dm snapshot: combine consecutive exceptions in memory

Provided sector_t is 64 bits, reduce the in-memory footprint of the
snapshot exception table by the simple method of using unused bits of
the chunk number to combine consecutive entries.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

2 files changed, 116 insertions, 16 deletions

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index fad84654b045..ae24eab8cd81 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -213,11 +213,15 @@ static void unregister_snapshot(struct dm_snapshot *s)
 
 /*
  * Implementation of the exception hash tables.
+ * The lowest hash_shift bits of the chunk number are ignored, allowing
+ * some consecutive chunks to be grouped together.
  */
-static int init_exception_table(struct exception_table *et, uint32_t size)
+static int init_exception_table(struct exception_table *et, uint32_t size,
+                                unsigned hash_shift)
 {
         unsigned int i;
 
+        et->hash_shift = hash_shift;
         et->hash_mask = size - 1;
         et->table = dm_vcalloc(size, sizeof(struct list_head));
         if (!et->table)
@@ -248,7 +252,7 @@ static void exit_exception_table(struct exception_table *et, struct kmem_cache *
 
 static uint32_t exception_hash(struct exception_table *et, chunk_t chunk)
 {
-        return chunk & et->hash_mask;
+        return (chunk >> et->hash_shift) & et->hash_mask;
 }
 
 static void insert_exception(struct exception_table *eh,
@@ -275,7 +279,8 @@ static struct dm_snap_exception *lookup_exception(struct exception_table *et,
 
         slot = &et->table[exception_hash(et, chunk)];
         list_for_each_entry (e, slot, hash_list)
-                if (e->old_chunk == chunk)
+                if (chunk >= e->old_chunk &&
+                    chunk <= e->old_chunk + dm_consecutive_chunk_count(e))
                         return e;
 
         return NULL;
@@ -307,6 +312,49 @@ static void free_pending_exception(struct dm_snap_pending_exception *pe)
         mempool_free(pe, pending_pool);
 }
 
+static void insert_completed_exception(struct dm_snapshot *s,
+                                       struct dm_snap_exception *new_e)
+{
+        struct exception_table *eh = &s->complete;
+        struct list_head *l;
+        struct dm_snap_exception *e = NULL;
+
+        l = &eh->table[exception_hash(eh, new_e->old_chunk)];
+
+        /* Add immediately if this table doesn't support consecutive chunks */
+        if (!eh->hash_shift)
+                goto out;
+
+        /* List is ordered by old_chunk */
+        list_for_each_entry_reverse(e, l, hash_list) {
+                /* Insert after an existing chunk? */
+                if (new_e->old_chunk == (e->old_chunk +
+                                         dm_consecutive_chunk_count(e) + 1) &&
+                    new_e->new_chunk == (dm_chunk_number(e->new_chunk) +
+                                         dm_consecutive_chunk_count(e) + 1)) {
+                        dm_consecutive_chunk_count_inc(e);
+                        free_exception(new_e);
+                        return;
+                }
+
+                /* Insert before an existing chunk? */
+                if (new_e->old_chunk == (e->old_chunk - 1) &&
+                    new_e->new_chunk == (dm_chunk_number(e->new_chunk) - 1)) {
+                        dm_consecutive_chunk_count_inc(e);
+                        e->old_chunk--;
+                        e->new_chunk--;
+                        free_exception(new_e);
+                        return;
+                }
+
+                if (new_e->old_chunk > e->old_chunk)
+                        break;
+        }
+
+out:
+        list_add(&new_e->hash_list, e ? &e->hash_list : l);
+}
+
 int dm_add_exception(struct dm_snapshot *s, chunk_t old, chunk_t new)
 {
         struct dm_snap_exception *e;
@@ -316,8 +364,12 @@ int dm_add_exception(struct dm_snapshot *s, chunk_t old, chunk_t new)
                 return -ENOMEM;
 
         e->old_chunk = old;
+
+        /* Consecutive_count is implicitly initialised to zero */
         e->new_chunk = new;
-        insert_exception(&s->complete, e);
+
+        insert_completed_exception(s, e);
+
         return 0;
 }
 
@@ -352,7 +404,8 @@ static int init_hash_tables(struct dm_snapshot *s)
         hash_size = min(hash_size, max_buckets);
 
         hash_size = rounddown_pow_of_two(hash_size);
-        if (init_exception_table(&s->complete, hash_size))
+        if (init_exception_table(&s->complete, hash_size,
+                                 DM_CHUNK_CONSECUTIVE_BITS))
                 return -ENOMEM;
 
         /*
@@ -363,7 +416,7 @@ static int init_hash_tables(struct dm_snapshot *s)
         if (hash_size < 64)
                 hash_size = 64;
 
