dm-snap-persistent.c

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/*
 * Copyright (C) 2001-2002 Sistina Software (UK) Limited.
 * Copyright (C) 2006-2008 Red Hat GmbH
 *
 * This file is released under the GPL.
 */

#include "dm-exception-store.h"

#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/vmalloc.h>
#include <linux/export.h>
#include <linux/slab.h>
#include <linux/dm-io.h>

#define DM_MSG_PREFIX "persistent snapshot"
#define DM_CHUNK_SIZE_DEFAULT_SECTORS 32    /* 16KB */

/*-----------------------------------------------------------------
 * Persistent snapshots, by persistent we mean that the snapshot
 * will survive a reboot.
 *---------------------------------------------------------------*/

/*
 * We need to store a record of which parts of the origin have
 * been copied to the snapshot device.  The snapshot code
 * requires that we copy exception chunks to chunk aligned areas
 * of the COW store.  It makes sense therefore, to store the
 * metadata in chunk size blocks.
 *
 * There is no backward or forward compatibility implemented,
 * snapshots with different disk versions than the kernel will
 * not be usable.  It is expected that "lvcreate" will blank out
 * the start of a fresh COW device before calling the snapshot
 * constructor.
 *
 * The first chunk of the COW device just contains the header.
 * After this there is a chunk filled with exception metadata,
 * followed by as many exception chunks as can fit in the
 * metadata areas.
 *
 * All on disk structures are in little-endian format.  The end
 * of the exceptions info is indicated by an exception with a
 * new_chunk of 0, which is invalid since it would point to the
 * header chunk.
 */

/*
 * Magic for persistent snapshots: "SnAp" - Feeble isn't it.
 */
#define SNAP_MAGIC 0x70416e53

/*
 * The on-disk version of the metadata.
 */
#define SNAPSHOT_DISK_VERSION 1

#define NUM_SNAPSHOT_HDR_CHUNKS 1

struct disk_header {
    __le32 magic;

    /*
     * Is this snapshot valid.  There is no way of recovering
     * an invalid snapshot.
     */
    __le32 valid;

    /*
     * Simple, incrementing version. no backward
     * compatibility.
     */
    __le32 version;

    /* In sectors */
    __le32 chunk_size;
} __packed;

struct disk_exception {
    __le64 old_chunk;
    __le64 new_chunk;
} __packed;

struct core_exception {
    uint64_t old_chunk;
    uint64_t new_chunk;
};

struct commit_callback {
    void (*callback)(void *, int success);
    void *context;
};

/*
 * The top level structure for a persistent exception store.
 */
struct pstore {
    struct dm_exception_store *store;
    int version;
    int valid;
    uint32_t exceptions_per_area;

    /*
     * Now that we have an asynchronous kcopyd there is no
     * need for large chunk sizes, so it wont hurt to have a
     * whole chunks worth of metadata in memory at once.
     */
    void *area;

    /*
     * An area of zeros used to clear the next area.
     */
    void *zero_area;

    /*
     * An area used for header. The header can be written
     * concurrently with metadata (when invalidating the snapshot),
     * so it needs a separate buffer.
     */
    void *header_area;

    /*
     * Used to keep track of which metadata area the data in
     * 'chunk' refers to.
     */
    chunk_t current_area;

    /*
     * The next free chunk for an exception.
     *
     * When creating exceptions, all the chunks here and above are
     * free.  It holds the next chunk to be allocated.  On rare
     * occasions (e.g. after a system crash) holes can be left in
     * the exception store because chunks can be committed out of
     * order.
     *
     * When merging exceptions, it does not necessarily mean all the
     * chunks here and above are free.  It holds the value it would
     * have held if all chunks had been committed in order of
     * allocation.  Consequently the value may occasionally be
     * slightly too low, but since it's only used for 'status' and
     * it can never reach its minimum value too early this doesn't
     * matter.
     */

    chunk_t next_free;

