bcachefs: Update btree ptrs after every write

This closes a significant hole (and last known hole) in our ability to
verify metadata. Previously, since btree nodes are log structured, we
couldn't detect lost btree writes that weren't the first write to a
given node. Additionally, this seems to have lead to some significant
metadata corruption on multi device filesystems with metadata
replication: since a write may have made it to one device and not
another, if we read that btree node back from the replica that did have
that write and started appending after that point, the other replica
would have a gap in the bset entries and reading from that replica
wouldn't find the rest of the bsets.

But, since updates to interior btree nodes are now journalled, we can
close this hole by updating pointers to btree nodes after every write
with the currently written number of sectors, without negatively
affecting performance. This means we will always detect lost or corrupt
metadata - it also means that our btree is now a curious hybrid of COW
and non COW btrees, with all the benefits of both (excluding
complexity).

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

1 files changed, 9 insertions, 2 deletions

diff --git a/fs/bcachefs/btree_io.h b/fs/bcachefs/btree_io.h
index 3732d135de8d..7fdcf879c7d4 100644
--- a/fs/bcachefs/btree_io.h
+++ b/fs/bcachefs/btree_io.h
@@ -32,6 +32,13 @@ static inline void clear_btree_node_dirty(struct bch_fs *c, struct btree *b)
 		atomic_dec(&c->btree_cache.dirty);
 }
 
+static inline unsigned btree_ptr_sectors_written(struct bkey_i *k)
+{
+	return k->k.type == KEY_TYPE_btree_ptr_v2
+		? le16_to_cpu(bkey_i_to_btree_ptr_v2(k)->v.sectors_written)
+		: 0;
+}
+
 struct btree_read_bio {
 	struct bch_fs		*c;
 	struct btree		*b;
@@ -48,7 +55,8 @@ struct btree_write_bio {
 	struct work_struct	work;
 	__BKEY_PADDED(key, BKEY_BTREE_PTR_VAL_U64s_MAX);
 	void			*data;
-	unsigned		bytes;
+	unsigned		data_bytes;
+	unsigned		sector_offset;
 	struct bch_write_bio	wbio;
 };
 
@@ -137,7 +145,6 @@ int bch2_btree_root_read(struct bch_fs *, enum btree_id,
 
 void bch2_btree_complete_write(struct bch_fs *, struct btree *,
 			      struct btree_write *);
-void bch2_btree_write_error_work(struct work_struct *);
 
 void __bch2_btree_node_write(struct bch_fs *, struct btree *, bool);
 bool bch2_btree_post_write_cleanup(struct bch_fs *, struct btree *);