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    Subject[PATCH] mm: make nr_scan_try_batch() more safe on races
    nr_scan_try_batch() can be called concurrently on the same zone
    and the non-atomic calculations can go wrong. This is not a big
    problem as long as the errors are small and won't impact the
    balanced zone aging noticeably.

    @nr_to_scan could be much larger values than @swap_cluster_max.
    So don't store such large values to *nr_saved_scan directly,
    which helps reducing possible errors on races.

    CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    mm/vmscan.c | 9 ++++-----
    1 file changed, 4 insertions(+), 5 deletions(-)

    --- linux.orig/mm/vmscan.c 2009-08-20 10:46:30.000000000 +0800
    +++ linux/mm/vmscan.c 2009-08-20 10:49:36.000000000 +0800
    @@ -1496,15 +1496,14 @@ static unsigned long nr_scan_try_batch(u
    unsigned long *nr_saved_scan,
    unsigned long swap_cluster_max)
    {
    - unsigned long nr;
    -
    - *nr_saved_scan += nr_to_scan;
    - nr = *nr_saved_scan;
    + unsigned long nr = *nr_saved_scan + nr_to_scan;

    if (nr >= swap_cluster_max)
    *nr_saved_scan = 0;
    - else
    + else {
    + *nr_saved_scan = nr;
    nr = 0;
    + }

    return nr;
    }

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