btrfs: reduce size of struct tree_mod_elem

Several members are used for specific types of tree mod log operations so
they can be placed in a union in order to reduce the structure's size.

This reduces the structure size from 112 bytes to 88 bytes on x86_64,
so we can now use the kmalloc-96 slab instead of the kmalloc-128 slab.

Reviewed-by: Boris Burkov <[e-mail]>
Signed-off-by: Filipe Manana <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/tree-mod-log.c

···
 	/* This is used for BTRFS_MOD_LOG_KEY* and BTRFS_MOD_LOG_ROOT_REPLACE. */
 	u64 generation;
 
-	/* Those are used for op == BTRFS_MOD_LOG_KEY_{REPLACE,REMOVE}. */
-	struct btrfs_disk_key key;
-	u64 blockptr;
+	union {
+		/*
+		 * This is used for the following op types:
+		 *
+		 *    BTRFS_MOD_LOG_KEY_REMOVE_WHILE_FREEING
+		 *    BTRFS_MOD_LOG_KEY_REMOVE_WHILE_MOVING
+		 *    BTRFS_MOD_LOG_KEY_REMOVE
+		 *    BTRFS_MOD_LOG_KEY_REPLACE
+		 */
+		struct {
+			struct btrfs_disk_key key;
+			u64 blockptr;
+		} slot_change;
 
-	/* This is used for op == BTRFS_MOD_LOG_MOVE_KEYS. */
-	struct {
-		int dst_slot;
-		int nr_items;
-	} move;
+		/* This is used for op == BTRFS_MOD_LOG_MOVE_KEYS. */
+		struct {
+			int dst_slot;
+			int nr_items;
+		} move;
 
-	/* This is used for op == BTRFS_MOD_LOG_ROOT_REPLACE. */
-	struct tree_mod_root old_root;
+		/* This is used for op == BTRFS_MOD_LOG_ROOT_REPLACE. */
+		struct tree_mod_root old_root;
+	};
 };
 
 /*
···
 {
 	struct tree_mod_elem *tm;
 
+	/* Can't be one of these types, due to union in struct tree_mod_elem. */
+	ASSERT(op != BTRFS_MOD_LOG_MOVE_KEYS);
+	ASSERT(op != BTRFS_MOD_LOG_ROOT_REPLACE);
+
 	tm = kzalloc(sizeof(*tm), GFP_NOFS);
 	if (!tm)
 		return NULL;
 
 	tm->logical = eb->start;
-	if (op != BTRFS_MOD_LOG_KEY_ADD) {
-		btrfs_node_key(eb, &tm->key, slot);
-		tm->blockptr = btrfs_node_blockptr(eb, slot);
-	}
+	btrfs_node_key(eb, &tm->slot_change.key, slot);
+	tm->slot_change.blockptr = btrfs_node_blockptr(eb, slot);
 	tm->op = op;
 	tm->slot = slot;
 	tm->generation = btrfs_node_ptr_generation(eb, slot);
···
 			fallthrough;
 		case BTRFS_MOD_LOG_KEY_REMOVE_WHILE_MOVING:
 		case BTRFS_MOD_LOG_KEY_REMOVE:
-			btrfs_set_node_key(eb, &tm->key, tm->slot);
-			btrfs_set_node_blockptr(eb, tm->slot, tm->blockptr);
+			btrfs_set_node_key(eb, &tm->slot_change.key, tm->slot);
+			btrfs_set_node_blockptr(eb, tm->slot, tm->slot_change.blockptr);
 			btrfs_set_node_ptr_generation(eb, tm->slot,
 						      tm->generation);
 			n++;
···
 			break;
 		case BTRFS_MOD_LOG_KEY_REPLACE:
 			BUG_ON(tm->slot >= n);
-			btrfs_set_node_key(eb, &tm->key, tm->slot);
-			btrfs_set_node_blockptr(eb, tm->slot, tm->blockptr);
+			btrfs_set_node_key(eb, &tm->slot_change.key, tm->slot);
+			btrfs_set_node_blockptr(eb, tm->slot, tm->slot_change.blockptr);
 			btrfs_set_node_ptr_generation(eb, tm->slot,
 						      tm->generation);
 			break;