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

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

On Sat, Sep 07, 2024 at 11:36:32AM +0100, Richard W.M. Jones wrote:
> On Sat, Sep 07, 2024 at 06:07:21PM +0800, Ming Lei wrote:
> > On Sat, Sep 07, 2024 at 11:02:13AM +0100, Richard W.M. Jones wrote:
> > > On Sat, Sep 07, 2024 at 05:48:44PM +0800, Ming Lei wrote:
> > > > On Sat, Sep 07, 2024 at 06:04:59PM +0900, 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
> > > > 
> > > > Right, I remember that the freezing starts to cover update of
> > > > max_sectors_kb.
> > > > 
> > > > > be to do the request_module() before freezing the queue.
> > > > > Something like below should work (totally untested and that may be overkill).
> > > > > 
> > > > > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> > > > > index 60116d13cb80..aef87f6b4a8a 100644
> > > > > --- a/block/blk-sysfs.c
> > > > > +++ b/block/blk-sysfs.c
> > > > > @@ -23,6 +23,7 @@
> > > > >  struct queue_sysfs_entry {
> > > > >         struct attribute attr;
> > > > >         ssize_t (*show)(struct gendisk *disk, char *page);
> > > > > +       int (*pre_store)(struct gendisk *disk, const char *page, size_t count);
> > > > 
> > > > It seems over-kill to add one new callback, and another way is just to
> > > > not freeze queue for storing elevator.
> > > > 
> > > > But if other attribute update needs to not freeze queue, 'pre_store'
> > > > looks one reasonable solution.
> > > > 
> > > > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> > > > index 60116d13cb80..c418edf66f0c 100644
> > > > --- a/block/blk-sysfs.c
> > > > +++ b/block/blk-sysfs.c
> > > > @@ -666,15 +666,24 @@ queue_attr_store(struct kobject *kobj, struct attribute *attr,
> > > >  	struct gendisk *disk = container_of(kobj, struct gendisk, queue_kobj);
> > > >  	struct request_queue *q = disk->queue;
> > > >  	ssize_t res;
> > > > +	bool need_freeze;
> > > >  
> > > >  	if (!entry->store)
> > > >  		return -EIO;
> > > >  
> > > > -	blk_mq_freeze_queue(q);
> > > > +	/*
> > > > +	 * storing scheduler freezes queue in its way, especially
> > > > +	 * loading scheduler module can't be done when queue is frozen
> > > > +	 */
> > > > +	need_freeze = (entry->store == elv_iosched_store);
> > > > +
> > > > +	if (need_freeze)
> > > > +		blk_mq_freeze_queue(q);
> > > >  	mutex_lock(&q->sysfs_lock);
> > > >  	res = entry->store(disk, page, length);
> > > >  	mutex_unlock(&q->sysfs_lock);
> > > > -	blk_mq_unfreeze_queue(q);
> > > > +	if (need_freeze)
> > > > +		blk_mq_unfreeze_queue(q);
> > > >  	return res;
> > > >  }
> > > >  
> > > 
> > > Unfortunately this doesn't fix the problem for me.  The test still
> > > hangs occasionally in the same way as before.
> > 
> > 'need_freeze' needs to be flipped by:
> > 
> > 	need_freeze = (entry->store != elv_iosched_store);
> 
> I'm still running the test (takes 5,000 boot iterations before I can
> be "sure"), but so far it seems flipping this test fixes the bug.

BTW, the issue can be reproduced 100% by:

echo "deadlock" > /sys/block/$ROOT_DISK/queue/scheduler

> 
> This seems like the neatest (or shortest) fix so far, but doesn't it
> "mix up layers" by checking elv_iosched_store?

It is just one exception for 'scheduler' sysfs attribute wrt. freezing
queue for storing, and the check can be done via the attribute
name("scheduler") too.


Thanks, 
Ming