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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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      Last update: 2012-08-20 07:01    [W:0.035 / U:30.440 seconds]
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