = dm_snap_cow(ps->store->snap)->bdev,
 234                 .sector = ps->store->chunk_size * chunk,
 235                 .count = ps->store->chunk_size,
 236         };
 237         struct dm_io_request io_req = {
 238                 .bi_op = op,
 239                 .bi_op_flags = op_flags,
 240                 .mem.type = DM_IO_VMA,
 241                 .mem.ptr.vma = area,
 242                 .client = ps->io_client,
 243                 .notify.fn = NULL,
 244         };
 245         struct mdata_req req;
 246 
 247         if (!metadata)
 248                 return dm_io(&io_req, 1, &where, NULL);
 249 
 250         req.where = &where;
 251         req.io_req = &io_req;
 252 
 253         /*
 254          * Issue the synchronous I/O from a different thread
 255          * to avoid generic_make_request recursion.
 256          */
 257         INIT_WORK_ONSTACK(&req.work, do_metadata);
 258         queue_work(ps->metadata_wq, &req.work);
 259         flush_workqueue(ps->metadata_wq);
 260         destroy_work_on_stack(&req.work);
 261 
 262         return req.result;
 263 }
 264 
 265 /*
 266  * Convert a metadata area index to a chunk index.
 267  */
 268 static chunk_t area_location(struct pstore *ps, chunk_t area)
 269 {
 270         return NUM_SNAPSHOT_HDR_CHUNKS + ((ps->exceptions_per_area + 1) * area);
 271 }
 272 
 273 static void skip_metadata(struct pstore *ps)
 274 {
 275         uint32_t stride = ps->exceptions_per_area + 1;
 276         chunk_t next_free = ps->next_free;
 277         if (sector_div(next_free, stride) == NUM_SNAPSHOT_HDR_CHUNKS)
 278                 ps->next_free++;
 279 }
 280 
 281 /*
 282  * Read or write a metadata area.  Remembering to skip the first
 283  * chunk which holds the header.
 284  */
 285 static int area_io(struct pstore *ps, int op, int op_flags)
 286 {
 287         int r;
 288         chunk_t chunk;
 289 
 290         chunk = area_location(ps, ps->current_area);
 291 
 292         r = chunk_io(ps, ps->area, chunk, op, op_flags, 0);
 293         if (r)
 294                 return r;
 295 
 296         return 0;
 297 }
 298 
 299 static void zero_memory_area(struct pstore *ps)
 300 {
 301         memset(ps->area, 0, ps->store->chunk_size << SECTOR_SHIFT);
 302 }
 303 
 304 static int zero_disk_area(struct pstore *ps, chunk_t area)
 305 {
 306         return chunk_io(ps, ps->zero_area, area_location(ps, area),
 307                         REQ_OP_WRITE, 0, 0);
 308 }
 309 
 310 static int read_header(struct pstore *ps, int *new_snapshot)
 311 {
 312         int r;
 313         struct disk_header *dh;
 314         unsigned chunk_size;
 315         int chunk_size_supplied = 1;
 316         char *chunk_err;
 317 
 318         /*
 319          * Use default chunk size (or logical_block_size, if larger)
 320          * if none supplied
 321          */
 322         if (!ps->store->chunk_size) {
 323                 ps->store->chunk_size = max(DM_CHUNK_SIZE_DEFAULT_SECTORS,
 324                     bdev_logical_block_size(dm_snap_cow(ps->store->snap)->
 325                                             bdev) >> 9);
 326                 ps->store->chunk_mask = ps->store->chunk_size - 1;
 327                 ps->store->chunk_shift = __ffs(ps->store->chunk_size);
 328                 chunk_size_supplied = 0;
 329         }
 330 
 331         ps->io_client = dm_io_client_create();
 332         if (IS_ERR(ps->io_client))
 333                 return PTR_ERR(ps->io_client);
 334 
 335         r = alloc_area(ps);
 336         if (r)
 337                 return r;
 338 
 339         r = chunk_io(ps, ps->header_area, 0, REQ_OP_READ, 0, 1);
 340         if (r)
 341                 goto bad;
 342 
 343         dh = ps->header_area;
 344 
 345         if (le32_to_cpu(dh->magic) == 0) {
 346                 *new_snapshot = 1;
 347                 return 0;
 348         }
 349 
 350         if (le32_to_cpu(dh->magic) != SNAP_MAGIC) {
 351                 DMWARN("Invalid or corrupt snapshot");
 352                 r = -ENXIO;
 353                 goto bad;
 354         }
 355 
 356         *new_snapshot = 0;
 357         ps->valid = le32_to_cpu(dh->valid);
 358         ps->version = le32_to_cpu(dh->version);
 359         chunk_size = le32_to_cpu(dh->chunk_size);
