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

To: <jake.wires@xxxxxxxxxx>,<dmeyer@xxxxxxxxx>
Subject: [Xen-devel] need for ClearPageReserved() in blktap2 code
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Mon, 09 Aug 2010 16:47:28 +0100
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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

Thanks for letting my know your thoughts on this,

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