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

[Izik Eidus]

Andrew Morton wrote:
> On Tue, 11 Nov 2008 20:48:16 +0200
> Avi Kivity <avi@xxxxxxxxxx> 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.
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