Re: xm mem-max and mem-set

[Anthony Liguori <aliguori@us.ibm.com>]

Ky Srinivasan wrote:
> It appears that these commands (and code backing these commands) do no
> sanity checking and could potentially get the system to crash if the
> values picked are not appropriate. For instance one can set the mem-max
> value to a value that appears reasonable and basically render the
> machine unusable. Consider the case where max value being set is less
> than what is currently allocated to the domain. All subsequent
> allocations will fail and these failures are considered fatal in Linux
> (look at hypervisor.c). Once the domain is up, does it even make sense
> to lower the max_mem parameter without risking crashing the system?
> Minimally, we should ensure that the mem_max value is at least equal to
> what the current domain allocation is. I have a trivial patch to xen
> that implements this logic. This patch fixes a bug we have in our
> bugzilla against SLES10. Would there be interest in such a patch. 
>   

I'm slightly concerned about the subtle race condition it would 
introduce.  If there's no reason to set max-mem below current 
reservation (if it causes crashes which I don't really understand why it 
would) then I think it would be something best enforced within the 
hypervisor.

Why, exactly, would setting max-mem below the current reservation cause 
problems in the guest?  I guess it may fail because of grant transfer 
ops (in which case, we really ought to enforce it at the hypervisor level).

Regards,

Anthony Liguori

> Regards,
>
> K. Y
>
>
>
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