 360 
 361         if (ps->store->chunk_size == chunk_size)
 362                 return 0;
 363 
 364         if (chunk_size_supplied)
 365                 DMWARN("chunk size %u in device metadata overrides "
 366                        "table chunk size of %u.",
 367                        chunk_size, ps->store->chunk_size);
 368 
 369         /* We had a bogus chunk_size. Fix stuff up. */
 370         free_area(ps);
 371 
 372         r = dm_exception_store_set_chunk_size(ps->store, chunk_size,
 373                                               &chunk_err);
 374         if (r) {
 375                 DMERR("invalid on-disk chunk size %u: %s.",
 376                       chunk_size, chunk_err);
 377                 return r;
 378         }
 379 
 380         r = alloc_area(ps);
 381         return r;
 382 
 383 bad:
 384         free_area(ps);
 385         return r;
 386 }
 387 
 388 static int write_header(struct pstore *ps)
 389 {
 390         struct disk_header *dh;
 391 
 392         memset(ps->header_area, 0, ps->store->chunk_size << SECTOR_SHIFT);
 393 
 394         dh = ps->header_area;
 395         dh->magic = cpu_to_le32(SNAP_MAGIC);
 396         dh->valid = cpu_to_le32(ps->valid);
 397         dh->version = cpu_to_le32(ps->version);
 398         dh->chunk_size = cpu_to_le32(ps->store->chunk_size);
 399 
 400         return chunk_io(ps, ps->header_area, 0, REQ_OP_WRITE, 0, 1);
 401 }
 402 
 403 /*
 404  * Access functions for the disk exceptions, these do the endian conversions.
 405  */
 406 static struct disk_exception *get_exception(struct pstore *ps, void *ps_area,
 407                                             uint32_t index)
 408 {
 409         BUG_ON(index >= ps->exceptions_per_area);
 410 
 411         return ((struct disk_exception *) ps_area) + index;
 412 }
 413 
 414 static void read_exception(struct pstore *ps, void *ps_area,
 415                            uint32_t index, struct core_exception *result)
 416 {
 417         struct disk_exception *de = get_exception(ps, ps_area, index);
 418 
 419         /* copy it */
 420         result->old_chunk = le64_to_cpu(de->old_chunk);
 421         result->new_chunk = le64_to_cpu(de->new_chunk);
 422 }
 423 
 424 static void write_exception(struct pstore *ps,
 425                             uint32_t index, struct core_exception *e)
 426 {
 427         struct disk_exception *de = get_exception(ps, ps->area, index);
 428 
 429         /* copy it */
 430         de->old_chunk = cpu_to_le64(e->old_chunk);
 431         de->new_chunk = cpu_to_le64(e->new_chunk);
 432 }
 433 
 434 static void clear_exception(struct pstore *ps, uint32_t index)
 435 {
 436         struct disk_exception *de = get_exception(ps, ps->area, index);
 437 
 438         /* clear it */
 439         de->old_chunk = 0;
 440         de->new_chunk = 0;
 441 }
 442 
 443 /*
 444  * Registers the exceptions that are present in the current area.
 445  * 'full' is filled in to indicate if the area has been
 446  * filled.
 447  */
 448 static int insert_exceptions(struct pstore *ps, void *ps_area,
 449                              int (*callback)(void *callback_context,
 450                                              chunk_t old, chunk_t new),
 451                              void *callback_context,
 452                              int *full)
 453 {
 454         int r;
 455         unsigned int i;
 456         struct core_exception e;
 457 
 458         /* presume the area is full */
 459         *full = 1;
 460 
 461         for (i = 0; i < ps->exceptions_per_area; i++) {
 462                 read_exception(ps, ps_area, i, &e);
 463 
 464                 /*
 465                  * If the new_chunk is pointing at the start of
 466                  * the COW device, where the first metadata area
 467                  * is we know that we've hit the end of the
 468                  * exceptions.  Therefore the area is not full.
 469                  */
 470                 if (e.new_chunk == 0LL) {
 471                         ps->current_committed = i;
 472                         *full = 0;
 473                         break;
 474                 }
 475 
 476                 /*
 477                  * Keep track of the start of the free chunks.
