bcachefs: bch2_propagate_key_to_snapshot_leaves()

If fsck finds a key that needs work done, the primary example being an
unlinked inode that needs to be deleted, and the key is in an internal
snapshot node, we have a bit of a conundrum.

The conundrum is that internal snapshot nodes are shared, and we in
general do updates in internal snapshot nodes because there may be
overwrites in some snapshots and not others, and this may affect other
keys referenced by this key (i.e. extents).

For example, we might be seeing an unlinked inode in an internal
snapshot node, but then in one child snapshot the inode might have been
reattached and might not be unlinked. Deleting the inode in the internal
snapshot node would be wrong, because then we'll delete all the extents
that the child snapshot references.

But if an unlinked inode does not have any overwrites in child
snapshots, we're fine: the inode is overwrritten in all child snapshots,
so we can do the deletion at the point of comonality in the snapshot
tree, i.e. the node where we found it.

This patch adds a new helper, bch2_propagate_key_to_snapshot_leaves(),
to handle the case where we need a to update a key that does have
overwrites in child snapshots: we copy the key to leaf snapshot nodes,
and then rewind fsck and process the needed updates there.

With this, fsck can now always correctly handle unlinked inodes found in
internal snapshot nodes.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/bcachefs/snapshot.h b/fs/bcachefs/snapshot.h
index 624a42d1c8b7..dabc9b9d921b 100644
--- a/fs/bcachefs/snapshot.h
+++ b/fs/bcachefs/snapshot.h
@@ -263,6 +263,9 @@ static inline int bch2_key_has_snapshot_overwrites(struct btree_trans *trans,
 	return __bch2_key_has_snapshot_overwrites(trans, id, pos);
 }
 
+int bch2_propagate_key_to_snapshot_leaves(struct btree_trans *, enum btree_id,
+					  struct bkey_s_c, struct bpos *);
+
 int bch2_snapshots_read(struct bch_fs *);
 void bch2_fs_snapshots_exit(struct bch_fs *);