    /*
     * The index of next free exception in the current
     * metadata area.
     */
    uint32_t current_committed;

    atomic_t pending_count;
    uint32_t callback_count;
    struct commit_callback *callbacks;
    struct dm_io_client *io_client;

    struct workqueue_struct *metadata_wq;
};

static int alloc_area(struct pstore *ps)
{
    int r = -ENOMEM;
    size_t len;

    len = ps->store->chunk_size << SECTOR_SHIFT;

    /*
     * Allocate the chunk_size block of memory that will hold
     * a single metadata area.
     */
    ps->area = vmalloc(len);
    if (!ps->area)
        goto err_area;

    ps->zero_area = vzalloc(len);
    if (!ps->zero_area)
        goto err_zero_area;

    ps->header_area = vmalloc(len);
    if (!ps->header_area)
        goto err_header_area;

    return 0;

err_header_area:
    vfree(ps->zero_area);

err_zero_area:
    vfree(ps->area);

err_area:
    return r;
}

static void free_area(struct pstore *ps)
{
    if (ps->area)
        vfree(ps->area);
    ps->area = NULL;

    if (ps->zero_area)
        vfree(ps->zero_area);
    ps->zero_area = NULL;

    if (ps->header_area)
        vfree(ps->header_area);
    ps->header_area = NULL;
}

struct mdata_req {
    struct dm_io_region *where;
    struct dm_io_request *io_req;
    struct work_struct work;
    int result;
};

static void do_metadata(struct work_struct *work)
{
    struct mdata_req *req = container_of(work, struct mdata_req, work);

    req->result = dm_io(req->io_req, 1, req->where, NULL);
}

/*
 * Read or write a chunk aligned and sized block of data from a device.
 */
static int chunk_io(struct pstore *ps, void *area, chunk_t chunk, int rw,
            int metadata)
{
    struct dm_io_region where = {
        .bdev = dm_snap_cow(ps->store->snap)->bdev,
        .sector = ps->store->chunk_size * chunk,
        .count = ps->store->chunk_size,
    };
    struct dm_io_request io_req = {
        .bi_rw = rw,
        .mem.type = DM_IO_VMA,
        .mem.ptr.vma = area,
        .client = ps->io_client,
        .notify.fn = NULL,
    };
    struct mdata_req req;

    if (!metadata)
        return dm_io(&io_req, 1, &where, NULL);

    req.where = &where;
    req.io_req = &io_req;

    /*
     * Issue the synchronous I/O from a different thread
     * to avoid generic_make_request recursion.
     */
    INIT_WORK_ONSTACK(&req.work, do_metadata);
    queue_work(ps->metadata_wq, &req.work);
    flush_work(&req.work);

    return req.result;
}

/*
 * Convert a metadata area index to a chunk index.
 */
static chunk_t area_location(struct pstore *ps, chunk_t area)
{
    return NUM_SNAPSHOT_HDR_CHUNKS + ((ps->exceptions_per_area + 1) * area);
}

/*
 * Read or write a metadata area.  Remembering to skip the first
 * chunk which holds the header.
 */
static int area_io(struct pstore *ps, int rw)
{
    int r;
    chunk_t chunk;

    chunk = area_location(ps, ps->current_area);

    r = chunk_io(ps, ps->area, chunk, rw, 0);
    if (r)
        return r;

    return 0;
}

static void zero_memory_area(struct pstore *ps)
{
    memset(ps->area, 0, ps->store->chunk_size << SECTOR_SHIFT);
}

static int zero_disk_area(struct pstore *ps, chunk_t area)
{
    return chunk_io(ps, ps->zero_area, area_location(ps, area), WRITE, 0);
}

static int read_header(struct pstore *ps, int *new_snapshot)
{
    int r;
    struct disk_header *dh;
    unsigned chunk_size;
    int chunk_size_supplied = 1;
    char *chunk_err;

