blk-iocost: abort weight transfer on invalid donation weights

transfer_surpluses() warns and then keeps going when the donation
calculation produced invalid weights.  The following propagation can then
divide by a zero hweight_inuse, so the warning is immediately followed by a
divide error.

When the check trips, abandon this period's donation transfer and release
the walk lists.  This trades one period without donation for not crashing;
the invalid state is not expected on the normal path.

Developed with assistance from Claude Code (Anthropic): the assistant
analysed the failure path, drafted the change and ran the checks listed
below.  The author reviewed the result and remains responsible for it.

Fixes: e08d02aa5fc9 ("blk-iocost: implement Andy's method for donation weight updates")
Cc: stable@vger.kernel.org
Signed-off-by: Kunhai Dai <daikunhai@didiglobal.com>
Tested-by: Tao Cui <cuitao@kylinos.cn>
Reviewed-by: Tao Cui <cuitao@kylinos.cn>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://patch.msgid.link/20261019125412.3123895-2-daikunhai@didiglobal.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 2 insertions, 0 deletions

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 47bea7f31408..1118a8a1f079 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -1961,6 +1961,7 @@ static void transfer_surpluses(struct list_head *surpluses, struct ioc_now *now)
 			pr_cont(": active=%u donating=%u after=%u\n",
 				iocg->hweight_active, iocg->hweight_donating,
 				iocg->hweight_after_donation);
+			goto out;
 		}
 	}
 
@@ -2059,6 +2060,7 @@ static void transfer_surpluses(struct list_head *surpluses, struct ioc_now *now)
 		__propagate_weights(iocg, iocg->active, inuse, true, now);
 	}
 
+out:
 	/* walk list should be dissolved after use */
 	list_for_each_entry_safe(iocg, tiocg, &inner_walk, walk_list)
 		list_del_init(&iocg->walk_list);