dm snapshot: add merging

Merging is started when origin is resumed and it is stopped when
origin is suspended or when the merging snapshot is destroyed or
errors are detected.

Merging is not yet interlocked with writes: this will be handled in
subsequent patches.

The code relies on callbacks from a private kcopyd thread.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-exception-store.h

@@ -154,6 +154,13 @@ static inline void dm_consecutive_chunk_count_inc(struct dm_exception *e)
 	BUG_ON(!dm_consecutive_chunk_count(e));
 }
 
+static inline void dm_consecutive_chunk_count_dec(struct dm_exception *e)
+{
+	BUG_ON(!dm_consecutive_chunk_count(e));
+
+	e->new_chunk -= (1ULL << DM_CHUNK_NUMBER_BITS);
+}
+
 #  else
 #    define DM_CHUNK_CONSECUTIVE_BITS 0
 
@@ -171,6 +178,10 @@ static inline void dm_consecutive_chunk_count_inc(struct dm_exception *e)
 {
 }
 
+static inline void dm_consecutive_chunk_count_dec(struct dm_exception *e)
+{
+}
+
 #  endif
 
 /*

drivers/md/dm-snap.c

@@ -106,8 +106,20 @@ struct dm_snapshot {
 	mempool_t *tracked_chunk_pool;
 	spinlock_t tracked_chunk_lock;
 	struct hlist_head tracked_chunk_hash[DM_TRACKED_CHUNK_HASH_SIZE];
+
+	/* Wait for events based on state_bits */
+	unsigned long state_bits;
 };
 
+/*
+ * state_bits:
+ *   RUNNING_MERGE  - Merge operation is in progress.
+ *   SHUTDOWN_MERGE - Set to signal that merge needs to be stopped;
+ *                    cleared afterwards.
+ */
+#define RUNNING_MERGE          0
+#define SHUTDOWN_MERGE         1
+
 struct dm_dev *dm_snap_cow(struct dm_snapshot *s)
 {
 	return s->cow;
@@ -386,6 +398,13 @@ static int __validate_exception_handover(struct dm_snapshot *snap)
 		return -EINVAL;
 	}
 
+	if (!snap_src->store->type->prepare_merge ||
+	    !snap_src->store->type->commit_merge) {
+		snap->ti->error = "Snapshot exception store does not "
+				  "support snapshot-merge.";
+		return -EINVAL;
+	}
+
 	return 1;
 }
 
@@ -721,6 +740,178 @@ static int init_hash_tables(struct dm_snapshot *s)
 	return 0;
 }
 
+static void merge_shutdown(struct dm_snapshot *s)
+{
+	clear_bit_unlock(RUNNING_MERGE, &s->state_bits);
+	smp_mb__after_clear_bit();
+	wake_up_bit(&s->state_bits, RUNNING_MERGE);
+}
+
+/*
+ * Remove one chunk from the index of completed exceptions.
+ */
+static int __remove_single_exception_chunk(struct dm_snapshot *s,
+					   chunk_t old_chunk)
+{
+	struct dm_exception *e;
+
+	/* FIXME: interlock writes to this chunk */
+
+	e = dm_lookup_exception(&s->complete, old_chunk);
+	if (!e) {
+		DMERR("Corruption detected: exception for block %llu is "
+		      "on disk but not in memory",
+		      (unsigned long long)old_chunk);
+		return -EINVAL;
+	}
+
+	/*
+	 * If this is the only chunk using this exception, remove exception.
+	 */
+	if (!dm_consecutive_chunk_count(e)) {
+		dm_remove_exception(e);
+		free_completed_exception(e);
+		return 0;
+	}
+
+	/*
+	 * The chunk may be either at the beginning or the end of a
+	 * group of consecutive chunks - never in the middle.  We are
+	 * removing chunks in the opposite order to that in which they
+	 * were added, so this should always be true.
+	 * Decrement the consecutive chunk counter and adjust the
+	 * starting point if necessary.
+	 */
+	if (old_chunk == e->old_chunk) {
+		e->old_chunk++;
+		e->new_chunk++;
+	} else if (old_chunk != e->old_chunk +
+		   dm_consecutive_chunk_count(e)) {
+		DMERR("Attempt to merge block %llu from the "
+		      "middle of a chunk range [%llu - %llu]",
+		      (unsigned long long)old_chunk,
+		      (unsigned long long)e->old_chunk,
+		      (unsigned long long)
+		      e->old_chunk + dm_consecutive_chunk_count(e));
+		return -EINVAL;
+	}
+
+	dm_consecutive_chunk_count_dec(e);
+
+	return 0;
+}
+
+static int remove_single_exception_chunk(struct dm_snapshot *s,
+					 chunk_t old_chunk)
+{
+	int r = 0;
+
+	down_write(&s->lock);
+	r = __remove_single_exception_chunk(s, old_chunk);
+	up_write(&s->lock);
+
+	return r;
+}
+
+static void merge_callback(int read_err, unsigned long write_err,
+			   void *context);
+
+static void snapshot_merge_next_chunks(struct dm_snapshot *s)
+{
+	int r;
+	chunk_t old_chunk, new_chunk;
+	struct dm_io_region src, dest;
+
+	BUG_ON(!test_bit(RUNNING_MERGE, &s->state_bits));
+	if (unlikely(test_bit(SHUTDOWN_MERGE, &s->state_bits)))
+		goto shut;
+
+	/*
+	 * valid flag never changes during merge, so no lock required.
+	 */
+	if (!s->valid) {
+		DMERR("Snapshot is invalid: can't merge");
+		goto shut;
+	}
+
+	r = s->store->type->prepare_merge(s->store, &old_chunk, &new_chunk);
+	if (r <= 0) {
+		if (r < 0)
+			DMERR("Read error in exception store: "
+			      "shutting down merge");
+		goto shut;
+	}
+
+	/* TODO: use larger I/O size once we verify that kcopyd handles it */
+
+	if (remove_single_exception_chunk(s, old_chunk) < 0)
+		goto shut;
+
+	dest.bdev = s->origin->bdev;
+	dest.sector = chunk_to_sector(s->store, old_chunk);
+	dest.count = min((sector_t)s->store->chunk_size,
+			 get_dev_size(dest.bdev) - dest.sector);
+
+	src.bdev = s->cow->bdev;
+	src.sector = chunk_to_sector(s->store, new_chunk);
+	src.count = dest.count;
+
+	dm_kcopyd_copy(s->kcopyd_client, &src, 1, &dest, 0, merge_callback, s);
+	return;
+
+shut:
+	merge_shutdown(s);
+}
+
+static void merge_callback(int read_err, unsigned long write_err, void *context)
+{
+	struct dm_snapshot *s = context;
+
+	if (read_err || write_err) {
+		if (read_err)
+			DMERR("Read error: shutting down merge.");
+		else
+			DMERR("Write error: shutting down merge.");
+		goto shut;
+	}
+
+	if (s->store->type->commit_merge(s->store, 1) < 0) {
+		DMERR("Write error in exception store: shutting down merge");
+		goto shut;
+	}
+
+	snapshot_merge_next_chunks(s);
+
+	return;
+
+shut:
+	merge_shutdown(s);
+}
+
+static void start_merge(struct dm_snapshot *s)
+{
+	if (!test_and_set_bit(RUNNING_MERGE, &s->state_bits))
+		snapshot_merge_next_chunks(s);
+}
+
+static int wait_schedule(void *ptr)
+{
+	schedule();
+
+	return 0;
+}
+
+/*
+ * Stop the merging process and wait until it finishes.
+ */
+static void stop_merge(struct dm_snapshot *s)
+{
+	set_bit(SHUTDOWN_MERGE, &s->state_bits);
+	wait_on_bit(&s->state_bits, RUNNING_MERGE, wait_schedule,
+		    TASK_UNINTERRUPTIBLE);
+	clear_bit(SHUTDOWN_MERGE, &s->state_bits);
+}
+
 /*
  * Construct a snapshot mapping: <origin_dev> <COW-dev> <p/n> <chunk-size>
  */
@@ -791,6 +982,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	init_rwsem(&s->lock);
 	INIT_LIST_HEAD(&s->list);
 	spin_lock_init(&s->pe_lock);
+	s->state_bits = 0;
 
