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Re: [PATCH v2 2/3] xen/mm: do not assign pages to a domain until they are scrubbed
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 26 Mar 2026 12:51:15 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 26 Mar 2026 11:51:19 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 26.03.2026 09:51, Roger Pau Monne wrote:
> Assigning pages to a domain make them the possible target of hypercalls
> like XENMEM_decrease_reservation ahead of such pages being scrubbed in
> populate_physmap() when the guest is running in PV mode. This might allow
> pages to be freed ahead of being scrubbed for example, as a stubdomain
> already running could target them by guessing their MFNs. It's also
> possible other action could set the page type ahead of scrubbing, which
> would be problematic.
>
> Prevent the pages pending scrub from being assigned to the domain, and only
> do the assign once the scrubbing has finished. This has the disadvantage
> that the allocated pages will be removed from the free pool, but not yet
> accounted towards the domain consumed page quota. However there can only
> be one stashed page in that state, and it's maximum size is bounded by the
> memop-max-order option. This is not too different from the current logic,
> where assigning pages to a domain (and thus checking whether such domain
> doesn't overflow it's quota) is also done after the memory has been
> allocated and removed from the pool of free pages.
>
> Fixes: 83a784a15b47 ("xen/mm: allow deferred scrub of physmap populate
> allocated pages")
> Reported-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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