bcachefs: rebalance_work btree is not a snapshots btree

rebalance_work entries may refer to entries in the extents btree, which
is a snapshots btree, or they may also refer to entries in the reflink
btree, which is not.

Hence rebalance_work keys may use the snapshot field but it's not
required to be nonzero - add a new btree flag to reflect this.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs_format.h

@@ -2260,7 +2260,8 @@ LE32_BITMASK(JSET_NO_FLUSH,	struct jset, flags, 5, 6);
 enum btree_id_flags {
 	BTREE_ID_EXTENTS	= BIT(0),
 	BTREE_ID_SNAPSHOTS	= BIT(1),
-	BTREE_ID_DATA		= BIT(2),
+	BTREE_ID_SNAPSHOT_FIELD	= BIT(2),
+	BTREE_ID_DATA		= BIT(3),
 };
 
 #define BCH_BTREE_IDS()								\
@@ -2315,12 +2316,12 @@ enum btree_id_flags {
 	  BIT_ULL(KEY_TYPE_bucket_gens))					\
 	x(snapshot_trees,	15,	0,					\
 	  BIT_ULL(KEY_TYPE_snapshot_tree))					\
-	x(deleted_inodes,	16,	BTREE_ID_SNAPSHOTS,			\
+	x(deleted_inodes,	16,	BTREE_ID_SNAPSHOT_FIELD,		\
 	  BIT_ULL(KEY_TYPE_set))						\
 	x(logged_ops,		17,	0,					\
 	  BIT_ULL(KEY_TYPE_logged_op_truncate)|					\
 	  BIT_ULL(KEY_TYPE_logged_op_finsert))					\
-	x(rebalance_work,	18,	BTREE_ID_SNAPSHOTS,			\
+	x(rebalance_work,	18,	BTREE_ID_SNAPSHOT_FIELD,		\
 	  BIT_ULL(KEY_TYPE_set)|BIT_ULL(KEY_TYPE_cookie))
 
 enum btree_id {

fs/bcachefs/bkey_methods.c

@@ -186,15 +186,20 @@ int __bch2_bkey_invalid(struct bch_fs *c, struct bkey_s_c k,
 	if (type != BKEY_TYPE_btree) {
 		enum btree_id btree = type - 1;
 
-		bkey_fsck_err_on(!btree_type_has_snapshots(btree) &&
-				 k.k->p.snapshot, c, err,
-				 bkey_snapshot_nonzero,
-				 "nonzero snapshot");
-
-		bkey_fsck_err_on(btree_type_has_snapshots(btree) &&
-				 !k.k->p.snapshot, c, err,
-				 bkey_snapshot_zero,
-				 "snapshot == 0");
+		if (btree_type_has_snapshots(btree)) {
+			bkey_fsck_err_on(!k.k->p.snapshot, c, err,
+					 bkey_snapshot_zero,
+					 "snapshot == 0");
+		} else if (!btree_type_has_snapshot_field(btree)) {
+			bkey_fsck_err_on(k.k->p.snapshot, c, err,
+					 bkey_snapshot_nonzero,
+					 "nonzero snapshot");
+		} else {
+			/*
+			 * btree uses snapshot field but it's not required to be
+			 * nonzero
+			 */
+		}
 
 		bkey_fsck_err_on(bkey_eq(k.k->p, POS_MAX), c, err,
 				 bkey_at_pos_max,

fs/bcachefs/btree_iter.h

@@ -416,6 +416,7 @@ static inline unsigned __bch2_btree_iter_flags(struct btree_trans *trans,
 		flags |= BTREE_ITER_IS_EXTENTS;
 
 	if (!(flags & __BTREE_ITER_ALL_SNAPSHOTS) &&
+	    !btree_type_has_snapshot_field(btree_id) &&
 	    !btree_type_has_snapshots(btree_id))
 		flags &= ~BTREE_ITER_ALL_SNAPSHOTS;
 

fs/bcachefs/btree_trans_commit.c

@@ -269,6 +269,7 @@ static inline void btree_insert_entry_checks(struct btree_trans *trans,
 	BUG_ON(i->level		!= i->path->level);
 	BUG_ON(i->btree_id	!= i->path->btree_id);
 	EBUG_ON(!i->level &&
+		btree_type_has_snapshots(i->btree_id) &&
 		!(i->flags & BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE) &&
 		test_bit(JOURNAL_REPLAY_DONE, &trans->c->journal.flags) &&
 		i->k->k.p.snapshot &&

fs/bcachefs/btree_types.h

@@ -710,6 +710,17 @@ static inline bool btree_type_has_snapshots(enum btree_id id)
 	return (1U << id) & mask;
 }
 
+static inline bool btree_type_has_snapshot_field(enum btree_id id)
+{
+	const unsigned mask = 0
+#define x(name, nr, flags, ...)	|((!!((flags) & BTREE_ID_SNAPSHOT_FIELD)) << nr)
+	BCH_BTREE_IDS()
+#undef x
+	;
+
+	return (1U << id) & mask;
+}
+
 static inline bool btree_type_has_ptrs(enum btree_id id)
 {
 	const unsigned mask = 0