linux-kernel - Re: [Xen-devel] [PATCH v3 1/2] xen/balloon: set a mapping for ballooned out pages

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Date:	Wed, 24 Jul 2013 18:37:35 +0100
From:	David Vrabel <dvrabel@...tab.net>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	xen-devel@...ts.xensource.com, ian.campbell@...rix.com,
	linux-kernel@...r.kernel.org, alex@...x.org.uk,
	dcrisan@...xiant.com
Subject: Re: [Xen-devel] [PATCH v3 1/2] xen/balloon: set a mapping for ballooned
 out pages

On 23/07/13 18:27, Stefano Stabellini wrote:
> Currently ballooned out pages are mapped to 0 and have INVALID_P2M_ENTRY
> in the p2m. These ballooned out pages are used to map foreign grants
> by gntdev and blkback (see alloc_xenballooned_pages).
> 
> Allocate a page per cpu and map all the ballooned out pages to the
> corresponding mfn. Set the p2m accordingly. This way reading from a
> ballooned out page won't cause a kernel crash (see
> http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html).

Reviewed-by: David Vrabel <david.vrabel@...rix.com>

A number of users of DEFINE_PER_CPU() initialize it with
for_each_possible_cpu() without registering a cpu notifier, so I think
there is no risk that offlining a CPU clears its per-cpu data and the
code as-is is fine.

David
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