Re: [PATCH] block: elevator: avoid to load iosched module from this disk

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

On Mon, Sep 09, 2024 at 10:59:00AM +0900, Damien Le Moal wrote:
> On 9/9/24 10:24, Ming Lei wrote:
> > On Sat, Sep 07, 2024 at 07:50:32AM -0600, Jens Axboe wrote:
> >> On 9/7/24 3:04 AM, Damien Le Moal wrote:
> >>> On 9/7/24 16:58, Ming Lei wrote:
> >>>> On Sat, Sep 07, 2024 at 08:35:22AM +0100, Richard W.M. Jones wrote:
> >>>>> On Sat, Sep 07, 2024 at 09:43:31AM +0800, Ming Lei wrote:
> >>>>>> When switching io scheduler via sysfs, 'request_module' may be called
> >>>>>> if the specified scheduler doesn't exist.
> >>>>>>
> >>>>>> This was has deadlock risk because the module may be stored on FS behind
> >>>>>> our disk since request queue is frozen before switching its elevator.
> >>>>>>
> >>>>>> Fix it by returning -EDEADLK in case that the disk is claimed, which
> >>>>>> can be thought as one signal that the disk is mounted.
> >>>>>>
> >>>>>> Some distributions(Fedora) simulates the original kernel command line of
> >>>>>> 'elevator=foo' via 'echo foo > /sys/block/$DISK/queue/scheduler', and boot
> >>>>>> hang is triggered.
> >>>>>>
> >>>>>> Cc: Richard Jones <rjones@redhat.com>
> >>>>>> Cc: Jeff Moyer <jmoyer@redhat.com>
> >>>>>> Cc: Jiri Jaburek <jjaburek@redhat.com>
> >>>>>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> >>>>>
> >>>>> I'd suggest also:
> >>>>>
> >>>>> Bug: https://bugzilla.kernel.org/show_bug.cgi?id=219166
> >>>>> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> >>>>> Reported-by: Jiri Jaburek <jjaburek@redhat.com>
> >>>>> Tested-by: Richard W.M. Jones <rjones@redhat.com>
> >>>>>
> >>>>> So I have tested this patch and it does fix the issue, at the possible
> >>>>> cost that now setting the scheduler can fail:
> >>>>>
> >>>>>   + for f in /sys/block/{h,s,ub,v}d*/queue/scheduler
> >>>>>   + echo noop
> >>>>>   /init: line 109: echo: write error: Resource deadlock avoided
> >>>>>
> >>>>> (I know I'm setting it to an impossible value here, but this could
> >>>>> also happen when setting it to a valid one.)
> >>>>
> >>>> Actually in most of dist, io-schedulers are built-in, so request_module
> >>>> is just a nop, but meta IO must be started.
> >>>>
> >>>>>
> >>>>> Since almost no one checks the result of 'echo foo > /sys/...'  that
> >>>>> would probably mean that sometimes a desired setting is silently not
> >>>>> set.
> >>>>
> >>>> As I mentioned, io-schedulers are built-in for most of dist, so
> >>>> request_module isn't called in case of one valid io-sched.
> >>>>
> >>>>>
> >>>>> Also I bisected this bug yesterday and found it was caused by (or,
> >>>>> more likely, exposed by):
> >>>>>
> >>>>>   commit af2814149883e2c1851866ea2afcd8eadc040f79
> >>>>>   Author: Christoph Hellwig <hch@lst.de>
> >>>>>   Date:   Mon Jun 17 08:04:38 2024 +0200
> >>>>>
> >>>>>     block: freeze the queue in queue_attr_store
> >>>>>     
> >>>>>     queue_attr_store updates attributes used to control generating I/O, and
> >>>>>     can cause malformed bios if changed with I/O in flight.  Freeze the queue
> >>>>>     in common code instead of adding it to almost every attribute.
> >>>>>
> >>>>> Reverting this commit on top of git head also fixes the problem.
> >>>>>
> >>>>> Why did this commit expose the problem?
> >>>>
> >>>> That is really the 1st bad commit which moves queue freezing before
> >>>> calling request_module(), originally we won't freeze queue until
> >>>> we have to do it.
> >>>>
> >>>> Another candidate fix is to revert it, or at least not do it
> >>>> for storing elevator attribute.
> >>>
> >>> I do not think that reverting is acceptable. Rather, a proper fix would simply
> >>> be to do the request_module() before freezing the queue.
> >>> Something like below should work (totally untested and that may be overkill).
> >>
> >> I like this approach, but let's please call it something descriptive
> >> like "load_module" or something like that.
> > 
> > But 'load_module' is too specific as interface, and we just only have
> > one case which need to load module exactly.
> 
> If another attr needs to do some prep work before freezing the queue and calling
> attr->store(), we can rename the load_module attribute method to something like
> "prepare_store" to be more generic.

'interface' is supposed to be generic from beginning, and I don't think
we will have another 'load_module' case here.


Thanks,
Ming