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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe