Re: [PATCH 08/10] blk-mq: fix blk_mq_tags double free while nr_requests grown

[Yu Kuai]

Hi,

在 2025/8/16 3:30, Nilay Shroff 写道:
> On 8/15/25 1:32 PM, Yu Kuai wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > In the case user trigger tags grow by queue sysfs attribute nr_requests,
> > hctx->sched_tags will be freed directly and replaced with a new
> > allocated tags, see blk_mq_tag_update_depth().
> >
> > The problem is that hctx->sched_tags is from elevator->et->tags, while
> > et->tags is still the freed tags, hence later elevator exist will try to
> > free the tags again, causing kernel panic.
> >
> > Fix this problem by using new allocated elevator_tags, also convert
> > blk_mq_update_nr_requests to void since this helper will never fail now.
> >
> > Meanwhile, there is a longterm problem can be fixed as well:
> >
> > If blk_mq_tag_update_depth() succeed for previous hctx, then bitmap depth
> > is updated, however, if following hctx failed, q->nr_requests is not
> > updated and the previous hctx->sched_tags endup bigger than q->nr_requests.
> >
> > Fixes: f5a6604f7a44 ("block: fix lockdep warning caused by lock dependency in elv_iosched_store")
> > Fixes: e3a2b3f931f5 ("blk-mq: allow changing of queue depth through sysfs")
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   block/blk-mq.c    | 19 ++++++-------------
> >   block/blk-mq.h    |  4 +++-
> >   block/blk-sysfs.c | 21 ++++++++++++++-------
> >   3 files changed, 23 insertions(+), 21 deletions(-)
> >
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 11c8baebb9a0..e9f037a25fe3 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -4917,12 +4917,12 @@ void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
> >   }
> >   EXPORT_SYMBOL(blk_mq_free_tag_set);
> >   
> > -int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
> > +void blk_mq_update_nr_requests(struct request_queue *q,
> > +			       struct elevator_tags *et, unsigned int nr)
> >   {
> >   	struct blk_mq_tag_set *set = q->tag_set;
> >   	struct blk_mq_hw_ctx *hctx;
> >   	unsigned long i;
> > -	int ret = 0;
> >   
> >   	blk_mq_quiesce_queue(q);
> >   
> > @@ -4946,24 +4946,17 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
> >   				nr - hctx->sched_tags->nr_reserved_tags);
> >   		}
> >   	} else {
> > -		queue_for_each_hw_ctx(q, hctx, i) {
> > -			if (!hctx->tags)
> > -				continue;
> > -			ret = blk_mq_tag_update_depth(hctx, &hctx->sched_tags,
> > -						      nr);
> > -			if (ret)
> > -				goto out;
> > -		}
> > +		blk_mq_free_sched_tags(q->elevator->et, set);
> I think you also need to ensure that elevator tags are freed after we unfreeze
> queue and release ->elevator_lock otherwise we may get into the lockdep splat
> for pcpu_lock dependency on ->freeze_lock and/or ->elevator_lock. Please note
> that blk_mq_free_sched_tags internally invokes sbitmap_free which invokes
> free_percpu which acquires pcpu_lock.

Ok, thanks for the notice. However, as Ming suggested, we might fix this 
problem

in the next merge window. I'll send one patch to fix this regression by 
replace

st->tags with reallocated new sched_tags as well.


Thanks,

Kuai

>   
> Thanks,
> --Nilay