dm snapshot: split out exception store implementations

Move the existing snapshot exception store implementations out into
separate files.  Later patches will place these behind a new
interface in preparation for alternative implementations.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 694 insertions, 0 deletions

diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
new file mode 100644
index 000000000000..57c946c69ee7
--- /dev/null
+++ b/drivers/md/dm-snap-persistent.c
@@ -0,0 +1,694 @@
+/*
+ * 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 "dm-snap.h"
+
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/vmalloc.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
+
+struct disk_header {
+        uint32_t magic;
+
+        /*
+         * Is this snapshot valid.  There is no way of recovering
+         * an invalid snapshot.
+         */
+        uint32_t valid;
+
+        /*
+         * Simple, incrementing version. no backward
+         * compatibility.
+         */
+        uint32_t version;
+
+        /* In sectors */
+        uint32_t chunk_size;
+};
+
+struct disk_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_snapshot *snap;       /* up pointer to my snapshot */
+        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;
+
+        /*
+         * 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.
+         */
+        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 unsigned sectors_to_pages(unsigned sectors)
+{
+        return DIV_ROUND_UP(sectors, PAGE_SIZE >> 9);
+}
+
+static int alloc_area(struct pstore *ps)
+{
+        int r = -ENOMEM;
+        size_t len;
+
+        len = ps->snap->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)
+                return r;
+
+        ps->zero_area = vmalloc(len);
+        if (!ps->zero_area) {
+                vfree(ps->area);
+                return r;
+        }
+        memset(ps->zero_area, 0, len);
+
+        return 0;
+}
+
+static void free_area(struct pstore *ps)
+{
+        vfree(ps->area);
+        ps->area = NULL;
+        vfree(ps->zero_area);
+        ps->zero_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, chunk_t chunk, int rw, int metadata)
+{
+        struct dm_io_region where = {
+                .bdev = ps->snap->cow->bdev,
+                .sector = ps->snap->chunk_size * chunk,
+                .count = ps->snap->chunk_size,
+        };
+        struct dm_io_request io_req = {
+                .bi_rw = rw,
+                .mem.type = DM_IO_VMA,
+                .mem.ptr.vma = ps->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(&req.work, do_metadata);
+        queue_work(ps->metadata_wq, &req.work);
+        flush_workqueue(ps->metadata_wq);
+
+        return req.result;
+}
+
+/*
+ * Convert a metadata area index to a chunk index.
+ */
+static chunk_t area_location(struct pstore *ps, chunk_t area)
+{
+        return 1 + ((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, chunk, rw, 0);
+        if (r)
+                return r;
+
+        return 0;
+}
+
+static void zero_memory_area(struct pstore *ps)
+{
+        memset(ps->area, 0, ps->snap->chunk_size << SECTOR_SHIFT);
+}
+
+static int zero_disk_area(struct pstore *ps, chunk_t area)
+{
+        struct dm_io_region where = {
+                .bdev = ps->snap->cow->bdev,
+                .sector = ps->snap->chunk_size * area_location(ps, area),
+                .count = ps->snap->chunk_size,
+        };
+        struct dm_io_request io_req = {
+                .bi_rw = WRITE,
+                .mem.type = DM_IO_VMA,
+                .mem.ptr.vma = ps->zero_area,
+                .client = ps->io_client,
+                .notify.fn = NULL,
+        };
+
+        return dm_io(&io_req, 1, &where, NULL);
+}
+
+static int read_header(struct pstore *ps, int *new_snapshot)
+{
+        int r;
+        struct disk_header *dh;
+        chunk_t chunk_size;
+        int chunk_size_supplied = 1;
+
+        /*
+         * Use default chunk size (or hardsect_size, if larger) if none supplied
+         */
+        if (!ps->snap->chunk_size) {
+                ps->snap->chunk_size = max(DM_CHUNK_SIZE_DEFAULT_SECTORS,
+                    bdev_hardsect_size(ps->snap->cow->bdev) >> 9);
+                ps->snap->chunk_mask = ps->snap->chunk_size - 1;
+                ps->snap->chunk_shift = ffs(ps->snap->chunk_size) - 1;
+                chunk_size_supplied = 0;
+        }
+
+        ps->io_client = dm_io_client_create(sectors_to_pages(ps->snap->
+                                                             chunk_size));
+        if (IS_ERR(ps->io_client))
+                return PTR_ERR(ps->io_client);
+
+        r = alloc_area(ps);
+        if (r)
+                return r;
+
+        r = chunk_io(ps, 0, READ, 1);
+        if (r)
+                goto bad;
+
+        dh = (struct disk_header *) ps->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 (!chunk_size_supplied || ps->snap->chunk_size == chunk_size)
+                return 0;
+
+        DMWARN("chunk size %llu in device metadata overrides "
+               "table chunk size of %llu.",
+               (unsigned long long)chunk_size,
+               (unsigned long long)ps->snap->chunk_size);
+
+        /* We had a bogus chunk_size. Fix stuff up. */
+        free_area(ps);
+
+        ps->snap->chunk_size = chunk_size;
+        ps->snap->chunk_mask = chunk_size - 1;
+        ps->snap->chunk_shift = ffs(chunk_size) - 1;
+
+        r = dm_io_client_resize(sectors_to_pages(ps->snap->chunk_size),
+                                ps->io_client);
+        if (r)
+                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->area, 0, ps->snap->chunk_size << SECTOR_SHIFT);
+
+        dh = (struct disk_header *) ps->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->snap->chunk_size);
+
+        return chunk_io(ps, 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 disk_exception *result)
+{
+        struct disk_exception *e = get_exception(ps, index);
+
+        /* copy it */
+        result->old_chunk = le64_to_cpu(e->old_chunk);
+        result->new_chunk = le64_to_cpu(e->new_chunk);
+}
+
+static void write_exception(struct pstore *ps,
+                            uint32_t index, struct disk_exception *de)
+{
+        struct disk_exception *e = get_exception(ps, index);
+
+        /* copy it */
+        e->old_chunk = cpu_to_le64(de->old_chunk);
+        e->new_chunk = cpu_to_le64(de->new_chunk);
+}
+
+/*
+ * 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 *full)
+{
+        int r;
+        unsigned int i;
+        struct disk_exception de;
+
+        /* presume the area is full */
+        *full = 1;
+
+        for (i = 0; i < ps->exceptions_per_area; i++) {
+                read_exception(ps, i, &de);
+
+                /*
+                 * 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 (de.new_chunk == 0LL) {
+                        ps->current_committed = i;
+                        *full = 0;
+                        break;
+                }
+
+                /*
+                 * Keep track of the start of the free chunks.
