On Thu, Feb 21, 2008 at 12:17:06PM +1100, Nigel Cunningham wrote:
quoted text
> Hi.
>
> Greg KH wrote:
>> On Thu, Feb 21, 2008 at 11:40:06AM +1100, Nigel Cunningham wrote:
>>> Hi.
>>>
>>> Matthew Garrett wrote:
>>>> On Thu, Feb 21, 2008 at 09:45:02AM +1100, Nigel Cunningham wrote:
>>>>> - people keep talking about hibernating to an ext3 fs mounted on fuse 
>>>>> as a limitation of the freezer. To do that with kexec, you're still 
>>>>> going to have to bmap the ext3 fs and pass the block list (in which 
>>>>> case we can also do it without kexec) or umount all the ext3/fuse part 
>>>>> and remount in the kexec'd kernel. Sort of defeats the purpose, doesn't 
>>>>> it?
>>>> No, with a freezer-based model you can basically *never* suspend to 
>>>> anything related to FUSE or a userspace USB device or anything involving 
>>>> userspace iSCSI initiators or whatever. Sure, there are cases where 
>>>> moving away from the current model doesn't buy you anything, but that 
>>>> doesn't mean that the current model is a good thing. It's not. The 
>>>> freezer is a fundamentally broken concept.
>>> Putting drivers and filesystems in userspace is the fundamentally broken 
>>> concept. Not just when it comes to the freezer. The whole idea is 
>>> inherently racy.
>> Racy with regards to other things becides trying to suspend a machine?
>> If so, what?
>
> That depends on what sort of tangled web you want to weave.

Lots of them :)

We have tanks running Linux using userspace USB drivers for vision
control systems (scary, I know...)  They seem to be successfully running
for many years now, and I'm interested in making sure those kinds of
things keep working.

We also have laser welding robots with userspace PCI drivers in car
manufacturing plants.  And other laser cutting robots slicing wood in
patterns moving at a rate of over 3 meters a second.  Again, with
userspace drivers and Linux.

Those users would also love to know of any potential problems you know
of for this situation.

quoted text
> Low memory situations is one other situation that occurs to me
> quickly, especially (though not only) if your ability to swap were to
> depend upon a userspace driver and/or filesystem.

Sure, swap over a userspace filesystem or driver isn't a sane idea.  And
neither is swaping over NFS over a PPP connection attached to a USB to
serial device.  Yes, it's possible, and all in the kernel, but not a
wise decision.

Other than foolish configurations, if you come up with other issues
surrounding userspace drivers that could cause problems, please let me
know.

thanks,

greg k-h
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