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Re: [Xen-devel] struct page_info

To: "xen-devel" <xen-devel@xxxxxxxxxxxxxxxxxxx>, <mukesh.rathor@xxxxxxxxxx>
Subject: Re: [Xen-devel] struct page_info
From: "Jan Beulich" <jbeulich@xxxxxxxxxx>
Date: Fri, 24 Apr 2009 07:50:41 +0100
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>>> Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> 24.04.09 05:15 >>>
>Now, looking at it, the shadow page is confusing.
>       /* Page is in use: ((count_info & PGC_count_mask) != 0).  */
>       ..
>         /* Page is in use as a shadow: count_info == 0. */
>       ..
>       /* Page is on a free list: ((count_info & PGC_count_mask) == 0). */
>       ..
>If count_info == 0, and it implies shadow page, how's that different
>from an unallocated page which also would've count_info=0 I presume?

These are not if-and-only-if conditions, so you can't conclude the use of
a page from just looking at count_info; only the other way around: from
e.g. knowing the page is in use as a shadow, you'll know count_info must
be zero.

>Also, if "Page is on a free list:", I assume one of the PGC_ fields would
>be set?

Not would, but might.


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