Re: [Xen-devel] [PATCH 1/2] Revert "xen: properly account for _PAGE_NUMA during xen pte translations"

[David Sutton <kantras@xxxxxxxxx>]

David,

> This reverts commit a9c8e4beeeb64c22b84c803747487857fe424b68.
>

Please note: this particular patch also helped to resolve an issue reported by some of the people using the xen package under Arch Linux, where they would see stability issues running certain software (particularly firefox) under dom0 - the software would start crashing under relatively light usage, with a kernel BUG message.

>
> PTEs in Xen PV guests must contain machine addresses if _PAGE_PRESENT
> is set and pseudo-physical addresses is _PAGE_PRESENT is clear.
>
> This is because during a domain save/restore (migration) the page
> table entries are "canonicalised" and uncanonicalised". i.e., MFNs are
> converted to PFNs during domain save so that on a restore the page
> table entries may be rewritten with the new MFNs on the destination.
>
> This change resulted in writing PTEs with MFNs if _PAGE_NUMA was set
> but _PAGE_PRESENT was clear. These PTEs would be migrated as-is which
> would result in unexpected behaviour in the destination domain.
> Either a) the MFN would be translated to the wrong PFN/page; b)
> setting the _PAGE_PRESENT bit would clear the PTE because the MFN is
> no longer owned by the domain; or c) the present bit would not get
> set.
>
> Symptoms include "Bad page" reports when unmapping after migrating a
> domain.
>
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Steven Noonan <steven@xxxxxxxxxxxxxx>
> Cc: Elena Ufimtseva <ufimtseva@xxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> [3.12+]
> ---

Regards,

  David

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