xen/p2m: Reuse existing P2M leafs if they are filled with 1:1 PFNs or INVALID.
If P2M leaf is completly packed with INVALID_P2M_ENTRY or with 1:1 PFNs (so IDENTITY_FRAME type PFNs), we can swap the P2M leaf with either a p2m_missing or p2m_identity respectively. The old page (which was created via extend_brk or was grafted on from the mfn_list) can be re-used for setting new PFNs. This also means we can remove git commit: 5bc6f988 xen/p2m: Reserve 8MB of _brk space for P2M leafs when populating back which tried to fix this. and make the amount that is required to be reserved much smaller. CC: stable@vger.kernel.org # for 3.5 only. Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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