bcachefs: Trust btree alloc info at runtime

This lets us avoid a cache miss in the write path.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -499,6 +499,7 @@ enum {
 	/* misc: */
 	BCH_FS_BDEV_MOUNTED,
 	BCH_FS_FIXED_GENS,
+	BCH_FS_ALLOC_WRITTEN,
 	BCH_FS_REBUILD_REPLICAS,
 	BCH_FS_HOLD_BTREE_WRITES,
 };

fs/bcachefs/buckets.c

@@ -1456,7 +1456,7 @@ static int bch2_trans_mark_pointer(struct btree_trans *trans,
 	if (ret < 0)
 		return ret;
 
-	if (!ret) {
+	if (!ret && unlikely(!test_bit(BCH_FS_ALLOC_WRITTEN, &c->flags))) {
 		/*
 		 * During journal replay, and if gc repairs alloc info at
 		 * runtime, the alloc info in the btree might not be up to date

fs/bcachefs/recovery.c

@@ -864,6 +864,8 @@ int bch2_fs_recovery(struct bch_fs *c)
 			goto err;
 		}
 		bch_verbose(c, "alloc write done");
+
+		set_bit(BCH_FS_ALLOC_WRITTEN, &c->flags);
 	}
 
 	if (!c->sb.clean) {