Permit in-use snapshot exception data to be 'handed over' from one
snapshot instance to another. This is a pre-requisite for patches
that allow the changes made in a snapshot device to be merged back into
its origin device and also allows device resizing.
The basic call sequence is:
dmsetup load new_snapshot (referencing the existing in-use cow device)
- the ctr code detects that the cow is already in use and links the
two snapshot target instances together
dmsetup suspend original_snapshot
dmsetup resume new_snapshot
- the new_snapshot becomes live, and if anything now tries to access
the original one it will receive EIO
dmsetup remove original_snapshot
(There can only be two snapshot targets referencing the same cow device
simultaneously.)
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Cc: Mikulas Patocka <mpatocka@xxxxxxxxxx>
---
drivers/md/dm-snap.c | 264 +++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 256 insertions(+), 8 deletions(-)
Index: linux-2.6/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-snap.c
+++ linux-2.6/drivers/md/dm-snap.c
@@ -75,6 +75,24 @@ struct dm_snapshot {
/* Whether or not owning mapped_device is suspended */
int suspended;
+ /*
+ * 'is_handover_destination' denotes another snapshot with the same
+ * cow block device (as identified with find_snapshot_using_cow)
+ * will hand over its exception store to this snapshot.
+ *
+ * 'is_handover_destination' is set in snapshot_ctr if an existing
+ * snapshot has the same cow device. The handover is performed,
+ * and 'is_handover_destination' is cleared, when the destination
+ * (new) snapshot, that is accepting the handover, is resumed.
+ */
+ int is_handover_destination;
+
+ /*
+ * reference to the other snapshot that will participate in the
+ * exception store handover; src references dest, dest references src
+ */
+ struct dm_snapshot *handover_snap;
+
mempool_t *pending_pool;
atomic_t pending_exceptions_count;
@@ -528,6 +546,31 @@ static int dm_add_exception(void *contex
return 0;
}
+static struct dm_snapshot *find_snapshot_using_cow(struct dm_snapshot *snap)
+{
+ struct dm_snapshot *s, *handover_snap = NULL;
+ struct origin *o;
+
+ down_read(&_origins_lock);
+
+ o = __lookup_origin(snap->origin->bdev);
+ if (!o)
+ goto out;
+
+ list_for_each_entry(s, &o->snapshots, list) {
+ if (s == snap || !bdev_equal(s->cow->bdev, snap->cow->bdev))
+ continue;
+
+ handover_snap = s;
+ break;
+ }
+
+out:
+ up_read(&_origins_lock);
+
+ return handover_snap;
+}
+
#define min_not_zero(l, r) (((l) == 0) ? (r) : (((r) == 0) ? (l) : min(l, r)))
/*
@@ -599,11 +642,105 @@ static int init_hash_tables(struct dm_sn
}
/*
+ * Ensure proper lock order for snapshots involved in handover:
+ * lock the source snapshot then lock the destination snapshot.
+ * Returns 0 if exception handover is not necessary.
+ * Returns 1 if both snapshots were locked, also sets pointers
+ * for which snapshot is the src and which is the dest.
+ */
+static int lock_snapshots_for_handover(struct dm_snapshot *s,
+ struct dm_snapshot **snap_src,
+ struct dm_snapshot **snap_dest)
+{
+ down_write(&s->lock);
+ if (!s->handover_snap) {
+ up_write(&s->lock);
+ return 0;
+ }
+
+ /*
+ * determine the src and dest snapshots from 's';
+ * lock the source then lock the destination
+ */
+ if (s->is_handover_destination) {
+ /* 's' is getting exceptions from another snapshot */
+ /* must drop lock and then get locks in proper order */
+ up_write(&s->lock);
+ *snap_src = s->handover_snap;
+ *snap_dest = s;
+ down_write_nested(&(*snap_src)->lock,
+ SINGLE_DEPTH_NESTING);
+ down_write(&(*snap_dest)->lock);
+ } else {
+ /* already have the 'snap_src' lock */
+ *snap_src = s;
+ *snap_dest = s->handover_snap;
+ down_write_nested(&(*snap_dest)->lock,
+ SINGLE_DEPTH_NESTING);
+ }
+
+ return 1;
+}
+
+static void unlock_snapshots_for_handover(struct dm_snapshot *snap_src,
+ struct dm_snapshot *snap_dest)
+{
+ up_write(&snap_dest->lock);
+ up_write(&snap_src->lock);
+}
+
+/*
+ * Reserve snap_src for handover to snap_dest.