 478                  */
 479                 if (ps->next_free <= e.new_chunk)
 480                         ps->next_free = e.new_chunk + 1;
 481 
 482                 /*
 483                  * Otherwise we add the exception to the snapshot.
 484                  */
 485                 r = callback(callback_context, e.old_chunk, e.new_chunk);
 486                 if (r)
 487                         return r;
 488         }
 489 
 490         return 0;
 491 }
 492 
 493 static int read_exceptions(struct pstore *ps,
 494                            int (*callback)(void *callback_context, chunk_t old,
 495                                            chunk_t new),
 496                            void *callback_context)
 497 {
 498         int r, full = 1;
 499         struct dm_bufio_client *client;
 500         chunk_t prefetch_area = 0;
 501 
 502         client = dm_bufio_client_create(dm_snap_cow(ps->store->snap)->bdev,
 503                                         ps->store->chunk_size << SECTOR_SHIFT,
 504                                         1, 0, NULL, NULL);
 505 
 506         if (IS_ERR(client))
 507                 return PTR_ERR(client);
 508 
 509         /*
 510          * Setup for one current buffer + desired readahead buffers.
 511          */
 512         dm_bufio_set_minimum_buffers(client, 1 + DM_PREFETCH_CHUNKS);
 513 
 514         /*
 515          * Keeping reading chunks and inserting exceptions until
 516          * we find a partially full area.
 517          */
 518         for (ps->current_area = 0; full; ps->current_area++) {
 519                 struct dm_buffer *bp;
 520                 void *area;
 521                 chunk_t chunk;
 522 
 523                 if (unlikely(prefetch_area < ps->current_area))
 524                         prefetch_area = ps->current_area;
 525 
 526                 if (DM_PREFETCH_CHUNKS) do {
 527                         chunk_t pf_chunk = area_location(ps, prefetch_area);
 528                         if (unlikely(pf_chunk >= dm_bufio_get_device_size(client)))
 529                                 break;
 530                         dm_bufio_prefetch(client, pf_chunk, 1);
 531                         prefetch_area++;
 532                         if (unlikely(!prefetch_area))
 533                                 break;
 534                 } while (prefetch_area <= ps->current_area + DM_PREFETCH_CHUNKS);
 535 
 536                 chunk = area_location(ps, ps->current_area);
 537 
 538                 area = dm_bufio_read(client, chunk, &bp);
 539                 if (IS_ERR(area)) {
 540                         r = PTR_ERR(area);
 541                         goto ret_destroy_bufio;
 542                 }
 543 
 544                 r = insert_exceptions(ps, area, callback, callback_context,
 545                                       &full);
 546 
 547                 if (!full)
 548                         memcpy(ps->area, area, ps->store->chunk_size << SECTOR_SHIFT);
 549 
 550                 dm_bufio_release(bp);
 551 
 552                 dm_bufio_forget(client, chunk);
 553 
 554                 if (unlikely(r))
 555                         goto ret_destroy_bufio;
 556         }
 557 
 558         ps->current_area--;
 559 
 560         skip_metadata(ps);
 561 
 562         r = 0;
 563 
 564 ret_destroy_bufio:
 565         dm_bufio_client_destroy(client);
 566 
 567         return r;
 568 }
 569 
 570 static struct pstore *get_info(struct dm_exception_store *store)
 571 {
 572         return (struct pstore *) store->context;
 573 }
 574 
 575 static void persistent_usage(struct dm_exception_store *store,
 576                              sector_t *total_sectors,
 577                              sector_t *sectors_allocated,
 578                              sector_t *metadata_sectors)
 579 {
 580         struct pstore *ps = get_info(store);
 581 
 582         *sectors_allocated = ps->next_free * store->chunk_size;
 583         *total_sectors = get_dev_size(dm_snap_cow(store->snap)->bdev);
 584 
 585         /*
 586          * First chunk is the fixed header.
 587          * Then there are (ps->current_area + 1) metadata chunks, each one
 588          * separated from the next by ps->exceptions_per_area data chunks.