    /*
     * Use default chunk size (or logical_block_size, if larger)
     * if none supplied
     */
    if (!ps->store->chunk_size) {
        ps->store->chunk_size = max(DM_CHUNK_SIZE_DEFAULT_SECTORS,
            bdev_logical_block_size(dm_snap_cow(ps->store->snap)->
                        bdev) >> 9);
        ps->store->chunk_mask = ps->store->chunk_size - 1;
        ps->store->chunk_shift = ffs(ps->store->chunk_size) - 1;
        chunk_size_supplied = 0;
    }

    ps->io_client = dm_io_client_create();
    if (IS_ERR(ps->io_client))
        return PTR_ERR(ps->io_client);

    r = alloc_area(ps);
    if (r)
        return r;

    r = chunk_io(ps, ps->header_area, 0, READ, 1);
    if (r)
        goto bad;

    dh = ps->header_area;

    if (le32_to_cpu(dh->magic) == 0) {
        *new_snapshot = 1;
        return 0;
    }

    if (le32_to_cpu(dh->magic) != SNAP_MAGIC) {
        DMWARN("Invalid or corrupt snapshot");
        r = -ENXIO;
        goto bad;
    }

    *new_snapshot = 0;
    ps->valid = le32_to_cpu(dh->valid);
    ps->version = le32_to_cpu(dh->version);
    chunk_size = le32_to_cpu(dh->chunk_size);

    if (ps->store->chunk_size == chunk_size)
        return 0;

    if (chunk_size_supplied)
        DMWARN("chunk size %u in device metadata overrides "
               "table chunk size of %u.",
               chunk_size, ps->store->chunk_size);

    /* We had a bogus chunk_size. Fix stuff up. */
    free_area(ps);

    r = dm_exception_store_set_chunk_size(ps->store, chunk_size,
                          &chunk_err);
    if (r) {
        DMERR("invalid on-disk chunk size %u: %s.",
              chunk_size, chunk_err);
        return r;
    }

    r = alloc_area(ps);
    return r;

bad:
    free_area(ps);
    return r;
}

static int write_header(struct pstore *ps)
{
    struct disk_header *dh;

    memset(ps->header_area, 0, ps->store->chunk_size << SECTOR_SHIFT);

    dh = ps->header_area;
    dh->magic = cpu_to_le32(SNAP_MAGIC);
    dh->valid = cpu_to_le32(ps->valid);
    dh->version = cpu_to_le32(ps->version);
    dh->chunk_size = cpu_to_le32(ps->store->chunk_size);

    return chunk_io(ps, ps->header_area, 0, WRITE, 1);
}

/*
 * Access functions for the disk exceptions, these do the endian conversions.
 */
static struct disk_exception *get_exception(struct pstore *ps, uint32_t index)
{
    BUG_ON(index >= ps->exceptions_per_area);

    return ((struct disk_exception *) ps->area) + index;
}

static void read_exception(struct pstore *ps,
               uint32_t index, struct core_exception *result)
{
    struct disk_exception *de = get_exception(ps, index);

    /* copy it */
    result->old_chunk = le64_to_cpu(de->old_chunk);
    result->new_chunk = le64_to_cpu(de->new_chunk);
}

static void write_exception(struct pstore *ps,
                uint32_t index, struct core_exception *e)
{
    struct disk_exception *de = get_exception(ps, index);

    /* copy it */
    de->old_chunk = cpu_to_le64(e->old_chunk);
    de->new_chunk = cpu_to_le64(e->new_chunk);
}

static void clear_exception(struct pstore *ps, uint32_t index)
{
    struct disk_exception *de = get_exception(ps, index);

    /* clear it */
    de->old_chunk = 0;
    de->new_chunk = 0;
}

/*
 * Registers the exceptions that are present in the current area.
 * 'full' is filled in to indicate if the area has been
 * filled.
 */
static int insert_exceptions(struct pstore *ps,
                 int (*callback)(void *callback_context,
                         chunk_t old, chunk_t new),
                 void *callback_context,
                 int *full)
{
    int r;
    unsigned int i;
    struct core_exception e;

    /* presume the area is full */
    *full = 1;

    for (i = 0; i < ps->exceptions_per_area; i++) {
        read_exception(ps, i, &e);

        /*
         * If the new_chunk is pointing at the start of
         * the COW device, where the first metadata area
         * is we know that we've hit the end of the
         * exceptions.  Therefore the area is not full.
         */
        if (e.new_chunk == 0LL) {
            ps->current_committed = i;
            *full = 0;
            break;
        }

