[PATCH v4 08/13] dm snapshot: the merge procedure

[Mike Snitzer]

From: Mikulas Patocka <mpatocka@xxxxxxxxxx>

Merging is started when origin is resumed and it is stopped when
origin is suspended or when the merging snapshot is destoyed.

We don't need a separate thread, kcopyd does the job just fine
(provided that we have a private kcopyd).

Merging is not yet interlocked with writes, so there is a race condition
with concurrent access.  It will be fixed in further patches.

Adds a supporting function to decrement consecutive chunk counter.
Care is taken to increment the exception's old_chunk and new_chunk,
prior to the dm_consecutive_chunk_count_dec() call, if the chunk is at
the start of an exception's consecutive chunk range.  This allows for
snapshot-merge to support chunks that are added to the 'complete'
exception hash table before existing chunks.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
---
 drivers/md/dm-exception-store.h |   11 +++
 drivers/md/dm-snap.c            |  179 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 184 insertions(+), 6 deletions(-)

diff --git a/drivers/md/dm-exception-store.h b/drivers/md/dm-exception-store.h
index 534427f..7b83002 100644
--- a/drivers/md/dm-exception-store.h
+++ b/drivers/md/dm-exception-store.h
@@ -153,6 +153,13 @@ 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
 
@@ -170,6 +177,10 @@ 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
 
 /*
diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index f4e9aa6..87c9033 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -101,6 +101,13 @@ struct dm_snapshot {
 	mempool_t *tracked_chunk_pool;
 	spinlock_t tracked_chunk_lock;
 	struct hlist_head tracked_chunk_hash[DM_TRACKED_CHUNK_HASH_SIZE];
+
+	/* Merge operation is in progress */
+	int merge_running;
+
+	/* It is requested to shut down merging */
+	/* Cleared back to 0 when the merging is stopped */
+	int merge_shutdown;
 };
 
 struct dm_dev *dm_snap_cow(struct dm_snapshot *s)
@@ -375,6 +382,14 @@ static int __validate_exception_handover(struct dm_snapshot *snap)
 				r = -EINVAL;
 				goto out;
 			}
+
+			if (!snap_src->store->type->prepare_merge ||
+			    !snap_src->store->type->commit_merge) {
+				snap->ti->error = "Merging snapshot store must "
+						  "support snapshot-merge";
+				r = -EINVAL;
+				goto out;
+			}
 		}
 
 		r = 1;
@@ -714,6 +729,123 @@ static int init_hash_tables(struct dm_snapshot *s)
 	return 0;
 }
 
+static void merge_callback(int read_err, unsigned long write_err,
+			   void *context);
+
+static void snapshot_merge_process(struct dm_snapshot *s)
+{
+	int r;
+	chunk_t old_chunk, new_chunk;
+	struct dm_exception *e;
+	struct dm_io_region src, dest;
+
+	BUG_ON(!s->merge_running);
+	if (s->merge_shutdown)
+		goto shut;
+
+	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 */
+
+	/* !!! FIXME: intelock writes to this chunk */
+	down_write(&s->lock);
+	e = dm_lookup_exception(&s->complete, old_chunk);
+	if (!e) {
+		DMERR("exception for block %llu is on disk but not in memory",
+		      (unsigned long long)old_chunk);
+		up_write(&s->lock);
+		goto shut;
+	}
+	if (dm_consecutive_chunk_count(e)) {
+		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("merge from the middle of a chunk range");
+			up_write(&s->lock);
+			goto shut;
+		}
+		dm_consecutive_chunk_count_dec(e);
+	} else {
+		dm_remove_exception(e);
+		free_completed_exception(e);
+	}
+	up_write(&s->lock);
+
+	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:
+	s->merge_running = 0;
+}
+
+static void merge_callback(int read_err, unsigned long write_err, void *context)
+{
+	int r;
+	struct dm_snapshot *s = context;
+
+	if (read_err || write_err) {
+		if (read_err)
+			DMERR("Read error in data, shutting down merge");
+		else
+			DMERR("Write error in data, shutting down merge");
+		goto shut;
+	}
+
+	r = s->store->type->commit_merge(s->store, 1);
+	if (r < 0) {
+		DMERR("Write error in exception store, shutting down merge");
+		goto shut;
+	}
+
+	snapshot_merge_process(s);
+	return;
+
+shut:
+	s->merge_running = 0;
+}
+
+static void start_merge(struct dm_snapshot *s)
+{
+	if (!s->merge_running && !s->merge_shutdown) {
+		s->merge_running = 1;
+		snapshot_merge_process(s);
+	}
+}
+
+/*
+ * Stop the merging process and wait until it finishes.
+ */
+static void stop_merge(struct dm_snapshot *s)
+{
+	while (s->merge_running) {
+		s->merge_shutdown = 1;
+		msleep(1);
+	}
+	s->merge_shutdown = 0;
+}
+
 /*
  * Construct a snapshot mapping: <origin_dev> <COW-dev> <p/n> <chunk-size>
  */
@@ -778,6 +910,8 @@ 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->merge_running = 0;
+	s->merge_shutdown = 0;
 
 	/* Allocate hash table for COW data */
 	if (init_hash_tables(s)) {
@@ -950,6 +1084,9 @@ static void snapshot_dtr(struct dm_target *ti)
 	}
 	up_read(&_origins_lock);
 
+	if (is_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);
@@ -1378,6 +1515,13 @@ 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;
@@ -1438,6 +1582,32 @@ static void snapshot_resume(struct dm_target *ti)
 	up_write(&s->lock);
 }
 
+static chunk_t get_origin_minimum_chunksize(struct block_device *bdev)
+{
+	chunk_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;
+
+	snapshot_resume(ti);
+	/*
+	 * snapshot-merge can take on the role of the origin too
+	 * - must adjust snapshot-merge's ti->split_io accordingly
+	 */
+	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)
 {
@@ -1682,11 +1852,7 @@ 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,
@@ -1750,9 +1916,10 @@ 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,
 };
-- 
1.6.5.2

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