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Subject[ 07/46] xen/p2m: Reserve 8MB of _brk space for P2M leafs when populating back.
From: Greg KH <>

3.5-stable review patch. If anyone has any objections, please let me know.


From: Konrad Rzeszutek Wilk <>

commit 5bc6f9888db5739abfa0cae279b4b442e4db8049 upstream.

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.

Without this, on certain compilation of the kernel we would hit:

(XEN) domain_crash_sync called from entry.S
(XEN) CPU: 0
(XEN) RIP: e033:[<ffffffff818aad3b>]
(XEN) RFLAGS: 0000000000000206 EM: 1 CONTEXT: pv guest
(XEN) rax: ffffffff81a7c000 rbx: 000000000000003d rcx: 0000000000001000
(XEN) rdx: ffffffff81a7b000 rsi: 0000000000001000 rdi: 0000000000001000
(XEN) rbp: ffffffff81801cd8 rsp: ffffffff81801c98 r8: 0000000000100000
(XEN) r9: ffffffff81a7a000 r10: 0000000000000001 r11: 0000000000000003
(XEN) r12: 0000000000000004 r13: 0000000000000004 r14: 000000000000003d
(XEN) r15: 00000000000001e8 cr0: 000000008005003b cr4: 00000000000006f0
(XEN) cr3: 0000000125803000 cr2: 0000000000000000
(XEN) ds: 0000 es: 0000 fs: 0000 gs: 0000 ss: e02b cs: e033
(XEN) Guest stack trace from rsp=ffffffff81801c98:

.. which is extend_brk hitting a BUG_ON.

Interestingly enough, most of the time we are not going to hit this
b/c the _brk space is quite large (v3.5):
ffffffff81a25000 B __brk_base
ffffffff81e43000 B __brk_limit
= ~4MB.

vs earlier kernels (with this back-ported), the space is smaller:
ffffffff81a25000 B __brk_base
ffffffff81a7b000 B __brk_limit
= 344 kBytes.

where we would certainly hit this and hit extend_brk.

Note that git commit c3d93f880197953f86ab90d9da4744e926b38e33
(xen: populate correct number of pages when across mem boundary (v2))
exposed this bug).

[v1: Made it 8MB of _brk space instead of 4MB per Jan's suggestion]

Signed-off-by: Konrad Rzeszutek Wilk <>
Signed-off-by: Greg Kroah-Hartman <>

arch/x86/xen/p2m.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -194,6 +194,11 @@ RESERVE_BRK(p2m_mid_mfn, PAGE_SIZE * (MA
* 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);

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