bcachefs: Add a comment for BTREE_INSERT_NOJOURNAL usage

BTREE_INSERT_NOJOURNAL is primarily used for a performance optimization
related to inode updates and fsync - document it.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -257,7 +257,7 @@ static void bch2_btree_iter_verify(struct btree_iter *iter)
 
 	BUG_ON(!(iter->flags & __BTREE_ITER_ALL_SNAPSHOTS) &&
 	       (iter->flags & BTREE_ITER_ALL_SNAPSHOTS) &&
-	       !btree_type_has_snapshots(iter->btree_id));
+	       !btree_type_has_snapshot_field(iter->btree_id));
 
 	if (iter->update_path)
 		bch2_btree_path_verify(trans, iter->update_path);

fs/bcachefs/btree_iter.h

@@ -416,8 +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))
+	    !btree_type_has_snapshot_field(btree_id))
 		flags &= ~BTREE_ITER_ALL_SNAPSHOTS;
 
 	if (!(flags & BTREE_ITER_ALL_SNAPSHOTS) &&

fs/bcachefs/btree_types.h

@@ -713,7 +713,7 @@ static inline bool btree_type_has_snapshots(enum btree_id id)
 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)
+#define x(name, nr, flags, ...)	|((!!((flags) & (BTREE_ID_SNAPSHOT_FIELD|BTREE_ID_SNAPSHOTS))) << nr)
 	BCH_BTREE_IDS()
 #undef x
 	;

fs/bcachefs/io_write.c

@@ -202,6 +202,17 @@ static inline int bch2_extent_update_i_size_sectors(struct btree_trans *trans,
 	struct btree_iter iter;
 	struct bkey_i *k;
 	struct bkey_i_inode_v3 *inode;
+	/*
+	 * Crazy performance optimization:
+	 * Every extent update needs to also update the inode: the inode trigger
+	 * will set bi->journal_seq to the journal sequence number of this
+	 * transaction - for fsync.
+	 *
+	 * But if that's the only reason we're updating the inode (we're not
+	 * updating bi_size or bi_sectors), then we don't need the inode update
+	 * to be journalled - if we crash, the bi_journal_seq update will be
+	 * lost, but that's fine.
+	 */
 	unsigned inode_update_flags = BTREE_UPDATE_NOJOURNAL;
 	int ret;