dm snapshot: trigger exceptions in remaining snapshots during merge

When there is one merging snapshot and other non-merging snapshots,
snapshot_merge_process() must make exceptions in the non-merging
snapshots.

Use a sequence count to resolve the race between I/O to chunks that are
about to be merged.  The count increases each time an exception
reallocation finishes.  Use wait_event() to wait until the count
changes.

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>

1 files changed, 83 insertions, 0 deletions

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index bc52776c69cc..1498704467a7 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -270,6 +270,10 @@ struct origin {
 static struct list_head *_origins;
 static struct rw_semaphore _origins_lock;
 
+static DECLARE_WAIT_QUEUE_HEAD(_pending_exceptions_done);
+static DEFINE_SPINLOCK(_pending_exceptions_done_spinlock);
+static uint64_t _pending_exceptions_done_count;
+
 static int init_origin_hash(void)
 {
         int i;
@@ -847,14 +851,38 @@ out:
         return r;
 }
 
+static int origin_write_extent(struct dm_snapshot *merging_snap,
+                               sector_t sector, unsigned chunk_size);
+
 static void merge_callback(int read_err, unsigned long write_err,
                            void *context);
 
+static uint64_t read_pending_exceptions_done_count(void)
+{
+        uint64_t pending_exceptions_done;
+
+        spin_lock(&_pending_exceptions_done_spinlock);
+        pending_exceptions_done = _pending_exceptions_done_count;
+        spin_unlock(&_pending_exceptions_done_spinlock);
+
+        return pending_exceptions_done;
+}
+
+static void increment_pending_exceptions_done_count(void)
+{
+        spin_lock(&_pending_exceptions_done_spinlock);
+        _pending_exceptions_done_count++;
+        spin_unlock(&_pending_exceptions_done_spinlock);
+
+        wake_up_all(&_pending_exceptions_done);
+}
+
 static void snapshot_merge_next_chunks(struct dm_snapshot *s)
 {
         int r;
         chunk_t old_chunk, new_chunk;
         struct dm_io_region src, dest;
+        uint64_t previous_count;
 
         BUG_ON(!test_bit(RUNNING_MERGE, &s->state_bits));
         if (unlikely(test_bit(SHUTDOWN_MERGE, &s->state_bits)))
@@ -887,6 +915,24 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
         src.sector = chunk_to_sector(s->store, new_chunk);
         src.count = dest.count;
 
+        /*
+         * Reallocate any exceptions needed in other snapshots then
+         * wait for the pending exceptions to complete.
+         * Each time any pending exception (globally on the system)
+         * completes we are woken and repeat the process to find out
+         * if we can proceed.  While this may not seem a particularly
+         * efficient algorithm, it is not expected to have any
+         * significant impact on performance.
+         */
+        previous_count = read_pending_exceptions_done_count();
+        while (origin_write_extent(s, dest.sector, s->store->chunk_size)) {
+                wait_event(_pending_exceptions_done,
+                           (read_pending_exceptions_done_count() !=
+                            previous_count));
+                /* Retry after the wait, until all exceptions are done. */
+                previous_count = read_pending_exceptions_done_count();
+        }
+
         down_write(&s->lock);
         s->first_merging_chunk = old_chunk;
         s->num_merging_chunks = 1;
@@ -1372,6 +1418,8 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
         origin_bios = bio_list_get(&pe->origin_bios);
         free_pending_exception(pe);
 
+        increment_pending_exceptions_done_count();
+
         up_write(&s->lock);
 
         /* Submit any pending write bios */
@@ -1963,6 +2011,41 @@ static int do_origin(struct dm_dev *origin, struct bio *bio)
 }
 
 /*
+ * Trigger exceptions in all non-merging snapshots.
+ *
+ * The chunk size of the merging snapshot may be larger than the chunk
+ * size of some other snapshot so we may need to reallocate multiple
+ * chunks in other snapshots.
+ *
+ * We scan all the overlapping exceptions in the other snapshots.
+ * Returns 1 if anything was reallocated and must be waited for,
+ * otherwise returns 0.
+ *
+ * size must be a multiple of merging_snap's chunk_size.
+ */
+static int origin_write_extent(struct dm_snapshot *merging_snap,
+                               sector_t sector, unsigned size)
+{
+        int must_wait = 0;
+        sector_t n;
+        struct origin *o;
+
+        /*
+         * The origin's __minimum_chunk_size() got stored in split_io
+         * by snapshot_merge_resume().
+         */
+        down_read(&_origins_lock);
+        o = __lookup_origin(merging_snap->origin->bdev);
+        for (n = 0; n < size; n += merging_snap->ti->split_io)
+                if (__origin_write(&o->snapshots, sector + n, NULL) ==
+                    DM_MAPIO_SUBMITTED)
+                        must_wait = 1;
+        up_read(&_origins_lock);
+
+        return must_wait;
+}
+
+/*
  * Origin: maps a linear range of a device, with hooks for snapshotting.
  */