 589          */
 590         *metadata_sectors = (ps->current_area + 1 + NUM_SNAPSHOT_HDR_CHUNKS) *
 591                             store->chunk_size;
 592 }
 593 
 594 static void persistent_dtr(struct dm_exception_store *store)
 595 {
 596         struct pstore *ps = get_info(store);
 597 
 598         destroy_workqueue(ps->metadata_wq);
 599 
 600         /* Created in read_header */
 601         if (ps->io_client)
 602                 dm_io_client_destroy(ps->io_client);
 603         free_area(ps);
 604 
 605         /* Allocated in persistent_read_metadata */
 606         vfree(ps->callbacks);
 607 
 608         kfree(ps);
 609 }
 610 
 611 static int persistent_read_metadata(struct dm_exception_store *store,
 612                                     int (*callback)(void *callback_context,
 613                                                     chunk_t old, chunk_t new),
 614                                     void *callback_context)
 615 {
 616         int r, uninitialized_var(new_snapshot);
 617         struct pstore *ps = get_info(store);
 618 
 619         /*
 620          * Read the snapshot header.
 621          */
 622         r = read_header(ps, &new_snapshot);
 623         if (r)
 624                 return r;
 625 
 626         /*
 627          * Now we know correct chunk_size, complete the initialisation.
 628          */
 629         ps->exceptions_per_area = (ps->store->chunk_size << SECTOR_SHIFT) /
 630                                   sizeof(struct disk_exception);
 631         ps->callbacks = dm_vcalloc(ps->exceptions_per_area,
 632                                    sizeof(*ps->callbacks));
 633         if (!ps->callbacks)
 634                 return -ENOMEM;
 635 
 636         /*
 637          * Do we need to setup a new snapshot ?
 638          */
 639         if (new_snapshot) {
 640                 r = write_header(ps);
 641                 if (r) {
 642                         DMWARN("write_header failed");
 643                         return r;
 644                 }
 645 
 646                 ps->current_area = 0;
 647                 zero_memory_area(ps);
 648                 r = zero_disk_area(ps, 0);
 649                 if (r)
 650                         DMWARN("zero_disk_area(0) failed");
 651                 return r;
 652         }
 653         /*
 654          * Sanity checks.
 655          */
 656         if (ps->version != SNAPSHOT_DISK_VERSION) {
 657                 DMWARN("unable to handle snapshot disk version %d",
 658                        ps->version);
 659                 return -EINVAL;
 660         }
 661 
 662         /*
 663          * Metadata are valid, but snapshot is invalidated
 664          */
 665         if (!ps->valid)
 666                 return 1;
 667 
 668         /*
 669          * Read the metadata.
 670          */
 671         r = read_exceptions(ps, callback, callback_context);
 672 
 673         return r;
 674 }
 675 
 676 static int persistent_prepare_exception(struct dm_exception_store *store,
 677                                         struct dm_exception *e)
 678 {
 679         struct pstore *ps = get_info(store);
 680         sector_t size = get_dev_size(dm_snap_cow(store->snap)->bdev);
 681 
 682         /* Is there enough room ? */
 683         if (size < ((ps->next_free + 1) * store->chunk_size))
 684                 return -ENOSPC;
 685 
 686         e->new_chunk = ps->next_free;
 687 
 688         /*
 689          * Move onto the next free pending, making sure to take
 690          * into account the location of the metadata chunks.
 691          */
 692         ps->next_free++;
 693         skip_metadata(ps);
 694 
 695         atomic_inc(&ps->pending_count);
 696         return 0;
 697 }
 698 
 699 static void persistent_commit_exception(struct dm_exception_store *store,
 700                                         struct dm_exception *e, int valid,
 701                                         void (*callback) (void *, int success),
 702                                         void *callback_context)
 703 {
 704         unsigned int i;
 705         struct pstore *ps = get_info(store);
 706         struct core_exception ce;
 707         struct commit_callback *cb;
 708 
 709         if (!valid)
 710                 ps->valid = 0;
 711 
 712         ce.old_chunk = e->old_chunk;
 713         ce.new_chunk = e->new_chunk;
 714         write_exception(ps, ps->current_committed++, &ce);
 715 
 716         /*
 717          * Add the callback to the back of the array.  This code
 718          * is the only place where the callback array is
 719          * manipulated, and we know that it will never be called
 720          * multiple times concurrently.
 721          */
 722         cb = ps->callbacks + ps->callback_count++;
 723         cb->callback = callback;
 724         cb->context = callback_context;
 725 
 726         /*
 727          * If there are exceptions in flight and we have not yet
 728          * filled this metadata area there's nothing more to do.
