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Re: [Xen-devel] Meaning of PG_foreign

  • To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
  • From: "Jacob Gorm Hansen" <jacobg@xxxxxxx>
  • Date: Fri, 6 Oct 2006 03:45:58 -0700
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Fri, 06 Oct 2006 03:46:26 -0700
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On 10/6/06, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:
On 6/10/06 10:54, "Jacob Gorm Hansen" <jacobg@xxxxxxx> wrote:

> can I count on all pages in a domU that are changed by Xen, backend
> drivers, or writable pagetable magic, being marked as PG_foreign?

I'm not sure what you mean by 'changed'. But I expect you misunderstand the
meaning of PG_foreign anyway. What it means is 'foreign' from the
point-of-view of Linux's buddy allocator -- i.e., it is a page that actually
comes from a different allocator, which is wanting to hook off of
free_pages(). It doesn't (necessarily) mean that the page is actually from a
foreign virtual machine.

I had a problem with self-checkpoint consistency -- page contents
would change without my knowledge (I would get no page faults, and the
pages did not seem to come from netfront or blkfront drivers), and I
'solved' that in a very ugly way by looping through all pages and
explicitly checkpointing those with the PG_foreign bit set.

The offending pages looked like page tables, they would be all zero,
and then a single entry would somehow sneak in without warning, even
though I was taking care to flush all writable pagetable actions
before taking the checkpoint.

This is in -testing, perhaps I should try -unstable, where writable
page tables are rumoured to have gone away.


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