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

[Ming Lei <ming.lei@redhat.com>]

On Sat, Aug 16, 2025 at 04:05:30PM +0800, 余快 wrote:
> Hi,
> 
> Ming Lei <ming.lei@redhat.com> 于2025年8月16日周六 12:05写道：
> >
> > On Sat, Aug 16, 2025 at 10:57:23AM +0800, Yu Kuai wrote:
> > > 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@huawei.com>
> > > > >
> > > > > 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@huawei.com>
> > > > > ---
> > > > >   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.
> >
> > There are two issues involved:
> >
> > - blk_mq_tags double free, introduced recently
> >
> > - long-term lock issue in queue_requests_store()
> >
> > IMO, the former is a bit serious, because kernel panic can be triggered,
> > so suggest to make it to v6.17. The latter looks less serious and has
> > existed for long time, but may need code refactor to get clean fix.
> >
> > > I'll send one patch to fix this regression by
> > > replace
> > >
> > > st->tags with reallocated new sched_tags as well.
> >
> > Patch 7 in this patchset and patch 8 in your 1st post looks enough to
> > fix this double free issue.
> >
> But without previous refactor, this looks hard. Can we consider the following

I feel the following one should be enough:


diff --git a/block/blk-mq.c b/block/blk-mq.c
index b67d6c02eceb..021155bb64fc 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4917,6 +4917,29 @@ void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
 }
 EXPORT_SYMBOL(blk_mq_free_tag_set);
 
+static int blk_mq_sched_grow_tags(struct request_queue *q, unsigned int nr)
+{
+	struct blk_mq_hw_ctx *hctx;
+	struct elevator_tags *et;
+	unsigned long i;
+
+	if (nr > MAX_SCHED_RQ)
+		return -EINVAL;
+
+	et = blk_mq_alloc_sched_tags(q->tag_set, q->nr_hw_queues, nr);
+	if (!et)
+		return -ENOMEM;
+
+	blk_mq_free_sched_tags(q->elevator->et, q->tag_set);
+	queue_for_each_hw_ctx(q, hctx, i) {
+		hctx->sched_tags = et->tags[i];
+		if (q->elevator->type->ops.depth_updated)
+			q->elevator->type->ops.depth_updated(hctx);
+	}
+	q->elevator->et = et;
+	return 0;
+}
+
 int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
 {
 	struct blk_mq_tag_set *set = q->tag_set;
@@ -4936,6 +4959,12 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
 	blk_mq_quiesce_queue(q);
 
 	ret = 0;
+
+	if (q->elevator && nr > q->elevator->et->nr_requests) {
+		ret = blk_mq_sched_grow_tags(q, nr);
+		goto update_nr_requests;
+	}
+
 	queue_for_each_hw_ctx(q, hctx, i) {
 		if (!hctx->tags)
 			continue;
@@ -4955,6 +4984,7 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
 		if (q->elevator && q->elevator->type->ops.depth_updated)
 			q->elevator->type->ops.depth_updated(hctx);
 	}
+update_nr_requests:
 	if (!ret) {
 		q->nr_requests = nr;
 		if (blk_mq_is_shared_tags(set->flags)) {

> one line patch for this merge window? just fix the first double free
> issue for now.
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index d880c50629d6..1e0ccf19295a 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -622,6 +622,7 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
>                         return -ENOMEM;
> 
>                 blk_mq_free_map_and_rqs(set, *tagsptr, hctx->queue_num);
> +               hctx->queue->elevator->et->tags[hctx->queue_num]= new;
>                 *tagsptr = new;

It is fine if this way can work, however old elevator->et may has lower
depth, then:

- the above change cause et->tags overflow

- meantime memory leak is caused in blk_mq_free_sched_tags()


Thanks,
Ming