dm exception store: add merge specific methods

Add functions that decide how many consecutive chunks of snapshot to
merge back into the origin next and to update the metadata afterwards.

prepare_merge provides a pointer to the most recent still-to-be-merged
chunk and returns how many previous ones are consecutive and can be
processed together.

commit_merge removes the nr_merged most-recent chunks permanently from
the exception store.  The number must not exceed that returned by
prepare_merge.

Introduce NUM_SNAPSHOT_HDR_CHUNKS to show where the snapshot header
chunk is accounted for.

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

2 files changed, 129 insertions, 2 deletions

diff --git a/drivers/md/dm-exception-store.h b/drivers/md/dm-exception-store.h
index bb8874653de..c53e08935b4 100644
--- a/drivers/md/dm-exception-store.h
+++ b/drivers/md/dm-exception-store.h
@@ -75,6 +75,23 @@ struct dm_exception_store_type {
                                   void *callback_context);
 
         /*
+         * Returns 0 if the exception store is empty.
+         *
+         * If there are exceptions still to be merged, sets
+         * *last_old_chunk and *last_new_chunk to the most recent
+         * still-to-be-merged chunk and returns the number of
+         * consecutive previous ones.
+         */
+        int (*prepare_merge) (struct dm_exception_store *store,
+                              chunk_t *last_old_chunk, chunk_t *last_new_chunk);
+
+        /*
+         * Clear the last n exceptions.
+         * nr_merged must be <= the value returned by prepare_merge.
+         */
+        int (*commit_merge) (struct dm_exception_store *store, int nr_merged);
+
+        /*
          * The snapshot is invalid, note this in the metadata.
          */
         void (*drop_snapshot) (struct dm_exception_store *store);
diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index 157999ebd23..7d08879689a 100644
--- a/drivers/md/dm-snap-persistent.c
+++ b/drivers/md/dm-snap-persistent.c
@@ -55,6 +55,8 @@
  */
 #define SNAPSHOT_DISK_VERSION 1
 
+#define NUM_SNAPSHOT_HDR_CHUNKS 1
+
 struct disk_header {
         uint32_t magic;
 
@@ -120,7 +122,22 @@ struct pstore {
 
         /*
          * The next free chunk for an exception.
+         *
+         * When creating exceptions, all the chunks here and above are
+         * free.  It holds the next chunk to be allocated.  On rare
+         * occasions (e.g. after a system crash) holes can be left in
+         * the exception store because chunks can be committed out of
+         * order.
+         *
+         * When merging exceptions, it does not necessarily mean all the
+         * chunks here and above are free.  It holds the value it would
+         * have held if all chunks had been committed in order of
+         * allocation.  Consequently the value may occasionally be
+         * slightly too low, but since it's only used for 'status' and
+         * it can never reach its minimum value too early this doesn't
+         * matter.
          */
+
         chunk_t next_free;
 
         /*
@@ -409,6 +426,15 @@ static void write_exception(struct pstore *ps,
         e->new_chunk = cpu_to_le64(de->new_chunk);
 }
 
+static void clear_exception(struct pstore *ps, uint32_t index)
+{
+        struct disk_exception *e = get_exception(ps, index);
+
+        /* clear it */
+        e->old_chunk = 0;
+        e->new_chunk = 0;
+}
+
 /*
  * Registers the exceptions that are present in the current area.
  * 'full' is filled in to indicate if the area has been
@@ -505,7 +531,8 @@ static void persistent_usage(struct dm_exception_store *store,
          * Then there are (ps->current_area + 1) metadata chunks, each one
          * separated from the next by ps->exceptions_per_area data chunks.
          */
-        *metadata_sectors = (ps->current_area + 2) * store->chunk_size;
+        *metadata_sectors = (ps->current_area + 1 + NUM_SNAPSHOT_HDR_CHUNKS) *
+                            store->chunk_size;
 }
 
 static void persistent_dtr(struct dm_exception_store *store)
@@ -680,6 +707,85 @@ static void persistent_commit_exception(struct dm_exception_store *store,
         ps->callback_count = 0;
 }
 