+ */
+static int link_snapshots_for_handover(struct dm_snapshot *snap_src,
+ struct dm_snapshot *snap_dest)
+{
+ int r = -EINVAL;
+
+ down_write(&snap_src->lock);
+
+ /* Another handover already set? */
+ if (snap_src->handover_snap)
+ goto out;
+
+ snap_src->handover_snap = snap_dest;
+
+ snap_dest->handover_snap = snap_src;
+ snap_dest->is_handover_destination = 1;
+
+ r = 0;
+
+out:
+ up_write(&snap_src->lock);
+ return r;
+}
+
+/*
+ * Unreserve snap_src for handover to snap_dest.
+ * Must take associated locks with lock_snapshots_for_handover().
+ */
+static int __unlink_snapshots_for_handover(struct dm_snapshot *snap_src,
+ struct dm_snapshot *snap_dest)
+{
+ /* make sure these snapshots are already linked */
+ if ((snap_src->handover_snap != snap_dest) ||
+ (snap_dest->handover_snap != snap_src))
+ return -EINVAL;
+
+ snap_src->handover_snap = NULL;
+
+ snap_dest->handover_snap = NULL;
+ snap_dest->is_handover_destination = 0;
+
+ return 0;
+}
+
+/*
* Construct a snapshot mapping: <origin_dev> <COW-dev> <p/n> <chunk-size>
*/
static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
- struct dm_snapshot *s;
+ struct dm_snapshot *s, *handover_snap;
int i;
int r = -EINVAL;
char *origin_path, *cow_path;
@@ -659,7 +796,10 @@ static int snapshot_ctr(struct dm_target
s->active = 0;
s->suspended = 0;
atomic_set(&s->pending_exceptions_count, 0);
+ s->is_handover_destination = 0;
+ s->handover_snap = NULL;
init_rwsem(&s->lock);
+ INIT_LIST_HEAD(&s->list);
spin_lock_init(&s->pe_lock);
/* Allocate hash table for COW data */
@@ -694,6 +834,27 @@ static int snapshot_ctr(struct dm_target
spin_lock_init(&s->tracked_chunk_lock);
+ /* Does snapshot need exceptions handed over to it? */
+ handover_snap = find_snapshot_using_cow(s);
+ if (handover_snap) {
+ r = link_snapshots_for_handover(handover_snap, s);
+ if (r) {
+ ti->error = "Unable to handover snapshot to "
+ "two devices at once.";
+ goto bad_load_and_register;
+ }
+ }
+
+ bio_list_init(&s->queued_bios);
+ INIT_WORK(&s->queued_bios_work, flush_queued_bios);
+
+ ti->private = s;
+ ti->num_flush_requests = 1;
+
+ if (handover_snap)
+ /* register_snapshot is deferred until after handover_exceptions */
+ return 0;
+
/* Metadata must only be loaded into one table at once */
r = s->store->type->read_metadata(s->store, dm_add_exception,
(void *)s);
@@ -705,13 +866,11 @@ static int snapshot_ctr(struct dm_target
DMWARN("Snapshot is marked invalid.");
}
- bio_list_init(&s->queued_bios);
- INIT_WORK(&s->queued_bios_work, flush_queued_bios);
-
if (!s->store->chunk_size) {
ti->error = "Chunk size not set";
goto bad_load_and_register;
}
+ ti->split_io = s->store->chunk_size;
/* Add snapshot to the list of snapshots for this origin */
/* Exceptions aren't triggered till snapshot_resume() is called */
@@ -721,10 +880,6 @@ static int snapshot_ctr(struct dm_target
goto bad_load_and_register;
}
- ti->private = s;
- ti->split_io = s->store->chunk_size;
- ti->num_flush_requests = 1;
-
return 0;
bad_load_and_register:
@@ -765,15 +920,61 @@ static void __free_exceptions(struct dm_
dm_exception_table_exit(&s->complete, exception_cache);
}
+static void handover_exceptions(struct dm_snapshot *snap_src,
+ struct dm_snapshot *snap_dest)
+{
+ union {
+ struct dm_exception_table table_swap;
+ struct dm_exception_store *store_swap;
+ } u;
+
+ BUG_ON((snap_src->handover_snap != snap_dest) ||
+ (snap_dest->handover_snap != snap_src));
+ BUG_ON((snap_src->is_handover_destination != 0) ||
