Re: mark pages in p2m_ram_paging_out state read-only

["Andres Lagar-Cavilla" <andres@lagarcavilla.org>]

> 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?
>
The stall is minuscule. A "minor fault" since the page hasn't been paged
out yet (in the condition this patch is concerned with) and thus there is
no IO readback.

The current code handles this nicely with its state machine. Further, it
lets the pager know with an additional event in the ring, which is
something I'd like to keep in place.

There are extra round-trip(s) to the pager, as is. Imho, it's not an
unreasonable price for a fringe condition.

Andres