        /*
         * Keep track of the start of the free chunks.
         */
        if (ps->next_free <= e.new_chunk)
            ps->next_free = e.new_chunk + 1;

        /*
         * Otherwise we add the exception to the snapshot.
         */
        r = callback(callback_context, e.old_chunk, e.new_chunk);
        if (r)
            return r;
    }

    return 0;
}

static int read_exceptions(struct pstore *ps,
               int (*callback)(void *callback_context, chunk_t old,
                       chunk_t new),
               void *callback_context)
{
    int r, full = 1;

    /*
     * Keeping reading chunks and inserting exceptions until
     * we find a partially full area.
     */
    for (ps->current_area = 0; full; ps->current_area++) {
        r = area_io(ps, READ);
        if (r)
            return r;

        r = insert_exceptions(ps, callback, callback_context, &full);
        if (r)
            return r;
    }

    ps->current_area--;

    return 0;
}

static struct pstore *get_info(struct dm_exception_store *store)
{
    return (struct pstore *) store->context;
}

static void persistent_usage(struct dm_exception_store *store,
                 sector_t *total_sectors,
                 sector_t *sectors_allocated,
                 sector_t *metadata_sectors)
{
    struct pstore *ps = get_info(store);

    *sectors_allocated = ps->next_free * store->chunk_size;
    *total_sectors = get_dev_size(dm_snap_cow(store->snap)->bdev);

    /*
     * First chunk is the fixed header.
     * Then there are (ps->current_area + 1) metadata chunks, each one
     * separated from the next by ps->exceptions_per_area data chunks.
     */
    *metadata_sectors = (ps->current_area + 1 + NUM_SNAPSHOT_HDR_CHUNKS) *
                store->chunk_size;
}

static void persistent_dtr(struct dm_exception_store *store)
{
    struct pstore *ps = get_info(store);

    destroy_workqueue(ps->metadata_wq);

    /* Created in read_header */
    if (ps->io_client)
        dm_io_client_destroy(ps->io_client);
    free_area(ps);

    /* Allocated in persistent_read_metadata */
    if (ps->callbacks)
        vfree(ps->callbacks);

    kfree(ps);
}

static int persistent_read_metadata(struct dm_exception_store *store,
                    int (*callback)(void *callback_context,
                            chunk_t old, chunk_t new),
                    void *callback_context)
{
    int r, uninitialized_var(new_snapshot);
    struct pstore *ps = get_info(store);

    /*
     * Read the snapshot header.
     */
    r = read_header(ps, &new_snapshot);
    if (r)
        return r;

    /*
     * Now we know correct chunk_size, complete the initialisation.
     */
    ps->exceptions_per_area = (ps->store->chunk_size << SECTOR_SHIFT) /
                  sizeof(struct disk_exception);
    ps->callbacks = dm_vcalloc(ps->exceptions_per_area,
                   sizeof(*ps->callbacks));
    if (!ps->callbacks)
        return -ENOMEM;

    /*
     * Do we need to setup a new snapshot ?
     */
    if (new_snapshot) {
        r = write_header(ps);
        if (r) {
            DMWARN("write_header failed");
            return r;
        }

        ps->current_area = 0;
        zero_memory_area(ps);
        r = zero_disk_area(ps, 0);
        if (r)
            DMWARN("zero_disk_area(0) failed");
        return r;
    }
    /*
     * Sanity checks.
     */
    if (ps->version != SNAPSHOT_DISK_VERSION) {
        DMWARN("unable to handle snapshot disk version %d",
               ps->version);
        return -EINVAL;
    }

    /*
     * Metadata are valid, but snapshot is invalidated
     */
    if (!ps->valid)
        return 1;

    /*
     * Read the metadata.
     */
    r = read_exceptions(ps, callback, callback_context);

    return r;
}

static int persistent_prepare_exception(struct dm_exception_store *store,
                    struct dm_exception *e)
{
    struct pstore *ps = get_info(store);
    uint32_t stride;
    chunk_t next_free;
    sector_t size = get_dev_size(dm_snap_cow(store->snap)->bdev);

