[PATCH] Add events for dm suspend / resume

[PATCH] Add events for dm suspend / resume

[Hannes Reinecke]

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Hi all,

currently device-mapper doesn't play well with udev & hotplug events.
The event is sent out on the initial device create; only at this stage
the device is not usable. Only after one did a 'table load' and a
'resume' the device is actually accessible from userland.
And thus it is purely coincidental if any 'dmsetup' call from udev
(which is triggered by the device create events) will return any useful
data.

This patch adds two additional events 'online' and 'offline' which get
send after 'resume' and 'suspend', respectively.
With this patch udev can hook on the 'online' event and will always get
valid data via the dmsetup call.

Comments etc. are welcome.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke			hare@suse.de
SuSE Linux Products GmbH		S390 & zSeries
Maxfeldstraße 5				+49 911 74053 688
90409 Nürnberg				http://www.suse.de

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--- linux-2.6.16/drivers/md/dm.c.orig	2006-05-23 12:18:09.000000000 +0200
+++ linux-2.6.16/drivers/md/dm.c	2006-05-26 14:09:16.000000000 +0200
@@ -1201,6 +1201,7 @@ int dm_suspend(struct mapped_device *md,
 	dm_table_postsuspend_targets(map);
 
 	set_bit(DMF_SUSPENDED, &md->flags);
+	kobject_uevent(&md->disk->kobj, KOBJ_OFFLINE);
 
 	r = 0;
 
@@ -1247,6 +1248,7 @@ int dm_resume(struct mapped_device *md)
 
 	dm_table_unplug_all(map);
 
+	kobject_uevent(&md->disk->kobj, KOBJ_ONLINE);
 	r = 0;
 
 out:

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Re: [PATCH] Add events for dm suspend / resume

[Alasdair G Kergon]

On Mon, May 29, 2006 at 10:04:09AM +0200, Hannes Reinecke wrote:
> currently device-mapper doesn't play well with udev & hotplug events.

That's a matter of opinion - I'd say it's udev that doesn't play well with
device-mapper:-)

> The event is sent out on the initial device create; only at this stage
> the device is not usable. Only after one did a 'table load' and a
> 'resume' the device is actually accessible from userland.
> And thus it is purely coincidental if any 'dmsetup' call from udev
> (which is triggered by the device create events) will return any useful
> data.
> This patch adds two additional events 'online' and 'offline' which get
> send after 'resume' and 'suspend', respectively.

> With this patch udev can hook on the 'online' event and will always get
> valid data via the dmsetup call.

'Always' is rather too strong: the device could already have changed again
or even disappeared or mutated into a completely different device...
 

Very little seems to use kobject online/offline events yet, but this looks
to me to be a reasonable use for them to reduce some of the existing
problems.

So the new protocol would be:

  add: Says nothing about whether or not it's ready to be used.
       Ignore it.  [Or create a /dev node if you want.]

  online: The device is usable.
       Trigger the multipath kpartx now, for example.

  offline: I can't think of a need for this yet (and triggering udev events
           when a device is suspended could cause problems) so I'd rather
           leave it out for now.


*But* before applying this, testing is needed to check the extra event can't
lead to the machine locking up.  For example, I notice some worrying
allocations without mempools (or PF_MEMALLOC) in kobject_uevent(): 'suspend'
and 'resume' are sections of device-mapper where such allocations are not
permitted.


Assuming the memory allocation can be avoided or done safely, the change is
a step in the right direction, but it still doesn't address the design
problem with using an asynchronous udev for anything connected with
device-mapper, namely that by the time the udev event gets around to being
processed, the device's state could have changed - or it could even be a
completely different device.

Alasdair.


> --- linux-2.6.16/drivers/md/dm.c.orig	2006-05-23 12:18:09.000000000 +0200
> +++ linux-2.6.16/drivers/md/dm.c	2006-05-26 14:09:16.000000000 +0200
> @@ -1201,6 +1201,7 @@ int dm_suspend(struct mapped_device *md,
>  	dm_table_postsuspend_targets(map);
>  
>  	set_bit(DMF_SUSPENDED, &md->flags);
> +	kobject_uevent(&md->disk->kobj, KOBJ_OFFLINE);
>  
>  	r = 0;
>  
> @@ -1247,6 +1248,7 @@ int dm_resume(struct mapped_device *md)
>  
>  	dm_table_unplug_all(map);
>  
> +	kobject_uevent(&md->disk->kobj, KOBJ_ONLINE);
>  	r = 0;
>  
>  out: