RE: [Xen-devel] struct page_info

["Cihula, Joseph" <joseph.cihula@xxxxxxxxx>]

> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On
> Behalf Of Jan Beulich
> Sent: Thursday, April 23, 2009 11:51 PM
>
> >>> 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.

Would:
        ((page->count_info & PGC_count_mask) != 0 || page->count_info == 0)
definitively indicate if a page was in use?

Joe

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