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 | 236 +++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 228 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 +75,27 @@ 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 one of the
+ * following occurs:
+ * - src (old) snapshot, that is handing over, is destructed
+ * - src (old) snapshot, that is handing over, is resumed
+ * - dest (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 +549,31 @@ 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 +645,64 @@ static int init_hash_tables(struct dm_sn
}
/*
+ * 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.
+ */
+static int unlink_snapshots_for_handover(struct dm_snapshot *snap_src,
+ struct dm_snapshot *snap_dest)
+{
+ int r = -EINVAL;
+
+ down_write(&snap_src->lock);
+
+ /* make sure these snapshots are already linked */
+ if ((snap_src->handover_snap != snap_dest) ||
+ (snap_dest->handover_snap != snap_src))
+ goto out;
+
+ snap_src->handover_snap = NULL;
+
+ snap_dest->handover_snap = NULL;
+ snap_dest->is_handover_destination = 0;
+
+ r = 0;
+
+out:
+ up_write(&snap_src->lock);
+ return r;
+}
+
+/*
* 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;
@@ -659,6 +758,8 @@ 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);
spin_lock_init(&s->pe_lock);
@@ -694,6 +795,27 @@ static int snapshot_ctr(struct dm_target
spin_lock_init(&s->tracked_chunk_lock);
+ /* Does snapshot need exceptions handing 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 +827,11 @@ 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 +841,6 @@ 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 +881,90 @@ static void __free_exceptions(struct dm_
dm_exception_table_exit(&s->complete, exception_cache);
}
+static void handover_exceptions(struct dm_snapshot *old,
+ struct dm_snapshot *new)
+{
+ union {
+ struct dm_exception_table table_swap;
+ struct dm_exception_store *store_swap;
+ } u;
+
+ BUG_ON((old->handover_snap != new) ||
+ (new->handover_snap != old));
+ BUG_ON((old->is_handover_destination != 0) ||
+ (new->is_handover_destination != 1));
+ BUG_ON(!old->suspended);
+
+ /* make sure old snapshot is still valid */
+ if (!old->valid) {
+ new->valid = 0;
+ DMERR("Unable to handover exceptions "
+ "from an invalid snapshot.");
+ return;
+ }
+
+ /* swap exceptions tables and stores */
+ u.table_swap = new->complete;
+ new->complete = old->complete;
+ old->complete = u.table_swap;
+ u.store_swap = new->store;
+ new->store = old->store;
+ old->store = u.store_swap;
+
+ new->store->snap = new;
+ old->store->snap = old;
+
+ /* reset split_io to store's chunk_size */
+ if (new->ti->split_io != new->store->chunk_size)
+ new->ti->split_io = new->store->chunk_size;
+
+ /* Mark old snapshot invalid and inactive */
+ old->valid = 0;
+ old->active = 0;
+
+ /* Reset handover_snap references */
+ old->handover_snap = NULL;
+ new->handover_snap = NULL;
+
+ new->is_handover_destination = 0;
+}
+
static void snapshot_dtr(struct dm_target *ti)
{
#ifdef CONFIG_DM_DEBUG
int i;
#endif
struct dm_snapshot *s = ti->private;
+ struct dm_snapshot *new_snap = NULL;
flush_workqueue(ksnapd);
+ /* This snapshot may need to handover its exception store */
+ down_write(&s->lock);
+ if (s->handover_snap) {
+ if (!s->is_handover_destination) {
+ /* Handover exceptions to another snapshot */
+ new_snap = s->handover_snap;
+
+ down_write_nested(&new_snap->lock,
+ SINGLE_DEPTH_NESTING);
+ handover_exceptions(s, new_snap);
+ up_write(&new_snap->lock);
+ } else {
+ /* allow table_clear to cancel handover */
+ unlink_snapshots_for_handover(s->handover_snap, s);
+ }
+ }
+ /* An incomplete exception handover is not allowed */
+ BUG_ON(s->handover_snap);
+ up_write(&s->lock);
+
+ if (new_snap && register_snapshot(new_snap)) {
+ DMERR("Unable to register snapshot "
+ "after exception handover.");
+ new_snap->valid = 0;
+ }
+
/* Prevent further origin writes from using this snapshot. */
/* After this returns there can be no new kcopyd jobs. */
unregister_snapshot(s);
@@ -1189,11 +1380,40 @@ static void snapshot_presuspend(struct d
static void snapshot_resume(struct dm_target *ti)
{
struct dm_snapshot *s = ti->private;
+ struct dm_snapshot *lock_snap, *old_snap, *new_snap = NULL;
down_write(&s->lock);
+ if (s->handover_snap) {
+ /*
+ * Initially assumes this snapshot will get
+ * exception store from another snapshot
+ */
+ old_snap = s->handover_snap;
+ new_snap = s;
+ lock_snap = old_snap;
+
+ if (!s->is_handover_destination) {
+ /* Handover exceptions to another snapshot */
+ old_snap = s;
+ new_snap = s->handover_snap;
+ lock_snap = new_snap;
+ }
+ down_write_nested(&lock_snap->lock,
+ SINGLE_DEPTH_NESTING);
+ handover_exceptions(old_snap, new_snap);
+ up_write(&lock_snap->lock);
+ }
+ /* An incomplete exception handover is not allowed */
+ BUG_ON(s->handover_snap);
s->active = 1;
s->suspended = 0;
up_write(&s->lock);
+
+ if (new_snap && register_snapshot(new_snap)) {
+ DMERR("Unable to register snapshot "
+ "after exception handover.");
+ new_snap->valid = 0;
+ }
}
static int snapshot_status(struct dm_target *ti, status_type_t type,
--
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