Re: [PATCH 00/10] blk-mq: fix blk_mq_tags double free while nr_requests grown

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

On Fri, Aug 15, 2025 at 05:05:34PM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2025/08/15 16:30, Ming Lei 写道:
> > On Fri, Aug 15, 2025 at 04:02:06PM +0800, 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.
> > > 
> > > patch 1-6 are prep cleanup and refactor patches for updating nr_requests
> > > patch 7,8 are the fix patches for the regression
> > > patch 9 is cleanup patch after patch 8
> > > patch 10 fix the stale nr_requests documentation
> > 
> > Please do not mix bug(regression) fix with cleanup.
> > 
> > The bug fix for updating nr_requests should have been simple enough in single
> > or two patches, why do you make 10-patches for dealing with the regression?
> 
> Ok, in short, my solution is:
> 
> - serialize switching elevator with updating nr_requests
> - check the case that nr_requests will grow and allocate elevator_tags
> before freezing the queue.
> - for the grow case, switch to new elevator_tags.

I'd suggest to make one or two commits to fix the recent regression
f5a6604f7a44 ("block: fix lockdep warning caused by lock dependency in elv_iosched_store")
first, because double free is one serious issue, and the fix should
belong to v6.17.

For other long-term or less serious issue, it may be fine to delay to v6.18
if the patchset is too big or complicated, which might imply new regression.


Thanks, 
Ming