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


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