+ (snap_dest->is_handover_destination != 1));
+
+ /* swap exceptions tables and stores */
+ u.table_swap = snap_dest->complete;
+ snap_dest->complete = snap_src->complete;
+ snap_src->complete = u.table_swap;
+ u.store_swap = snap_dest->store;
+ snap_dest->store = snap_src->store;
+ snap_src->store = u.store_swap;
+
+ snap_dest->store->snap = snap_dest;
+ snap_src->store->snap = snap_src;
+
+ /* reset split_io to store's chunk_size */
+ if (snap_dest->ti->split_io != snap_dest->store->chunk_size)
+ snap_dest->ti->split_io = snap_dest->store->chunk_size;
+
+ /* transfer 'valid' state, mark snap_src snapshot invalid */
+ snap_dest->valid = snap_src->valid;
+
+ __unlink_snapshots_for_handover(snap_src, snap_dest);
+}
+
static void snapshot_dtr(struct dm_target *ti)
{
#ifdef CONFIG_DM_DEBUG
int i;
#endif
struct dm_snapshot *s = ti->private;
+ struct dm_snapshot *snap_src, *snap_dest;
flush_workqueue(ksnapd);
+ if (lock_snapshots_for_handover(s, &snap_src, &snap_dest)) {
+ if (s == snap_src) {
+ DMERR("Unable to handover exceptions to another "
+ "snapshot from dtr, cancelling handover.");
+ s->valid = 0;
+ }
+ /* allow table_clear to cancel handover */
+ __unlink_snapshots_for_handover(snap_src, snap_dest);
+ unlock_snapshots_for_handover(snap_src, snap_dest);
+ }
+
/* Prevent further origin writes from using this snapshot. */
/* After this returns there can be no new kcopyd jobs. */
unregister_snapshot(s);
@@ -1186,11 +1387,57 @@ static void snapshot_presuspend(struct d
up_write(&s->lock);
}
+static int snapshot_preresume(struct dm_target *ti)
+{
+ struct dm_snapshot *s = ti->private;
+ struct dm_snapshot *snap_src, *snap_dest;
+
+ if (lock_snapshots_for_handover(s, &snap_src, &snap_dest)) {
+ if (s == snap_dest && !snap_src->suspended) {
+ /* make sure snap_src is suspended */
+ DMERR("Unable to accept exceptions from a "
+ "snapshot that is not suspended, "
+ "cancelling handover.");
+ __unlink_snapshots_for_handover(snap_src, snap_dest);
+ snap_dest->valid = 0;
+ } else if (s == snap_src) {
+ /*
+ * snap_dest is invalid if snap_src is
+ * resumed before it
+ */
+ DMERR("Unable to handover exceptions to another "
+ "snapshot on resume, cancelling handover.");
+ __unlink_snapshots_for_handover(snap_src, snap_dest);
+ snap_dest->valid = 0;
+ }
+ unlock_snapshots_for_handover(snap_src, snap_dest);
+ }
+
+ /* returning failure leaves target suspended, best to avoid hung IO */
+ return 0;
+}
+
static void snapshot_resume(struct dm_target *ti)
{
struct dm_snapshot *s = ti->private;
+ struct dm_snapshot *snap_src, *snap_dest;
+
+ if (lock_snapshots_for_handover(s, &snap_src, &snap_dest)) {
+ BUG_ON(s == snap_src);
+ /* Get exception store from another snapshot */
+ handover_exceptions(snap_src, snap_dest);
+ unlock_snapshots_for_handover(snap_src, snap_dest);
+
+ if (register_snapshot(snap_dest)) {
+ DMERR("Unable to register snapshot "
+ "after exception handover.");
+ snap_dest->valid = 0;
+ }
+ }
down_write(&s->lock);
+ /* An incomplete exception handover is not allowed */
+ BUG_ON(s->handover_snap);
s->active = 1;
s->suspended = 0;
up_write(&s->lock);
@@ -1506,6 +1753,7 @@ static struct target_type snapshot_targe
.map = snapshot_map,
.end_io = snapshot_end_io,
.presuspend = snapshot_presuspend,
+ .preresume = snapshot_preresume,
.resume = snapshot_resume,
.status = snapshot_status,
.iterate_devices = snapshot_iterate_devices,
--
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