    /* Is there enough room ? */
    if (size < ((ps->next_free + 1) * store->chunk_size))
        return -ENOSPC;

    e->new_chunk = ps->next_free;

    /*
     * Move onto the next free pending, making sure to take
     * into account the location of the metadata chunks.
     */
    stride = (ps->exceptions_per_area + 1);
    next_free = ++ps->next_free;
    if (sector_div(next_free, stride) == 1)
        ps->next_free++;

    atomic_inc(&ps->pending_count);
    return 0;
}

static void persistent_commit_exception(struct dm_exception_store *store,
                    struct dm_exception *e,
                    void (*callback) (void *, int success),
                    void *callback_context)
{
    unsigned int i;
    struct pstore *ps = get_info(store);
    struct core_exception ce;
    struct commit_callback *cb;

    ce.old_chunk = e->old_chunk;
    ce.new_chunk = e->new_chunk;
    write_exception(ps, ps->current_committed++, &ce);

    /*
     * Add the callback to the back of the array.  This code
     * is the only place where the callback array is
     * manipulated, and we know that it will never be called
     * multiple times concurrently.
     */
    cb = ps->callbacks + ps->callback_count++;
    cb->callback = callback;
    cb->context = callback_context;

    /*
     * If there are exceptions in flight and we have not yet
     * filled this metadata area there's nothing more to do.
     */
    if (!atomic_dec_and_test(&ps->pending_count) &&
        (ps->current_committed != ps->exceptions_per_area))
        return;

    /*
     * If we completely filled the current area, then wipe the next one.
     */
    if ((ps->current_committed == ps->exceptions_per_area) &&
        zero_disk_area(ps, ps->current_area + 1))
        ps->valid = 0;

    /*
     * Commit exceptions to disk.
     */
    if (ps->valid && area_io(ps, WRITE_FLUSH_FUA))
        ps->valid = 0;

    /*
     * Advance to the next area if this one is full.
     */
    if (ps->current_committed == ps->exceptions_per_area) {
        ps->current_committed = 0;
        ps->current_area++;
        zero_memory_area(ps);
    }

    for (i = 0; i < ps->callback_count; i++) {
        cb = ps->callbacks + i;
        cb->callback(cb->context, ps->valid);
    }

    ps->callback_count = 0;
}

static int persistent_prepare_merge(struct dm_exception_store *store,
                    chunk_t *last_old_chunk,
                    chunk_t *last_new_chunk)
{
    struct pstore *ps = get_info(store);
    struct core_exception ce;
    int nr_consecutive;
    int r;

    /*
     * When current area is empty, move back to preceding area.
     */
    if (!ps->current_committed) {
        /*
         * Have we finished?
         */
        if (!ps->current_area)
            return 0;

        ps->current_area--;
        r = area_io(ps, READ);
        if (r < 0)
            return r;
        ps->current_committed = ps->exceptions_per_area;
    }

    read_exception(ps, ps->current_committed - 1, &ce);
    *last_old_chunk = ce.old_chunk;
    *last_new_chunk = ce.new_chunk;

    /*
     * Find number of consecutive chunks within the current area,
     * working backwards.
     */
    for (nr_consecutive = 1; nr_consecutive < ps->current_committed;
         nr_consecutive++) {
        read_exception(ps, ps->current_committed - 1 - nr_consecutive,
                   &ce);
        if (ce.old_chunk != *last_old_chunk - nr_consecutive ||
            ce.new_chunk != *last_new_chunk - nr_consecutive)
            break;
    }

    return nr_consecutive;
}

static int persistent_commit_merge(struct dm_exception_store *store,
                   int nr_merged)
{
    int r, i;
    struct pstore *ps = get_info(store);

    BUG_ON(nr_merged > ps->current_committed);

    for (i = 0; i < nr_merged; i++)
        clear_exception(ps, ps->current_committed - 1 - i);

    r = area_io(ps, WRITE_FLUSH_FUA);
    if (r < 0)
        return r;

    ps->current_committed -= nr_merged;