+static int persistent_prepare_merge(struct dm_exception_store *store,
+                                    chunk_t *last_old_chunk,
+                                    chunk_t *last_new_chunk)
+{
+        struct pstore *ps = get_info(store);
+        struct disk_exception de;
+        int nr_consecutive;
+        int r;
+
+        /*
+         * When current area is empty, move back to preceding area.
+         */
+        if (!ps->current_committed) {
+                /*
+                 * Have we finished?
+                 */
+                if (!ps->current_area)
+                        return 0;
+
+                ps->current_area--;
+                r = area_io(ps, READ);
+                if (r < 0)
+                        return r;
+                ps->current_committed = ps->exceptions_per_area;
+        }
+
+        read_exception(ps, ps->current_committed - 1, &de);
+        *last_old_chunk = de.old_chunk;
+        *last_new_chunk = de.new_chunk;
+
+        /*
+         * Find number of consecutive chunks within the current area,
+         * working backwards.
+         */
+        for (nr_consecutive = 1; nr_consecutive < ps->current_committed;
+             nr_consecutive++) {
+                read_exception(ps, ps->current_committed - 1 - nr_consecutive,
+                               &de);
+                if (de.old_chunk != *last_old_chunk - nr_consecutive ||
+                    de.new_chunk != *last_new_chunk - nr_consecutive)
+                        break;
+        }
+
+        return nr_consecutive;
+}
+
+static int persistent_commit_merge(struct dm_exception_store *store,
+                                   int nr_merged)
+{
+        int r, i;
+        struct pstore *ps = get_info(store);
+
+        BUG_ON(nr_merged > ps->current_committed);
+
+        for (i = 0; i < nr_merged; i++)
+                clear_exception(ps, ps->current_committed - 1 - i);
+
+        r = area_io(ps, WRITE);
+        if (r < 0)
+                return r;
+
+        ps->current_committed -= nr_merged;
+
+        /*
+         * At this stage, only persistent_usage() uses ps->next_free, so
+         * we make no attempt to keep ps->next_free strictly accurate
+         * as exceptions may have been committed out-of-order originally.
+         * Once a snapshot has become merging, we set it to the value it
+         * would have held had all the exceptions been committed in order.
+         *
+         * ps->current_area does not get reduced by prepare_merge() until
+         * after commit_merge() has removed the nr_merged previous exceptions.
+         */
+        ps->next_free = (area_location(ps, ps->current_area) - 1) +
+                        (ps->current_committed + 1) + NUM_SNAPSHOT_HDR_CHUNKS;
+
+        return 0;
+}
+
 static void persistent_drop_snapshot(struct dm_exception_store *store)
 {
         struct pstore *ps = get_info(store);
@@ -705,7 +811,7 @@ static int persistent_ctr(struct dm_exception_store *store,
         ps->area = NULL;
         ps->zero_area = NULL;
         ps->header_area = NULL;
-        ps->next_free = 2;      /* skipping the header and first area */
+        ps->next_free = NUM_SNAPSHOT_HDR_CHUNKS + 1; /* header and 1st area */
         ps->current_committed = 0;
 
         ps->callback_count = 0;
@@ -748,6 +854,8 @@ static struct dm_exception_store_type _persistent_type = {
         .read_metadata = persistent_read_metadata,
         .prepare_exception = persistent_prepare_exception,
         .commit_exception = persistent_commit_exception,
+        .prepare_merge = persistent_prepare_merge,
+        .commit_merge = persistent_commit_merge,
         .drop_snapshot = persistent_drop_snapshot,
         .usage = persistent_usage,
         .status = persistent_status,
@@ -761,6 +869,8 @@ static struct dm_exception_store_type _persistent_compat_type = {
         .read_metadata = persistent_read_metadata,
         .prepare_exception = persistent_prepare_exception,
         .commit_exception = persistent_commit_exception,
+        .prepare_merge = persistent_prepare_merge,
+        .commit_merge = persistent_commit_merge,
         .drop_snapshot = persistent_drop_snapshot,
         .usage = persistent_usage,
         .status = persistent_status,