Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux

Izik Eidus
2008-11-11T19:19:09Z

Andrew Morton wrote:
> On Tue, 11 Nov 2008 20:48:16 +0200
> Avi Kivity <avi@redhat.com> wrote:
>
>
>> Andrew Morton wrote:
>>
>>> The whole approach seems wrong to me. The kernel lost track of these
>>> pages and then we run around post-facto trying to fix that up again.
>>> Please explain (for the changelog) why the kernel cannot get this right
>>> via the usual sharing, refcounting and COWing approaches.
>>>
>>>
>> For kvm, the kernel never knew those pages were shared. They are loaded
>> from independent (possibly compressed and encrypted) disk images. These
>> images are different; but some pages happen to be the same because they
>> came from the same installation media.
>>
>
> What userspace-only changes could fix this? Identify the common data,
> write it to a flat file and mmap it, something like that?
>
>
>> For OpenVZ the situation is less clear, but if you allow users to
>> independently upgrade their chroots you will eventually arrive at the
>> same scenario (unless of course you apply the same merging strategy at
>> the filesystem level).
>>
>
> hm.
>
> There has been the occasional discussion about idenfifying all-zeroes
> pages and scavenging them, repointing them at the zero page. Could
> this infrastructure be used for that? (And how much would we gain from
> it?)
>
> [I'm looking for reasons why this is more than a muck-up-the-vm-for-kvm
> thing here ;) ]
KSM is separate driver , it doesn't change anything in the VM but adding
two helper functions.