 	/* Allocate hash table for COW data */
 	if (init_hash_tables(s)) {
@@ -963,6 +1155,9 @@ static void snapshot_dtr(struct dm_target *ti)
 	}
 	up_read(&_origins_lock);
 
+	if (dm_target_is_snapshot_merge(ti))
+		stop_merge(s);
+
 	/* Prevent further origin writes from using this snapshot. */
 	/* After this returns there can be no new kcopyd jobs. */
 	unregister_snapshot(s);
@@ -1404,6 +1599,13 @@ static int snapshot_end_io(struct dm_target *ti, struct bio *bio,
 	return 0;
 }
 
+static void snapshot_merge_presuspend(struct dm_target *ti)
+{
+	struct dm_snapshot *s = ti->private;
+
+	stop_merge(s);
+}
+
 static void snapshot_postsuspend(struct dm_target *ti)
 {
 	struct dm_snapshot *s = ti->private;
@@ -1464,6 +1666,34 @@ static void snapshot_resume(struct dm_target *ti)
 	up_write(&s->lock);
 }
 
+static sector_t get_origin_minimum_chunksize(struct block_device *bdev)
+{
+	sector_t min_chunksize;
+
+	down_read(&_origins_lock);
+	min_chunksize = __minimum_chunk_size(__lookup_origin(bdev));
+	up_read(&_origins_lock);
+
+	return min_chunksize;
+}
+
+static void snapshot_merge_resume(struct dm_target *ti)
+{
+	struct dm_snapshot *s = ti->private;
+
+	/*
+	 * Handover exceptions from existing snapshot.
+	 */
+	snapshot_resume(ti);
+
+	/*
+	 * snapshot-merge acts as an origin, so set ti->split_io
+	 */
+	ti->split_io = get_origin_minimum_chunksize(s->origin->bdev);
+
+	start_merge(s);
+}
+
 static int snapshot_status(struct dm_target *ti, status_type_t type,
 			   char *result, unsigned int maxlen)
 {
@@ -1722,11 +1952,7 @@ static void origin_resume(struct dm_target *ti)
 {
 	struct dm_dev *dev = ti->private;
 
-	down_read(&_origins_lock);
-
-	ti->split_io = __minimum_chunk_size(__lookup_origin(dev->bdev));
-
-	up_read(&_origins_lock);
+	ti->split_io = get_origin_minimum_chunksize(dev->bdev);
 }
 
 static int origin_status(struct dm_target *ti, status_type_t type, char *result,
@@ -1790,9 +2016,10 @@ static struct target_type merge_target = {
 	.dtr     = snapshot_dtr,
 	.map     = snapshot_merge_map,
 	.end_io  = snapshot_end_io,
+	.presuspend = snapshot_merge_presuspend,
 	.postsuspend = snapshot_postsuspend,
 	.preresume  = snapshot_preresume,
-	.resume  = snapshot_resume,
+	.resume  = snapshot_merge_resume,
 	.status  = snapshot_status,
 	.iterate_devices = snapshot_iterate_devices,
 };