Re: Virtual Console Hotplug Leads to OOM

[Prarit Bhargava <prarit@sgi.com>]

Greg,

Thanks for the reply.

Greg KH wrote:

>>If you run the tty2_test which opens and closes ttys (attached to this 
>>email) you'll see THOUSANDS of hotplug events.  After running for 1 
>>minute on a 16p ia64 I had well over 4000 hotplug events.
>>    
>>
>
>Guess you need a faster machine, my little x86-64 had way more than that :)
>  
>
(sheepish grin) I did put a few sleeps in there so I could actually read 
the output streaming out :)

>  
>
>>If we're unlucky enough to fall below the low-water memory point at the 
>>same time tty2_test is spawning, the OOM-killer starts up.  Memory is 
>>taken up by the hotplug events ... and we're off into the weeds.
>>    
>>
>
>Yup.
>  
>
No offense, but I'm not sure we should be so cavalier on this issue -- 
I'm pointing out that we have a potential fork bomb within the kernel.

Aside from that, I'm think that hotplug is more of a victim here than 
the cause.  I'm also noticing that on this machine with 100,000 
outstanding task_structs, approximately 99,000 of them are hotplug 
task_structs.  Something is definately very wrong here.  These should 
have been reaped.  These tasks are not in the pid table but are left 
lying around on the task_struct list.

>  
>
>>The issue with the virtual console code is that it uses the class_simple 
>>library.  This is a problem as the library pre-defines a global kset 
>>parent as class_obj (or at least AFAICT).  This parent is used by ALL 
>>subsystems using the class_simple, not just the virtual console subsystem.
>>    
>>
>
>If you look at the -mm tree, class_simple is now gone.
>  
>
Will do.  Was there some signifigant reason for removing class_simple? 
(Just curious ...)

>  
>
>>This parent has a pointer to allow hotplug slot operations for all it's 
>>children (including the virtual consoles).  IMO zero-ing this pointer 
>>out is not a valid option as this would break hotplug op's for many 
>>other subystems.
>>    
>>
>
>Very true.
>
>  
>
>>** I did notice that the class struct has it's own hotplug value -- 
>>can/should this be used to override the parent's hotplug operations? **
>>    
>>
>
>Why would you want to override this?  We need those hotplug events.  All
>this means is that an admin better be careful who he lets have access to
>the tty group :)
>  
>
Absolutely correct -- but on some machines the group can be uucp ... 
which could *potentially* cause a security hole.  The tty2_test is 
(very) contrived but still ... it should be something we're aware of as 
it could cause problems on machines at critical stress points.

>thanks,
>
>greg k-h
>
>  
>


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