bcachefs: Increase size of btree node reserve

Also tweak the allocator to be more aggressive about keeping it full.
The recent changes to make updates to interior nodes transactional (and
thus generate updates to the alloc btree) all put more stress on the
btree node reserves.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -501,6 +501,7 @@ static void bch2_bucket_clock_init(struct bch_fs *c, int rw)
 static int wait_buckets_available(struct bch_fs *c, struct bch_dev *ca)
 {
 	unsigned long gc_count = c->gc_count;
+	u64 available;
 	int ret = 0;
 
 	ca->allocator_state = ALLOCATOR_BLOCKED;
@@ -516,9 +517,11 @@ static int wait_buckets_available(struct bch_fs *c, struct bch_dev *ca)
 		if (gc_count != c->gc_count)
 			ca->inc_gen_really_needs_gc = 0;
 
-		if ((ssize_t) (dev_buckets_available(c, ca) -
-			       ca->inc_gen_really_needs_gc) >=
-		    (ssize_t) fifo_free(&ca->free_inc))
+		available = max_t(s64, 0, dev_buckets_available(c, ca) -
+				  ca->inc_gen_really_needs_gc);
+
+		if (available > fifo_free(&ca->free_inc) ||
+		    (available && !fifo_full(&ca->free[RESERVE_BTREE])))
 			break;
 
 		up_read(&c->gc_lock);

fs/bcachefs/bcachefs.h

@@ -340,7 +340,7 @@ enum bch_time_stats {
 #define BTREE_RESERVE_MAX	(BTREE_MAX_DEPTH + (BTREE_MAX_DEPTH - 1))
 
 /* Size of the freelist we allocate btree nodes from: */
-#define BTREE_NODE_RESERVE	BTREE_RESERVE_MAX
+#define BTREE_NODE_RESERVE	(BTREE_RESERVE_MAX * 4)
 
 #define BTREE_NODE_OPEN_BUCKET_RESERVE	(BTREE_RESERVE_MAX * BCH_REPLICAS_MAX)