[PATCH 2/3] xen/p2m: Reserve 4MB of _brk space for P2M leafs when populating back.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

When we release pages back during bootup:

Freeing  9d-100 pfn range: 99 pages freed
Freeing  9cf36-9d0d2 pfn range: 412 pages freed
Freeing  9f6bd-9f6bf pfn range: 2 pages freed
Freeing  9f714-9f7bf pfn range: 171 pages freed
Freeing  9f7e0-9f7ff pfn range: 31 pages freed
Freeing  9f800-100000 pfn range: 395264 pages freed
Released 395979 pages of unused memory

We then try to populate those pages back. In the P2M tree however
the space for those leafs must be reserved - as such we use extend_brk.
We reserve 4MB of _brk space, which means we can fit over
524288 PFNs - which should be more than enough to cover. If we do
run over that number, the code is smart enough to stop and not
allocate anymore leafs.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 arch/x86/xen/p2m.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 1658af0..0aa61ef 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -194,6 +194,12 @@ RESERVE_BRK(p2m_mid_mfn, PAGE_SIZE * (MAX_DOMAIN_PAGES / (P2M_PER_PAGE * P2M_MID
  * boundary violation will require three middle nodes. */
 RESERVE_BRK(p2m_mid_identity, PAGE_SIZE * 2 * 3);
 
+/* When we populate back during bootup, the amount of pages can vary. The
+ * max we have is seen is 395979, but that does not mean it can't be more.
+ * So reserve enough for 524288 pages (2G of I/O and E820 holes). Note:
+ * We are smart enough to stop exhausting the _brk if we are close to
+ * exhaustion. */
+RESERVE_BRK(p2m_populated, PMD_SIZE * 2);
 static inline unsigned p2m_top_index(unsigned long pfn)
 {
 	BUG_ON(pfn >= MAX_P2M_PFN);
-- 
1.7.7.6