Re: [Xen-devel] mark pages in p2m_ram_paging_out state read-only

[Olaf Hering <olaf@xxxxxxxxx>]

On Thu, Nov 24, Olaf Hering wrote:

> On Thu, Nov 24, Tim Deegan wrote:
> 
> > > Is this a good idea? If the guest is accessing the page, then paging out
> > > should stop immediately. We're risking complex races for a tiny tiny gain.
> > 
> > I don't think that this would introduce any new races, but yes - this is
> > probably a hint that this page is a poor candidate to be paged out.
> > We might as well abandon the page-out rather than probably having to
> > page it back in immediately.
> 
> In the current state there will be a stall in the guest due to the
> fault, until the page comes back. 
> With this change the guest can still make progress. 
> Can the read-execute access be detected anyway to stop paging?

(I hit send too soon..)

Right now nominate would cause an immediate fault. populate would be
called at some point, hopefully before evict. If populate were smart
enough to bring the page back from p2m_ram_paging_out to p2m_ram_rw and
let the callers know about that, the guest could proceed right away. I
have already added some code to populate to detect p2m_ram_paging_out.
Perhaps p2m_mem_paging_populate shouldnt be void. It could return -EBUSY
to its callers, they should get a valid mfn for their gfn and handle the
gfn as valid ram.

Olaf

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