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

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

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.

I guess there may be same risk in queue_wb_lat_store() which calls into
GFP_KERNEL allocation which implies direct reclaim & IO.

Thanks,
Ming