-        if (init_exception_table(&s->pending, hash_size)) {
+        if (init_exception_table(&s->pending, hash_size, 0)) {
                 exit_exception_table(&s->complete, exception_cache);
                 return -ENOMEM;
         }
@@ -722,7 +775,7 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
          * Add a proper exception, and remove the
          * in-flight exception from the list.
          */
-        insert_exception(&s->complete, e);
+        insert_completed_exception(s, e);
 
  out:
         remove_exception(&pe->e);
@@ -856,11 +909,12 @@ __find_pending_exception(struct dm_snapshot *s, struct bio *bio)
 }
 
 static void remap_exception(struct dm_snapshot *s, struct dm_snap_exception *e,
-                            struct bio *bio)
+                            struct bio *bio, chunk_t chunk)
 {
         bio->bi_bdev = s->cow->bdev;
-        bio->bi_sector = chunk_to_sector(s, e->new_chunk) +
-                (bio->bi_sector & s->chunk_mask);
+        bio->bi_sector = chunk_to_sector(s, dm_chunk_number(e->new_chunk) +
+                         (chunk - e->old_chunk)) +
+                         (bio->bi_sector & s->chunk_mask);
 }
 
 static int snapshot_map(struct dm_target *ti, struct bio *bio,
@@ -891,7 +945,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
         /* If the block is already remapped - use that, else remap it */
         e = lookup_exception(&s->complete, chunk);
         if (e) {
-                remap_exception(s, e, bio);
+                remap_exception(s, e, bio, chunk);
                 goto out_unlock;
         }
 
@@ -908,7 +962,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
                         goto out_unlock;
                 }
 
-                remap_exception(s, &pe->e, bio);
+                remap_exception(s, &pe->e, bio, chunk);
                 bio_list_add(&pe->snapshot_bios, bio);
 
                 r = DM_MAPIO_SUBMITTED;
@@ -1196,7 +1250,7 @@ static int origin_status(struct dm_target *ti, status_type_t type, char *result,
 
 static struct target_type origin_target = {
         .name    = "snapshot-origin",
-        .version = {1, 5, 0},
+        .version = {1, 6, 0},
         .module  = THIS_MODULE,
         .ctr     = origin_ctr,
         .dtr     = origin_dtr,
@@ -1207,7 +1261,7 @@ static struct target_type origin_target = {
 
 static struct target_type snapshot_target = {
         .name    = "snapshot",
-        .version = {1, 5, 0},
+        .version = {1, 6, 0},
         .module  = THIS_MODULE,
         .ctr     = snapshot_ctr,
         .dtr     = snapshot_dtr,
diff --git a/drivers/md/dm-snap.h b/drivers/md/dm-snap.h
index 650e0f1f51d8..93bce5d49742 100644
--- a/drivers/md/dm-snap.h
+++ b/drivers/md/dm-snap.h
@@ -16,19 +16,22 @@
 
 struct exception_table {
         uint32_t hash_mask;
+        unsigned hash_shift;
         struct list_head *table;
 };
 
 /*
  * The snapshot code deals with largish chunks of the disk at a
- * time. Typically 64k - 256k.
+ * time. Typically 32k - 512k.
  */
-/* FIXME: can we get away with limiting these to a uint32_t ? */
 typedef sector_t chunk_t;
 
 /*
  * An exception is used where an old chunk of data has been
  * replaced by a new one.
+ * If chunk_t is 64 bits in size, the top 8 bits of new_chunk hold the number
+ * of chunks that follow contiguously.  Remaining bits hold the number of the
+ * chunk within the device.
  */
 struct dm_snap_exception {
         struct list_head hash_list;
@@ -38,6 +41,49 @@ struct dm_snap_exception {
 };
 
 /*
+ * Funtions to manipulate consecutive chunks
+ */
+#  if defined(CONFIG_LBD) || (BITS_PER_LONG == 64)
+#    define DM_CHUNK_CONSECUTIVE_BITS 8
+#    define DM_CHUNK_NUMBER_BITS 56
+
+static inline chunk_t dm_chunk_number(chunk_t chunk)
+{
+        return chunk & (chunk_t)((1ULL << DM_CHUNK_NUMBER_BITS) - 1ULL);
+}
+
+static inline unsigned dm_consecutive_chunk_count(struct dm_snap_exception *e)
+{
+        return e->new_chunk >> DM_CHUNK_NUMBER_BITS;
+}
+
+static inline void dm_consecutive_chunk_count_inc(struct dm_snap_exception *e)
+{
+        e->new_chunk += (1ULL << DM_CHUNK_NUMBER_BITS);
+
+        BUG_ON(!dm_consecutive_chunk_count(e));
+}
+
+#  else
+#    define DM_CHUNK_CONSECUTIVE_BITS 0
+
+static inline chunk_t dm_chunk_number(chunk_t chunk)
+{
+        return chunk;
+}
+
+static inline unsigned dm_consecutive_chunk_count(struct dm_snap_exception *e)
+{
+        return 0;
+}
+
+static inline void dm_consecutive_chunk_count_inc(struct dm_snap_exception *e)
+{
+}
+
+#  endif
+
+/*
  * Abstraction to handle the meta/layout of exception stores (the
  * COW device).
  */