Re: [PATCH] sys_getppid oopses on debug kernel

[Kirill Korotaev <dev@sw.ru>]

>>>Accessing freed memory is a bug, always, not just *only* when slab
>>>debugging is on, right?  Doesn't this mean we could get junk, or that
>>>the reader could potentially run off a bad pointer?
>>
>>no, read the comment in sys_getppid.
>>It is a valid optimization. _safe_ and alowing to bypass taking the lock.
>>BUT! This optimization relies on the fact that kernel memory (DMA + normal zone)
>>is always mapped into virtual address space.
>>Which is invalid for debug kernels only.
> 
> 
> Actually, it might also be invalid in hypervisor environments.  s390 and
> Xen use ballooning drivers to tell the hypervisor which pages are not
> currently in use by the OS so that they may be used in virtual machines
> elsewhere.
> 
> I'm cc'ing the s390 guys.  Will the s390 kernel oops if it accesses a
> page which was ballooned back to the hypervisor?

Yeah, a minute after my reply I got your idea and sent a new patch
which always takes the lock :)))

Thanks,
Kirill