xen/p2m: Reserve 8MB of _brk space for P2M leafs when populating back.

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 8MB of _brk space, which means we can fit over
1048576 PFNs - which is more than we should ever need.

[v1: Made it 8MB of _brk space instead of 4MB per Jan's suggestion]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit 99266871de5006ba7ad0bfece6bb283ede4094b9)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index ff98308d24cf403f303e180bd6eedb3cca4c1537..f95a0debadbdd24f960a30b4d66c71a8ae86413c 100644 (file)
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -194,6 +194,11 @@ 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.
+ * But some machines can have 3GB I/O holes even. So lets reserve enough
+ * for 4GB of I/O and E820 holes. */
+RESERVE_BRK(p2m_populated, PMD_SIZE * 4);
 static inline unsigned p2m_top_index(unsigned long pfn)
 {
        BUG_ON(pfn >= MAX_P2M_PFN);