bcachefs: Reserve some open buckets for btree allocations

This reverts part of the change from "bcachefs: Don't use
BTREE_INSERT_USE_RESERVE so much" - it turns out we still should be
reserving open buckets for btree node allocations, because otherwise
data bucket allocations (especially with erasure coding enabled) can use
up all our open buckets and we won't be able to do the metadata update
that lets us release those open bucket references. Oops.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_foreground.c

@@ -204,8 +204,11 @@ long bch2_bucket_alloc_new_fs(struct bch_dev *ca)
 static inline unsigned open_buckets_reserved(enum alloc_reserve reserve)
 {
 	switch (reserve) {
-	case RESERVE_MOVINGGC:
+	case RESERVE_BTREE:
+	case RESERVE_BTREE_MOVINGGC:
 		return 0;
+	case RESERVE_MOVINGGC:
+		return OPEN_BUCKETS_COUNT / 4;
 	default:
 		return OPEN_BUCKETS_COUNT / 2;
 	}
@@ -261,6 +264,7 @@ struct open_bucket *bch2_bucket_alloc(struct bch_fs *c, struct bch_dev *ca,
 		goto out;
 
 	switch (reserve) {
+	case RESERVE_BTREE_MOVINGGC:
 	case RESERVE_MOVINGGC:
 		if (fifo_pop(&ca->free[RESERVE_MOVINGGC], bucket))
 			goto out;

fs/bcachefs/alloc_types.h

@@ -34,9 +34,9 @@ struct bucket_clock {
 	struct mutex		lock;
 };
 
-/* There is one reserve for each type of btree, one for prios and gens
- * and one for moving GC */
 enum alloc_reserve {
+	RESERVE_BTREE_MOVINGGC	= -2,
+	RESERVE_BTREE		= -1,
 	RESERVE_MOVINGGC	= 0,
 	RESERVE_NONE		= 1,
 	RESERVE_NR		= 2,

fs/bcachefs/btree_update_interior.c

@@ -203,10 +203,10 @@ static struct btree *__bch2_btree_node_alloc(struct bch_fs *c,
 
 	if (flags & BTREE_INSERT_USE_RESERVE) {
 		nr_reserve	= 0;
-		alloc_reserve	= RESERVE_MOVINGGC;
+		alloc_reserve	= RESERVE_BTREE_MOVINGGC;
 	} else {
 		nr_reserve	= BTREE_NODE_RESERVE;
-		alloc_reserve	= RESERVE_NONE;
+		alloc_reserve	= RESERVE_BTREE;
 	}
 
 	mutex_lock(&c->btree_reserve_cache_lock);