linux-kernel - Re: [PATCH BUGFIX] block, bfq: fix delayed stable merge check

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Date:   Tue, 25 May 2021 12:40:04 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Holger Hoffstätte <holger@...lied-asynchrony.com>
Cc:     Luca Mariotti <mariottiluca1@...mail.it>,
        Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Pietro Pedroni <pedroni.pietro.96@...il.com>
Subject: Re: [PATCH BUGFIX] block, bfq: fix delayed stable merge check



> Il giorno 24 mag 2021, alle ore 20:45, Holger Hoffstätte <holger@...lied-asynchrony.com> ha scritto:
> 
> On 2021-05-24 19:41, Paolo Valente wrote:
>>> Il giorno 24 mag 2021, alle ore 19:13, Holger Hoffstätte <holger@...lied-asynchrony.com> ha scritto:
>>> 
>>> On 2021-05-24 18:57, Paolo Valente wrote:
>>>>> Il giorno 20 mag 2021, alle ore 09:15, Holger Hoffstätte <holger@...lied-asynchrony.com> ha scritto:
>>>>> 
>>>>> On 2021-05-18 12:43, Luca Mariotti wrote:
>>>>>> When attempting to schedule a merge of a given bfq_queue with the currently
>>>>>> in-service bfq_queue or with a cooperating bfq_queue among the scheduled
>>>>>> bfq_queues, delayed stable merge is checked for rotational or non-queueing
>>>>>> devs. For this stable merge to be performed, some conditions must be met.
>>>>>> If the current bfq_queue underwent some split from some merged bfq_queue,
>>>>>> one of these conditions is that two hundred milliseconds must elapse from
>>>>>> split, otherwise this condition is always met.
>>>>>> Unfortunately, by mistake, time_is_after_jiffies() was written instead of
>>>>>> time_is_before_jiffies() for this check, verifying that less than two
>>>>>> hundred milliseconds have elapsed instead of verifying that at least two
>>>>>> hundred milliseconds have elapsed.
>>>>>> Fix this issue by replacing time_is_after_jiffies() with
>>>>>> time_is_before_jiffies().
>>>>>> Signed-off-by: Luca Mariotti <mariottiluca1@...mail.it>
>>>>>> Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
>>>>>> Signed-off-by: Pietro Pedroni <pedroni.pietro.96@...il.com>
>>>>>> ---
>>>>>>  block/bfq-iosched.c | 2 +-
>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>>>>>> index acd1f881273e..2adb1e69c9d2 100644
>>>>>> --- a/block/bfq-iosched.c
>>>>>> +++ b/block/bfq-iosched.c
>>>>>> @@ -2697,7 +2697,7 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>>>>>>  	if (unlikely(!bfqd->nonrot_with_queueing)) {
>>>>>>  		if (bic->stable_merge_bfqq &&
>>>>>>  		    !bfq_bfqq_just_created(bfqq) &&
>>>>>> -		    time_is_after_jiffies(bfqq->split_time +
>>>>>> +		    time_is_before_jiffies(bfqq->split_time +
>>>>>>  					  msecs_to_jiffies(200))) {
>>>>>>  			struct bfq_queue *stable_merge_bfqq =
>>>>>>  				bic->stable_merge_bfqq;
>>>>> 
>>>>> Not sure why but with this patch I quickly got a division-by-zero in BFQ and
>>>>> complete system halt. Unfortunately I couldn't capture the exact stack trace,
>>>>> but it read something like bfq_calc_weight() or something ike that.
>>>>> I looked through the code and found bfq_delta(), so maybe weight got
>>>>> reduced to 0?
>>>>> 
>>>> Hi Holger,
>>>> is this (easily) reproducible for you?  If so, I'd like to propose you
>>>> a candidate fix.
>>> 
>>> Yes, it's easily reproducible (should be reproducible on 5.13-rc as well).
>>> Simple read/write I/O on a cold FS (rotational disk obviously) will crash
>>> pretty much immediately; without it everything works fine, likely because the
>>> bug (in the recent queue merging patches?) is never triggered due to the
>>> accidentally-wrong time calculation.
>> Exactly!
>> Unfortunately, no crash happens on my systems.  Or, actually, crashes
>> stopped after the attached fix.
>>> Will gladly test your patch! :)
>>> 
>> Here it is!
>> I'll make a proper commit after your early tests.
>> Crossing my fingers,
>> Paolo
> 
> That did it - it now survived a bunch of heavy read/write/mixed I/O that
> would previously crash right away. Maybe it's because btrfs uses several
> workers and so different IOs got mixed together? Anyway:
> 
> Fixes: 430a67f9d616 ("block, bfq: merge bursts of newly-created queues")
> Tested-by: Holger Hoffstätte <holger@...lied-asynchrony.com>
> 

Great!

Thank you very much!

I will put this fix in an upcoming patch series.

Paolo

> Thanks!
> Holger