Re: [PATCH 3/4] dm-snapshot-exception-handover

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Oct 05 2009 at  3:00pm -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> From: Mikulas Patocka <mpatocka@redhat.com>
> 
> Handover of the exception store. This is needed for merging (to allow reload of
> the table) and it also enables origin or snapshot resize.
> 
> The supported call sequences are:
> 
> new_snapshot->ctr
> old_snapshot->suspend
> old_snapshot->dtr
> new_snapshot->resume
> 
> and
> 
> new_snapshot->ctr
> old_snapshot->suspend
> new_snapshot->resume
> old_snapshot->dtr
> 
> There may not be more than two instances of a given snapshot. LVM always creates
> at most two; if there are more, the user misuses dmsetup.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>

...
> +/* _origins_lock must be held */
> +static struct dm_snapshot *find_duplicate(struct dm_snapshot *snap)
> +{
...

I just noticed that find_duplicate should be renamed to __find_duplicate
to use the convention of a '__' prefix on all dm-snap.c functions that
require the _origins_lock.

Here is an updated patch (doesn't change any snapshot-merge patches that
follow):

Handover of the exception store. This is needed for merging (to allow reload of
the table) and it also enables origin or snapshot resize.

The supported call sequences are:

new_snapshot->ctr
old_snapshot->suspend
old_snapshot->dtr
new_snapshot->resume

and

new_snapshot->ctr
old_snapshot->suspend
new_snapshot->resume
old_snapshot->dtr

There may not be more than two instances of a given snapshot. LVM always creates
at most two; if there are more, the user misuses dmsetup.

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

---
 drivers/md/dm-snap.c |   85 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)

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
@@ -72,6 +72,9 @@ struct dm_snapshot {
 	/* Origin writes don't trigger exceptions until this is set */
 	int active;
 
+	/* Exception store will be handed over from another snapshot */
+	int handover;
+
 	mempool_t *pending_pool;
 
 	atomic_t pending_exceptions_count;
@@ -505,6 +508,33 @@ static int dm_add_exception(void *contex
 	return 0;
 }
 
+/* _origins_lock must be held */
+static struct dm_snapshot *__find_duplicate(struct dm_snapshot *snap)
+{
+	struct dm_snapshot *dup;
+	struct dm_snapshot *l;
+	struct origin *o;
+
+	o = __lookup_origin(snap->origin->bdev);
+	if (!o)
+		return NULL;
+
+	dup = NULL;
+	list_for_each_entry(l, &o->snapshots, list)
+		if (l != snap && bdev_equal(l->cow->bdev,
+					    snap->cow->bdev)) {
+			if (!dup) {
+				dup = l;
+			} else {
+				DMERR("Multiple active duplicates, "
+				      "user misuses dmsetup.");
+				return NULL;
+			}
+		}
+
+	return dup;
+}
+
 #define min_not_zero(l, r) (((l) == 0) ? (r) : (((r) == 0) ? (l) : min(l, r)))
 
 /*
@@ -635,6 +665,7 @@ static int snapshot_ctr(struct dm_target
 	s->valid = 1;
 	s->active = 0;
 	atomic_set(&s->pending_exceptions_count, 0);
+	s->handover = 0;
 	init_rwsem(&s->lock);
 	spin_lock_init(&s->pe_lock);
 
@@ -670,6 +701,11 @@ static int snapshot_ctr(struct dm_target
 
 	spin_lock_init(&s->tracked_chunk_lock);
 
+	down_read(&_origins_lock);
+	if (__find_duplicate(s))
+		s->handover = 1;
+	up_read(&_origins_lock);
+
 	/* Metadata must only be loaded into one table at once */
 	r = s->store->type->read_metadata(s->store, dm_add_exception,
 					  (void *)s);
@@ -741,15 +777,49 @@ static void __free_exceptions(struct dm_
 	exit_exception_table(&s->complete, exception_cache);
 }
 
+static void handover_exceptions(struct dm_snapshot *old,
+				struct dm_snapshot *new)
+{
+	union {
+		struct exception_table table_swap;
+		struct dm_exception_store *store_swap;
+	} u;
+
+	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;
+
+	old->active = 0;
+	new->handover = 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 *dup;
 
 	flush_workqueue(ksnapd);
 
+	down_write(&_origins_lock);
+	down_write(&s->lock);
+	dup = __find_duplicate(s);
+	if (dup && dup->handover) {
+		down_write_nested(&dup->lock, SINGLE_DEPTH_NESTING);
+		handover_exceptions(s, dup);
+		up_write(&dup->lock);
+	}
+	up_write(&s->lock);
+	up_write(&_origins_lock);
+
 	/* Prevent further origin writes from using this snapshot. */
 	/* After this returns there can be no new kcopyd jobs. */
 	unregister_snapshot(s);
@@ -1144,9 +1214,24 @@ static void snapshot_resume(struct dm_ta
 {
 	struct dm_snapshot *s = ti->private;
 
+	down_write(&_origins_lock);
 	down_write(&s->lock);
+	if (s->handover) {
+		struct dm_snapshot *dup;
+		dup = __find_duplicate(s);
+		if (!dup) {
+			DMERR("duplicate not found");
+			s->valid = 0;
+			goto ret;
+		}
+		down_write_nested(&dup->lock, SINGLE_DEPTH_NESTING);
+		handover_exceptions(dup, s);
+		up_write(&dup->lock);
+	}
 	s->active = 1;
+ ret:
 	up_write(&s->lock);
+	up_write(&_origins_lock);
 }
 
 static int snapshot_status(struct dm_target *ti, status_type_t type,