Re: [PATCH] sys_getppid oopses on debug kernel

[Kirill Korotaev]

> > > 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

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