Re: [PATCH] block: iocost: ensure hweight_inuse is at least 1

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi

在 2024/11/24 21:44, 戴坤海 Tony Dai 写道:
> In fact, we did encounter such a special situation where the kernel printed out `iocg: invalid donation weights in /a/b/c: active=1 donating=1 after=0`, and then it immediately panic. I analyzed the code but could not figure out how this happened; it might be a concurrency issue or some other hidden bug.

Do you have a reporducer for this? I'd like to take a look at the
WARN first.

Thanks,
Kuai

> 
> Our kernel is not the latest, but it includes the patch edaa26334c117a584add6053f48d63a988d25a6e (iocost: Fix divide-by-zero on donation from low hweight cgroup).
> 
> 在 2024/11/22 16:16，“Yu Kuai”<yukuai1@huaweicloud.com <mailto:yukuai1@huaweicloud.com>> 写入:
> 
> 
> Hi,
> 
> 
> 在 2024/11/22 15:26, Kunhai Dai 写道:
>> The hweight_inuse calculation in transfer_surpluses() could potentially
>> result in a value of 0, which would lead to division by zero errors in
>> subsequent calculations that use this value as a divisor.
>>
>> Signed-off-by: Kunhai Dai <daikunhai@didiglobal.com <mailto:daikunhai@didiglobal.com>>
>> ---
>> block/blk-iocost.c | 7 ++++---
>> 1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
>> index 384aa15e8260..65cdb55d30cc 100644
>> --- a/block/blk-iocost.c
>> +++ b/block/blk-iocost.c
>> @@ -1999,9 +1999,10 @@ static void transfer_surpluses(struct list_head *surpluses, struct ioc_now *now)
>> parent = iocg->ancestors[iocg->level - 1];
>>
>> /* b' = gamma * b_f + b_t' */
>> - iocg->hweight_inuse = DIV64_U64_ROUND_UP(
>> - (u64)gamma * (iocg->hweight_active - iocg->hweight_donating),
>> - WEIGHT_ONE) + iocg->hweight_after_donation;
>> + iocg->hweight_inuse = max_t(u64, 1,
>> + DIV64_U64_ROUND_UP(
>> + (u64)gamma * (iocg->hweight_active - iocg->hweight_donating),
>> + WEIGHT_ONE) + iocg->hweight_after_donation);
> 
> 
> I'm confused, how could DIV64_U64_Round_UP() end up less than 1?
> 
> 
> #define DIV64_U64_ROUND_UP(ll, d) \
> ({ u64 _tmp = (d); div64_u64((ll) + _tmp - 1, _tmp); })
> 
> 
> AFAIK, the only case that could happen is that
> iocg->hweight_active - iocg->hweight_donating is 0, then I don't
> get it now how cound active iocg donate all the hweight, if this
> really happend perhaps the better solution is to avoid such case.
> 
> 
> Thanks,
> Kuai
> 
> 
>>
>> /* w' = s' * b' / b'_p */
>> inuse = DIV64_U64_ROUND_UP(
>>
> 
> 
> 
> 
>