bcachefs: Rework btree node pinning

In backpointers fsck, we do a seqential scan of one btree, and check
references to another: extents <-> backpointers

Checking references generates random lookups, so we want to pin that
btree in memory (or only a range, if it doesn't fit in ram).

Previously, this was done with a simple check in the shrinker - "if
btree node is in range being pinned, don't free it" - but this generated
OOMs, as our shrinker wasn't well behaved if there was less memory
available than expected.

Instead, we now have two different shrinkers and lru lists; the second
shrinker being for pinned nodes, with seeks set much higher than normal
- so they can still be freed if necessary, but we'll prefer not to.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/bcachefs/btree_cache.h b/fs/bcachefs/btree_cache.h
index f82064007127..367acd217c6a 100644
--- a/fs/bcachefs/btree_cache.h
+++ b/fs/bcachefs/btree_cache.h
@@ -19,6 +19,9 @@ int __bch2_btree_node_hash_insert(struct btree_cache *, struct btree *);
 int bch2_btree_node_hash_insert(struct btree_cache *, struct btree *,
 				unsigned, enum btree_id);
 
+void bch2_node_pin(struct bch_fs *, struct btree *);
+void bch2_btree_cache_unpin(struct bch_fs *);
+
 void bch2_btree_node_update_key_early(struct btree_trans *, enum btree_id, unsigned,
 				      struct bkey_s_c, struct bkey_i *);