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[Xen-devel] RE: need for ClearPageReserved() in blktap2 code

To: Jan Beulich <JBeulich@xxxxxxxxxx>, "dmeyer@xxxxxxxxx" <dmeyer@xxxxxxxxx>
Subject: [Xen-devel] RE: need for ClearPageReserved() in blktap2 code
From: Jake Wires <Jake.Wires@xxxxxxxxxx>
Date: Mon, 9 Aug 2010 10:59:22 -0700
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Thread-topic: need for ClearPageReserved() in blktap2 code

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxxxx]
> Sent: Monday, August 09, 2010 8:47 AM
> To: Jake Wires; dmeyer@xxxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: need for ClearPageReserved() in blktap2 code
> Jake, Dutch,
> while I can see the need for this on the ring pages, I have some
> difficulty understanding why it needs to be invoked on the individual
> data pages as they get removed from ring->foreign_map.map: I
> understand that SetPageReserved() when inserting them is needed
> (or at least desirable) since in the 2.6.18 and pv-ops trees the
> balloon driver doesn't guarantee pages being returned from
> alloc_empty_pages_and_page_vec() are marked reserved (which
> minimally I consider an inconsistency). In our trees, the balloon
> driver does guarantee this, but it expects the pages to also be
> returned in reserved state - which blktap2 breaks because of
> the arbitrary(?) clearing of the flag.
> The question hence really is whether that clearing is really
> necessary, i.e. whether the pages, when not in some
> ring->foreign_map.map[], validly and reasonably can (should) be
> considered non-reserved - to me it appears there's no need for
> this.

Yup, seems reasonable for these pages to remain reserved the whole time. 

> Thanks for letting my know your thoughts on this,
> Jan

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