[PATCH v2 1/2] blk-iocost: abort weight transfer on invalid donation weights

[Kunhai Dai <daikunhai@didiglobal.com>]

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>
---
The failure was originally observed on a vendor 4.18 kernel and reproduced
on v7.3-rc7 by a kprobe widening the current_hweight() publication window.
The guard is a fail-safe: a hand-constructed input can trigger the divide
at the function level, but runtime reachability without a race is not
confirmed, so this patch does not claim to fix a reachable no-race divide.
---
 block/blk-iocost.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 2745bffcd..7b8af094a 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -1956,6 +1956,7 @@ 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;
 		}
 	}
 
@@ -2054,6 +2055,7 @@ 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);
-- 
2.27.0