Re: [Xen-devel] Proposed new "memory capacity claim" hypercall/feature

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 08/11/2012 19:16, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

>> Yes, this clearly prohibits unlimited batches. But not being able to
>> schedule should be less restrictive than not being able to run
>> softirqs, so I'd still put under question whether the limit shouldn't
>> be bumped.
> 
> Wait, please define unlimited.
> 
> I think we are in agreement from previous discussion that, to solve
> the TOCTOU race, the heap_lock must be held for the entire allocation
> for a domain creation.  True?

It's pretty obvious that this isn't going to be possible in the general
case. E.g., a 40G domain being created out of 4k pages (eg. Because memory
is fragmented) is going to be at least 40G/4k == 10M heap operations. Say
each takes 10ns, which would be quick, we're talking 100ms of cpu work.
Holding a lock that long can't be recommended really.

 -- Keir



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