Messages in this thread | | | Subject | Re: [PATCH BUGFIX] block, bfq: fix delayed stable merge check | From | Holger Hoffstätte <> | Date | Mon, 24 May 2021 19:13:59 +0200 |
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On 2021-05-24 18:57, Paolo Valente wrote: > > >> Il giorno 20 mag 2021, alle ore 09:15, Holger Hoffstätte <holger@applied-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@hotmail.it> >>> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> >>> Signed-off-by: Pietro Pedroni <pedroni.pietro.96@gmail.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. Will gladly test your patch! :)
cheers Holger
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