bcachefs: Don't block on allocations when only writing to specific device

Since the copygc thread is now global and not per device, we're not
freeing up space on any one device in bounded time - and indeed we never
really were, since rebalance wasn't moving data around between devices
with that objective.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io.c

@@ -1090,6 +1090,11 @@ static void __bch2_write(struct closure *cl)
 			goto err;
 		}
 
+		/*
+		 * The copygc thread is now global, which means it's no longer
+		 * freeing up space on specific disks, which means that
+		 * allocations for specific disks may hang arbitrarily long:
+		 */
 		wp = bch2_alloc_sectors_start(c,
 			op->target,
 			op->opts.erasure_code,
@@ -1099,7 +1104,8 @@ static void __bch2_write(struct closure *cl)
 			op->nr_replicas_required,
 			op->alloc_reserve,
 			op->flags,
-			(op->flags & BCH_WRITE_ALLOC_NOWAIT) ? NULL : cl);
+			(op->flags & (BCH_WRITE_ALLOC_NOWAIT|
+				      BCH_WRITE_ONLY_SPECIFIED_DEVS)) ? NULL : cl);
 		EBUG_ON(!wp);
 
 		if (unlikely(IS_ERR(wp))) {