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

[Holger Hoffstätte]

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.
Will gladly test your patch! :)

cheers
Holger