bcachefs: Change when we allow overwrites

Originally, we'd check for -ENOSPC when getting a disk reservation
whenever the new extent took up more space on disk than the old extent.

Erasure coding screwed this up, because with erasure coding writes are
initially replicated, and then in the background the extra replicas are
dropped when the stripe is created. This means that with erasure coding
enabled, writes will always take up more space on disk than the data
they're overwriting - but, according to posix, overwrites aren't
supposed to return ENOSPC.

So, in this patch we fudge things: if the new extent has more replicas
than the _effective_ replicas of the old extent, or if the old extent is
compressed and the new one isn't, we check for ENOSPC when getting the
disk reservation - otherwise, we don't.

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

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/bcachefs/io.h b/fs/bcachefs/io.h
index 379263a935fa..6721440e8bc7 100644
--- a/fs/bcachefs/io.h
+++ b/fs/bcachefs/io.h
@@ -64,6 +64,8 @@ static inline struct workqueue_struct *index_update_wq(struct bch_write_op *op)
 		: op->c->wq;
 }
 
+int bch2_sum_sector_overwrites(struct btree_trans *, struct btree_iter *,
+			       struct bkey_i *, bool *, bool *, s64 *, s64 *);
 int bch2_extent_update(struct btree_trans *, struct btree_iter *,
 		       struct bkey_i *, struct disk_reservation *,
 		       u64 *, u64, s64 *);