dm bitset: only flush the current word if it has been dirtied

This change offers a big performance boost for dm-era.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/persistent-data/dm-bitset.c

@@ -65,7 +65,7 @@ int dm_bitset_flush(struct dm_disk_bitset *info, dm_block_t root,
 	int r;
 	__le64 value;
 
-	if (!info->current_index_set)
+	if (!info->current_index_set || !info->dirty)
 		return 0;
 
 	value = cpu_to_le64(info->current_bits);
@@ -77,6 +77,8 @@ int dm_bitset_flush(struct dm_disk_bitset *info, dm_block_t root,
 		return r;
 
 	info->current_index_set = false;
+	info->dirty = false;
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(dm_bitset_flush);
@@ -94,6 +96,8 @@ static int read_bits(struct dm_disk_bitset *info, dm_block_t root,
 	info->current_bits = le64_to_cpu(value);
 	info->current_index_set = true;
 	info->current_index = array_index;
+	info->dirty = false;
+
 	return 0;
 }
 
@@ -126,6 +130,8 @@ int dm_bitset_set_bit(struct dm_disk_bitset *info, dm_block_t root,
 		return r;
 
 	set_bit(b, (unsigned long *) &info->current_bits);
+	info->dirty = true;
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(dm_bitset_set_bit);
@@ -141,6 +147,8 @@ int dm_bitset_clear_bit(struct dm_disk_bitset *info, dm_block_t root,
 		return r;
 
 	clear_bit(b, (unsigned long *) &info->current_bits);
+	info->dirty = true;
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(dm_bitset_clear_bit);

drivers/md/persistent-data/dm-bitset.h

@@ -71,6 +71,7 @@ struct dm_disk_bitset {
 	uint64_t current_bits;
 
 	bool current_index_set:1;
+	bool dirty:1;
 };
 
 /*