bcachefs: Journal updates to interior nodes

Previously, the btree has always been self contained and internally
consistent on disk without anything from the journal - the journal just
contained pointers to the btree roots.

However, this meant that btree node split or compact operations - i.e.
anything that changes btree node topology and involves updates to
interior nodes - would require that interior btree node to be written
immediately, which means emitting a btree node write that's mostly empty
(using 4k of space on disk if the filesystemm blocksize is 4k to only
write perhaps ~100 bytes of new keys).

More importantly, this meant most btree node writes had to be FUA, and
consumer drives have a history of slow and/or buggy FUA support - other
filesystes have been bit by this.

This patch changes the interior btree update path to journal updates to
interior nodes, after the writes for the new btree nodes have completed.
Best of all, it turns out to simplify the interior node update path
somewhat.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 5 insertions, 4 deletions

diff --git a/fs/bcachefs/btree_io.h b/fs/bcachefs/btree_io.h
index 43fa8a6dbee5..a02e261c2eb2 100644
--- a/fs/bcachefs/btree_io.h
+++ b/fs/bcachefs/btree_io.h
@@ -102,19 +102,20 @@ bool bch2_btree_post_write_cleanup(struct bch_fs *, struct btree *);
 void bch2_btree_node_write(struct bch_fs *, struct btree *,
 			  enum six_lock_type);
 
-static inline void btree_node_write_if_need(struct bch_fs *c, struct btree *b)
+static inline void btree_node_write_if_need(struct bch_fs *c, struct btree *b,
+					    enum six_lock_type lock_held)
 {
 	while (b->written &&
 	       btree_node_need_write(b) &&
 	       btree_node_may_write(b)) {
 		if (!btree_node_write_in_flight(b)) {
-			bch2_btree_node_write(c, b, SIX_LOCK_read);
+			bch2_btree_node_write(c, b, lock_held);
 			break;
 		}
 
 		six_unlock_read(&b->c.lock);
 		btree_node_wait_on_io(b);
-		btree_node_lock_type(c, b, SIX_LOCK_read);
+		btree_node_lock_type(c, b, lock_held);
 	}
 }
 
@@ -131,7 +132,7 @@ do {									\
 		new |= (1 << BTREE_NODE_need_write);			\
 	} while ((v = cmpxchg(&(_b)->flags, old, new)) != old);		\
 									\
-	btree_node_write_if_need(_c, _b);				\
+	btree_node_write_if_need(_c, _b, SIX_LOCK_read);		\
 } while (0)
 
 void bch2_btree_flush_all_reads(struct bch_fs *);