+                 */
+                if (ps->next_free <= de.new_chunk)
+                        ps->next_free = de.new_chunk + 1;
+
+                /*
+                 * Otherwise we add the exception to the snapshot.
+                 */
+                r = dm_add_exception(ps->snap, de.old_chunk, de.new_chunk);
+                if (r)
+                        return r;
+        }
+
+        return 0;
+}
+
+static int read_exceptions(struct pstore *ps)
+{
+        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, &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_fraction_full(struct dm_exception_store *store,
+                                     sector_t *numerator, sector_t *denominator)
+{
+        *numerator = get_info(store)->next_free * store->snap->chunk_size;
+        *denominator = get_dev_size(store->snap->cow->bdev);
+}
+
+static void persistent_destroy(struct dm_exception_store *store)
+{
+        struct pstore *ps = get_info(store);
+
+        destroy_workqueue(ps->metadata_wq);
+        dm_io_client_destroy(ps->io_client);
+        vfree(ps->callbacks);
+        free_area(ps);
+        kfree(ps);
+}
+
+static int persistent_read_metadata(struct dm_exception_store *store)
+{
+        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->snap->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;
+                }
+        } else {
+                /*
+                 * 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);
+                if (r)
+                        return r;
+        }
+
+        return 0;
+}
+
+static int persistent_prepare(struct dm_exception_store *store,
+                              struct dm_snap_exception *e)
+{
+        struct pstore *ps = get_info(store);
+        uint32_t stride;
+        chunk_t next_free;
+        sector_t size = get_dev_size(store->snap->cow->bdev);
+
+        /* Is there enough room ? */
+        if (size < ((ps->next_free + 1) * store->snap->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(struct dm_exception_store *store,
+                              struct dm_snap_exception *e,
+                              void (*callback) (void *, int success),
+                              void *callback_context)
+{
+        unsigned int i;
+        struct pstore *ps = get_info(store);
+        struct disk_exception de;
+        struct commit_callback *cb;
+
+        de.old_chunk = e->old_chunk;
+        de.new_chunk = e->new_chunk;
+        write_exception(ps, ps->current_committed++, &de);
+
+        /*
+         * 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))
+                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 void persistent_drop(struct dm_exception_store *store)
+{
+        struct pstore *ps = get_info(store);
+
+        ps->valid = 0;
+        if (write_header(ps))
+                DMWARN("write header failed");
+}
+
+int dm_create_persistent(struct dm_exception_store *store)
+{
+        struct pstore *ps;
+
+        /* allocate the pstore */
+        ps = kmalloc(sizeof(*ps), GFP_KERNEL);
+        if (!ps)
+                return -ENOMEM;
+
+        ps->snap = store->snap;
+        ps->valid = 1;
+        ps->version = SNAPSHOT_DISK_VERSION;
+        ps->area = NULL;
+        ps->next_free = 2;      /* skipping the header and first area */
+        ps->current_committed = 0;
+
+        ps->callback_count = 0;
+        atomic_set(&ps->pending_count, 0);
+        ps->callbacks = NULL;
+
+        ps->metadata_wq = create_singlethread_workqueue("ksnaphd");
+        if (!ps->metadata_wq) {
+                kfree(ps);
+                DMERR("couldn't start header metadata update thread");
+                return -ENOMEM;
+        }
+
+        store->destroy = persistent_destroy;
+        store->read_metadata = persistent_read_metadata;
+        store->prepare_exception = persistent_prepare;
+        store->commit_exception = persistent_commit;
+        store->drop_snapshot = persistent_drop;
+        store->fraction_full = persistent_fraction_full;
+        store->context = ps;
+
+        return 0;
+}
+
+int dm_persistent_snapshot_init(void)
+{
+        return 0;
+}
+
+void dm_persistent_snapshot_exit(void)
+{
+}