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    Subject[PATCH 5.2 252/413] xen: let alloc_xenballooned_pages() fail if not enough memory free
    Date
    From: Juergen Gross <jgross@suse.com>

    commit a1078e821b605813b63bf6bca414a85f804d5c66 upstream.

    Instead of trying to allocate pages with GFP_USER in
    add_ballooned_pages() check the available free memory via
    si_mem_available(). GFP_USER is far less limiting memory exhaustion
    than the test via si_mem_available().

    This will avoid dom0 running out of memory due to excessive foreign
    page mappings especially on ARM and on x86 in PVH mode, as those don't
    have a pre-ballooned area which can be used for foreign mappings.

    As the normal ballooning suffers from the same problem don't balloon
    down more than si_mem_available() pages in one iteration. At the same
    time limit the default maximum number of retries.

    This is part of XSA-300.

    Signed-off-by: Juergen Gross <jgross@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/xen/balloon.c | 16 +++++++++++++---
    1 file changed, 13 insertions(+), 3 deletions(-)

    --- a/drivers/xen/balloon.c
    +++ b/drivers/xen/balloon.c
    @@ -538,8 +538,15 @@ static void balloon_process(struct work_
    state = reserve_additional_memory();
    }

    - if (credit < 0)
    - state = decrease_reservation(-credit, GFP_BALLOON);
    + if (credit < 0) {
    + long n_pages;
    +
    + n_pages = min(-credit, si_mem_available());
    + state = decrease_reservation(n_pages, GFP_BALLOON);
    + if (state == BP_DONE && n_pages != -credit &&
    + n_pages < totalreserve_pages)
    + state = BP_EAGAIN;
    + }

    state = update_schedule(state);

    @@ -578,6 +585,9 @@ static int add_ballooned_pages(int nr_pa
    }
    }

    + if (si_mem_available() < nr_pages)
    + return -ENOMEM;
    +
    st = decrease_reservation(nr_pages, GFP_USER);
    if (st != BP_DONE)
    return -ENOMEM;
    @@ -710,7 +720,7 @@ static int __init balloon_init(void)
    balloon_stats.schedule_delay = 1;
    balloon_stats.max_schedule_delay = 32;
    balloon_stats.retry_count = 1;
    - balloon_stats.max_retry_count = RETRY_UNLIMITED;
    + balloon_stats.max_retry_count = 4;

    #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
    set_online_page_callback(&xen_online_page);

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