Re: [PATCH] KVM: MMU: Fix rmap_remove() race

Avi Kivity
2008-03-27T14:35:55Z

Andrea Arcangeli wrote:
> On Thu, Mar 27, 2008 at 03:56:56PM +0200, Avi Kivity wrote:
>
>> That's not good. We need to support the older userspace, for a while yet.
>>
>> Why is there a problem? IIRC it's just anonymous memory.
>>
>
> Problem is that for it to be unmapped __do_fault must call
> page_add_new_anon_rmap on it. Even anon would be 1, that would mean
> cow and that's clearly not what you want as it would lose the
> visibility on the future guest writes. anon will be 0 if qemu is
> reading for example, leaving pinned anonymous memory that mmu
> notifiers won't be able to swap anymore. This is about providing the
> guarantee to the admin, that if he enables the mmu notifiers in the
> kernel, giving access to /dev/kvm won't give full mlock privileges
> too. Not sure if anybody will care, but it's a bit like removing the
> limit on tmpfs that only half ram can be pinned.
>

We can put it under a Kconfig option.

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