bcachefs: Fragmentation LRU

Now that we have much more efficient updates to the LRU btree, this
patch adds a new LRU that indexes buckets by fragmentation.

This means copygc no longer has to scan every bucket to find buckets
that need to be evacuated.

Changes:
 - A new field in bch_alloc_v4, fragmentation_lru - this corresponds to
   the bucket's position in the fragmentation LRU. We add a new field
   for this instead of calculating it as needed because we may make the
   fragmentation LRU optional; this field indicates whether a bucket is
   on the fragmentation LRU.

   Also, zoned devices will introduce variable bucket sizes; explicitly
   recording the LRU position will be safer for them.

 - A new copygc path for using the fragmentation LRU instead of
   scanning every bucket and building up an in-memory heap.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/bcachefs/recovery.c b/fs/bcachefs/recovery.c
index 178f06424460..1976d5fa3427 100644
--- a/fs/bcachefs/recovery.c
+++ b/fs/bcachefs/recovery.c
@@ -1105,6 +1105,9 @@ int bch2_fs_recovery(struct bch_fs *c)
 			c->opts.version_upgrade	= true;
 			c->opts.fsck		= true;
 			c->opts.fix_errors	= FSCK_OPT_YES;
+		} else if (c->sb.version < bcachefs_metadata_version_fragmentation_lru) {
+			bch_info(c, "version prior to backpointers, upgrade required");
+			c->opts.version_upgrade	= true;
 		}
 	}