dm persistent data: stop using dm_bm_unlock_move when shadowing blocks in tm

Stop using dm_bm_unlock_move when shadowing blocks in the transaction
manager as an optimisation and remove the function as it is then no
longer used.

Some code, such as the space maps, keeps using on-disk data structures
from the previous transaction.  It can do this because blocks won't
be reallocated until the subsequent transaction.  Using
dm_bm_unlock_move to copy blocks sounds like a win, but it forces a
synchronous read should the old block be accessed.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/drivers/md/persistent-data/dm-transaction-manager.c b/drivers/md/persistent-data/dm-transaction-manager.c
index b4f05830af0..d247a35da3c 100644
--- a/drivers/md/persistent-data/dm-transaction-manager.c
+++ b/drivers/md/persistent-data/dm-transaction-manager.c
@@ -219,13 +219,24 @@ static int __shadow_block(struct dm_transaction_manager *tm, dm_block_t orig,
         if (r < 0)
                 return r;
 
-        r = dm_bm_unlock_move(orig_block, new);
-        if (r < 0) {
+        /*
+         * It would be tempting to use dm_bm_unlock_move here, but some
+         * code, such as the space maps, keeps using the old data structures
+         * secure in the knowledge they won't be changed until the next
+         * transaction.  Using unlock_move would force a synchronous read
+         * since the old block would no longer be in the cache.
+         */
+        r = dm_bm_write_lock_zero(tm->bm, new, v, result);
+        if (r) {
                 dm_bm_unlock(orig_block);
                 return r;
         }
 
-        return dm_bm_write_lock(tm->bm, new, v, result);
+        memcpy(dm_block_data(*result), dm_block_data(orig_block),
+               dm_bm_block_size(tm->bm));
+
+        dm_bm_unlock(orig_block);
+        return r;
 }
 
 int dm_tm_shadow_block(struct dm_transaction_manager *tm, dm_block_t orig,