 729          */
 730         if (!atomic_dec_and_test(&ps->pending_count) &&
 731             (ps->current_committed != ps->exceptions_per_area))
 732                 return;
 733 
 734         /*
 735          * If we completely filled the current area, then wipe the next one.
 736          */
 737         if ((ps->current_committed == ps->exceptions_per_area) &&
 738             zero_disk_area(ps, ps->current_area + 1))
 739                 ps->valid = 0;
 740 
 741         /*
 742          * Commit exceptions to disk.
 743          */
 744         if (ps->valid && area_io(ps, REQ_OP_WRITE,
 745                                  REQ_PREFLUSH | REQ_FUA | REQ_SYNC))
 746                 ps->valid = 0;
 747 
 748         /*
 749          * Advance to the next area if this one is full.
 750          */
 751         if (ps->current_committed == ps->exceptions_per_area) {
 752                 ps->current_committed = 0;
 753                 ps->current_area++;
 754                 zero_memory_area(ps);
 755         }
 756 
 757         for (i = 0; i < ps->callback_count; i++) {
 758                 cb = ps->callbacks + i;
 759                 cb->callback(cb->context, ps->valid);
 760         }
 761 
 762         ps->callback_count = 0;
 763 }
 764 
 765 static int persistent_prepare_merge(struct dm_exception_store *store,
 766                                     chunk_t *last_old_chunk,
 767                                     chunk_t *last_new_chunk)
 768 {
 769         struct pstore *ps = get_info(store);
 770         struct core_exception ce;
 771         int nr_consecutive;
 772         int r;
 773 
 774         /*
 775          * When current area is empty, move back to preceding area.
 776          */
 777         if (!ps->current_committed) {
 778                 /*
 779                  * Have we finished?
 780                  */
 781                 if (!ps->current_area)
 782                         return 0;
 783 
 784                 ps->current_area--;
 785                 r = area_io(ps, REQ_OP_READ, 0);
 786                 if (r < 0)
 787                         return r;
 788                 ps->current_committed = ps->exceptions_per_area;
 789         }
 790 
 791         read_exception(ps, ps->area, ps->current_committed - 1, &ce);
 792         *last_old_chunk = ce.old_chunk;
 793         *last_new_chunk = ce.new_chunk;
 794 
 795         /*
 796          * Find number of consecutive chunks within the current area,
 797          * working backwards.
 798          */
 799         for (nr_consecutive = 1; nr_consecutive < ps->current_committed;
 800              nr_consecutive++) {
 801                 read_exception(ps, ps->area,
 802                                ps->current_committed - 1 - nr_consecutive, &ce);
 803                 if (ce.old_chunk != *last_old_chunk - nr_consecutive ||
 804                     ce.new_chunk != *last_new_chunk - nr_consecutive)
 805                         break;
 806         }
 807 
 808         return nr_consecutive;
 809 }
 810 
 811 static int persistent_commit_merge(struct dm_exception_store *store,
 812                                    int nr_merged)
 813 {
 814         int r, i;
 815         struct pstore *ps = get_info(store);
 816 
 817         BUG_ON(nr_merged > ps->current_committed);
 818 
 819         for (i = 0; i < nr_merged; i++)
 820                 clear_exception(ps, ps->current_committed - 1 - i);
 821 
 822         r = area_io(ps, REQ_OP_WRITE, REQ_PREFLUSH | REQ_FUA);
 823         if (r < 0)
 824                 return r;
 825 
 826         ps->current_committed -= nr_merged;
 827 
 828         /*
 829          * At this stage, only persistent_usage() uses ps->next_free, so
 830          * we make no attempt to keep ps->next_free strictly accurate
 831          * as exceptions may have been committed out-of-order originally.
 832          * Once a snapshot has become merging, we set it to the value it
 833          * would have held had all the exceptions been committed in order.
 834          *
 835          * ps->current_area does not get reduced by prepare_merge() until
 836          * after commit_merge() has removed the nr_merged previous exceptions.
