dm-snap-persistent.c

/*
 * 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->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)
		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->store->chunk_size * chunk,
		.count = ps->snap->store->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->store->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->store->chunk_size * area_location(ps, area),
		.count = ps->snap->store->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->store->chunk_size) {
		ps->snap->store->chunk_size = max(DM_CHUNK_SIZE_DEFAULT_SECTORS,
		    bdev_hardsect_size(ps->snap->cow->bdev) >> 9);
		ps->snap->store->chunk_mask = ps->snap->store->chunk_size - 1;
		ps->snap->store->chunk_shift = ffs(ps->snap->store->chunk_size)
					       - 1;
		chunk_size_supplied = 0;
	}

	ps->io_client = dm_io_client_create(sectors_to_pages(ps->snap->
							    store->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->store->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->store->chunk_size);

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

	ps->snap->store->chunk_size = chunk_size;
	ps->snap->store->chunk_mask = chunk_size - 1;
	ps->snap->store->chunk_shift = ffs(chunk_size) - 1;

	r = dm_io_client_resize(sectors_to_pages(ps->snap->store->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->store->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->store->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 (*callback)(void *callback_context,
					     chunk_t old, chunk_t new),
			     void *callback_context,
			     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 = callback(callback_context, de.old_chunk, de.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_fraction_full(struct dm_exception_store *store,
				     sector_t *numerator, sector_t *denominator)
{
	*numerator = get_info(store)->next_free * store->chunk_size;
	*denominator = get_dev_size(store->snap->cow->bdev);
}

static void persistent_dtr(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 (*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->snap->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;
		}
	} 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, callback, callback_context);
		if (r)
			return r;
	}

	return 0;
}

static int persistent_prepare_exception(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->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_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_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 = 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->context = ps;

	return 0;
}

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

	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,
	.drop_snapshot = persistent_drop_snapshot,
	.fraction_full = persistent_fraction_full,
	.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,
	.drop_snapshot = persistent_drop_snapshot,
	.fraction_full = persistent_fraction_full,
	.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);
}