Re: [kvm-devel] mmu notifiers

[Christoph Lameter <clameter@sgi.com>]

On Tue, 15 Jan 2008, Avi Kivity wrote:

> > But each guest has its own page structs. They cannot share page structs.
> > Concurrent access of two independent kernel instances for synchronization
> > and status maintenance to a single page struct?
> >   
> 
> There's a host page struct (that the guest know nothing about and cannot
> touch), and optionally a guest page struct for each guest (that the host and
> the other guest know nothing about).

Ok so if two linux guests want to share memory three page structs are 
involved:

1. Host page struct
2. Guest #1 page struct
3. Guest #2 page struct

I can understand that 1 and 2 point to the same physical page. Even all 
three could point to the same page if the page is readonly. 

However, lets say that Guest #1 allocates some anonymous memory and wants
to share it with Guest #2. In that case something like PFNMAP is likely
going to be used? Or are you remapping the physical page so that #1 and #2 
share it? In that case two page struct describe state of the same physical
page and we have no effective synchronization for writeback etc.

> The host page struct may disappear if the host decides to swap the page into
> its backing store and free the page.  The guest page structs (if any) would
> remain.

Page structs never disappear. The pte's may disappear and the page may be 
unmapped from an address space of a process but the page struct stays. 
Page struct can only disappear if memory hotplug is activated and memory 
is taken out of the system.


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