Simplify exception handover so that the duplicate snapshot is found in
one location (snapshot_ctr) and recorded in the associated (old and new)
snapshots that will be participating in the handover.
Confines handover to be from a specific snapshot to another specific
snapshot without additional __find_duplicate() calls.
Documents the exception handover state diagram with relevant comments
in the associated code.
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Cc: Alasdair G Kergon <agk@xxxxxxxxxx>
Cc: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Cc: Jonathan Brassow <jbrassow@xxxxxxxxxx>
---
drivers/md/dm-snap.c | 103 ++++++++++++++++++++++++++++++++++-----------------
1 file changed, 70 insertions(+), 33 deletions(-)
Index: linux-2.6-dev/drivers/md/dm-snap.c
===================================================================
--- linux-2.6-dev.orig/drivers/md/dm-snap.c
+++ linux-2.6-dev/drivers/md/dm-snap.c
@@ -75,9 +75,26 @@ struct dm_snapshot {
/* Set if the parent device is suspended */
int suspended;
- /* Exception store will be handed over from another snapshot */
+ /*
+ * 'handover' denotes exception store will be handed over from
+ * another snapshot with the same cow block device (as identified
+ * with __find_duplicate).
+ *
+ * 'handover' is set during a new snapshot's constructor if it finds
+ * a single old snapshot is using the same cow block device as it.
+ * Handover operation is performed, and 'handover' is cleared,
+ * when either of the following occurs:
+ * - old snapshot, that is handing over, is destructed
+ * - new snapshot, that is accepting the handover, is resumed
+ */
int handover;
+ /*
+ * reference to the other snapshot that will participate in the
+ * exception store handover; new references old, old references new
+ */
+ struct dm_snapshot *handover_snap;
+
mempool_t *pending_pool;
atomic_t pending_exceptions_count;
@@ -510,7 +527,8 @@ static int dm_add_exception(void *contex
}
/* _origins_lock must be held */
-static struct dm_snapshot *__find_duplicate(struct dm_snapshot *snap)
+static struct dm_snapshot *__find_duplicate(struct dm_snapshot *snap,
+ int *duplicate_count)
{
struct dm_snapshot *dup;
struct dm_snapshot *l;
@@ -521,16 +539,13 @@ static struct dm_snapshot *__find_duplic
return NULL;
dup = NULL;
+ *duplicate_count = 0;
list_for_each_entry(l, &o->snapshots, list)
if (l != snap && bdev_equal(l->cow->bdev,
snap->cow->bdev)) {
- if (!dup) {
+ if (!dup)
dup = l;
- } else {
- DMERR("Multiple active duplicates, "
- "user misuses dmsetup.");
- return NULL;
- }
+ (*duplicate_count)++;
}
return dup;
@@ -611,8 +626,8 @@ static int init_hash_tables(struct dm_sn
*/
static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
- struct dm_snapshot *s;
- int i;
+ struct dm_snapshot *s, *dup;
+ int i, duplicate_count = 0;
int r = -EINVAL;
char *origin_path, *cow_path;
unsigned args_used;
@@ -668,6 +683,7 @@ static int snapshot_ctr(struct dm_target
s->suspended = 0;
atomic_set(&s->pending_exceptions_count, 0);
s->handover = 0;
+ s->handover_snap = NULL;
init_rwsem(&s->lock);
spin_lock_init(&s->pe_lock);
@@ -703,10 +719,26 @@ static int snapshot_ctr(struct dm_target
spin_lock_init(&s->tracked_chunk_lock);
+ /* Snapshot may need to have exceptions handed over to it */
+ r = -EINVAL;
down_read(&_origins_lock);
- if (__find_duplicate(s))
- s->handover = 1;
+ dup = __find_duplicate(s, &duplicate_count);
up_read(&_origins_lock);
+ if (dup) {
+ if (duplicate_count > 1) {
+ ti->error = "More than one other snapshot has been "
+ "constructed with the same cow device.";
+ goto bad_load_and_register;
+ }
+ /* cross reference snapshots that will do handover */
+ down_write(&dup->lock);
+ dup->handover_snap = s;
+ up_write(&dup->lock);
+ s->handover_snap = dup;
+
+ /* this new snapshot will accept the handover */
+ s->handover = 1;
+ }
/* Metadata must only be loaded into one table at once */
r = s->store->type->read_metadata(s->store, dm_add_exception,
@@ -787,6 +819,11 @@ static void handover_exceptions(struct d
struct dm_exception_store *store_swap;
} u;
+ BUG_ON((old->handover_snap != new) ||
+ (new->handover_snap != old));
+ BUG_ON((old->handover != 0) || (new->handover != 1));
+ BUG_ON(!old->suspended);
+
u.table_swap = new->complete;
new->complete = old->complete;
old->complete = u.table_swap;
@@ -797,7 +834,14 @@ static void handover_exceptions(struct d
new->store->snap = new;
old->store->snap = old;
+ /* 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->handover = 0;
}
@@ -807,20 +851,18 @@ static void snapshot_dtr(struct dm_targe
int i;
#endif
struct dm_snapshot *s = ti->private;
- struct dm_snapshot *dup;
flush_workqueue(ksnapd);
- down_write(&_origins_lock);
+ /* This snapshot may need to handover its exception store */
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);
+ if (s->handover_snap) {
+ struct dm_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);
}
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. */
@@ -1225,25 +1267,20 @@ 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);
+ /* Get exception store from another snapshot */
+ struct dm_snapshot *old_snap = s->handover_snap;
+ BUG_ON(!old_snap);
+ down_write_nested(&old_snap->lock, SINGLE_DEPTH_NESTING);
+ handover_exceptions(old_snap, s);
+ up_write(&old_snap->lock);
}
+ /* An incomplete exception handover is not allowed */
+ BUG_ON(s->handover || s->handover_snap);
s->active = 1;
s->suspended = 0;
- ret:
up_write(&s->lock);
- up_write(&_origins_lock);
}
static int snapshot_status(struct dm_target *ti, status_type_t type,
--
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