    /*
     * At this stage, only persistent_usage() uses ps->next_free, so
     * we make no attempt to keep ps->next_free strictly accurate
     * as exceptions may have been committed out-of-order originally.
     * Once a snapshot has become merging, we set it to the value it
     * would have held had all the exceptions been committed in order.
     *
     * ps->current_area does not get reduced by prepare_merge() until
     * after commit_merge() has removed the nr_merged previous exceptions.
     */
    ps->next_free = area_location(ps, ps->current_area) +
            ps->current_committed + 1;

    return 0;
}

static void persistent_drop_snapshot(struct dm_exception_store *store)
{
    struct pstore *ps = get_info(store);

    ps->valid = 0;
    if (write_header(ps))
        DMWARN("write header failed");
}

static int persistent_ctr(struct dm_exception_store *store,
              unsigned argc, char **argv)
{
    struct pstore *ps;

    /* allocate the pstore */
    ps = kzalloc(sizeof(*ps), GFP_KERNEL);
    if (!ps)
        return -ENOMEM;

    ps->store = store;
    ps->valid = 1;
    ps->version = SNAPSHOT_DISK_VERSION;
    ps->area = NULL;
    ps->zero_area = NULL;
    ps->header_area = NULL;
    ps->next_free = NUM_SNAPSHOT_HDR_CHUNKS + 1; /* header and 1st area */
    ps->current_committed = 0;

    ps->callback_count = 0;
    atomic_set(&ps->pending_count, 0);
    ps->callbacks = NULL;

    ps->metadata_wq = alloc_workqueue("ksnaphd", WQ_MEM_RECLAIM, 0);
    if (!ps->metadata_wq) {
        kfree(ps);
        DMERR("couldn't start header metadata update thread");
        return -ENOMEM;
    }

    store->context = ps;

    return 0;
}

static unsigned persistent_status(struct dm_exception_store *store,
                  status_type_t status, char *result,
                  unsigned maxlen)
{
    unsigned sz = 0;

    switch (status) {
    case STATUSTYPE_INFO:
        break;
    case STATUSTYPE_TABLE:
        DMEMIT(" P %llu", (unsigned long long)store->chunk_size);
    }

    return sz;
}

static struct dm_exception_store_type _persistent_type = {
    .name = "persistent",
    .module = THIS_MODULE,
    .ctr = persistent_ctr,
    .dtr = persistent_dtr,
    .read_metadata = persistent_read_metadata,
    .prepare_exception = persistent_prepare_exception,
    .commit_exception = persistent_commit_exception,
    .prepare_merge = persistent_prepare_merge,
    .commit_merge = persistent_commit_merge,
    .drop_snapshot = persistent_drop_snapshot,
    .usage = persistent_usage,
    .status = persistent_status,
};

static struct dm_exception_store_type _persistent_compat_type = {
    .name = "P",
    .module = THIS_MODULE,
    .ctr = persistent_ctr,
    .dtr = persistent_dtr,
    .read_metadata = persistent_read_metadata,
    .prepare_exception = persistent_prepare_exception,
    .commit_exception = persistent_commit_exception,
    .prepare_merge = persistent_prepare_merge,
    .commit_merge = persistent_commit_merge,
    .drop_snapshot = persistent_drop_snapshot,
    .usage = persistent_usage,
    .status = persistent_status,
};

int dm_persistent_snapshot_init(void)
{
    int r;

    r = dm_exception_store_type_register(&_persistent_type);
    if (r) {
        DMERR("Unable to register persistent exception store type");
        return r;
    }

    r = dm_exception_store_type_register(&_persistent_compat_type);
    if (r) {
        DMERR("Unable to register old-style persistent exception "
              "store type");
        dm_exception_store_type_unregister(&_persistent_type);
        return r;
    }

    return r;
}

void dm_persistent_snapshot_exit(void)
{
    dm_exception_store_type_unregister(&_persistent_type);
    dm_exception_store_type_unregister(&_persistent_compat_type);
}