 837          */
 838         ps->next_free = area_location(ps, ps->current_area) +
 839                         ps->current_committed + 1;
 840 
 841         return 0;
 842 }
 843 
 844 static void persistent_drop_snapshot(struct dm_exception_store *store)
 845 {
 846         struct pstore *ps = get_info(store);
 847 
 848         ps->valid = 0;
 849         if (write_header(ps))
 850                 DMWARN("write header failed");
 851 }
 852 
 853 static int persistent_ctr(struct dm_exception_store *store, char *options)
 854 {
 855         struct pstore *ps;
 856         int r;
 857 
 858         /* allocate the pstore */
 859         ps = kzalloc(sizeof(*ps), GFP_KERNEL);
 860         if (!ps)
 861                 return -ENOMEM;
 862 
 863         ps->store = store;
 864         ps->valid = 1;
 865         ps->version = SNAPSHOT_DISK_VERSION;
 866         ps->area = NULL;
 867         ps->zero_area = NULL;
 868         ps->header_area = NULL;
 869         ps->next_free = NUM_SNAPSHOT_HDR_CHUNKS + 1; /* header and 1st area */
 870         ps->current_committed = 0;
 871 
 872         ps->callback_count = 0;
 873         atomic_set(&ps->pending_count, 0);
 874         ps->callbacks = NULL;
 875 
 876         ps->metadata_wq = alloc_workqueue("ksnaphd", WQ_MEM_RECLAIM, 0);
 877         if (!ps->metadata_wq) {
 878                 DMERR("couldn't start header metadata update thread");
 879                 r = -ENOMEM;
 880                 goto err_workqueue;
 881         }
 882 
 883         if (options) {
 884                 char overflow = toupper(options[0]);
 885                 if (overflow == 'O')
 886                         store->userspace_supports_overflow = true;
 887                 else {
 888                         DMERR("Unsupported persistent store option: %s", options);
 889                         r = -EINVAL;
 890                         goto err_options;
 891                 }
 892         }
 893 
 894         store->context = ps;
 895 
 896         return 0;
 897 
 898 err_options:
 899         destroy_workqueue(ps->metadata_wq);
 900 err_workqueue:
 901         kfree(ps);
 902 
 903         return r;
 904 }
 905 
 906 static unsigned persistent_status(struct dm_exception_store *store,
 907                                   status_type_t status, char *result,
 908                                   unsigned maxlen)
 909 {
 910         unsigned sz = 0;
 911 
 912         switch (status) {
 913         case STATUSTYPE_INFO:
 914                 break;
 915         case STATUSTYPE_TABLE:
 916                 DMEMIT(" %s %llu", store->userspace_supports_overflow ? "PO" : "P",
 917                        (unsigned long long)store->chunk_size);
 918         }
 919 
 920         return sz;
 921 }
 922 
 923 static struct dm_exception_store_type _persistent_type = {
 924         .name = "persistent",
 925         .module = THIS_MODULE,
 926         .ctr = persistent_ctr,
 927         .dtr = persistent_dtr,
 928         .read_metadata = persistent_read_metadata,
 929         .prepare_exception = persistent_prepare_exception,
 930         .commit_exception = persistent_commit_exception,
 931         .prepare_merge = persistent_prepare_merge,
 932         .commit_merge = persistent_commit_merge,
 933         .drop_snapshot = persistent_drop_snapshot,
 934         .usage = persistent_usage,
 935         .status = persistent_status,
 936 };
 937 
 938 static struct dm_exception_store_type _persistent_compat_type = {
 939         .name = "P",
 940         .module = THIS_MODULE,
 941         .ctr = persistent_ctr,
 942         .dtr = persistent_dtr,
 943         .read_metadata = persistent_read_metadata,
 944         .prepare_exception = persistent_prepare_exception,
 945         .commit_exception = persistent_commit_exception,
 946         .prepare_merge = persistent_prepare_merge,
 947         .commit_merge = persistent_commit_merge,
 948         .drop_snapshot = persistent_drop_snapshot,
 949         .usage = persistent_usage,
 950         .status = persistent_status,
 951 };
 952 
 953 int dm_persistent_snapshot_init(void)
 954 {
 955         int r;
 956 
 957         r = dm_exception_store_type_register(&_persistent_type);
 958         if (r) {
 959                 DMERR("Unable to register persistent exception store type");
 960                 return r;
 961         }
 962 
 963         r = dm_exception_store_type_register(&_persistent_compat_type);
 964         if (r) {
 965                 DMERR("Unable to register old-style persistent exception "
 966                       "store type");
 967                 dm_exception_store_type_unregister(&_persistent_type);
 968                 return r;
 969         }
 970 
 971         return r;
 972 }
 973 
 974 void dm_persistent_snapshot_exit(void)
 975 {
 976         dm_exception_store_type_unregister(&_persistent_type);
 977         dm_exception_store_type_unregister(&_persistent_compat_type);
 978 }