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

[Paolo Valente]

> Il giorno 24 mag 2021, alle ore 19:13, Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx> ha scritto:
> 
> On 2021-05-24 18:57, Paolo Valente wrote:
>>> Il giorno 20 mag 2021, alle ore 09:15, Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
>>>> Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
>>>> Signed-off-by: Pietro Pedroni <pedroni.pietro.96@xxxxxxxxx>
>>>> ---
>>>>  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

Attachment: 0001-block-bfq-avoid-delayed-merge-of-async-queues.patch.gz
Description: GNU Zip compressed data

> cheers
> Holger