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

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

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.


Thanks,
Ming