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    Subject[PATCH 3.10 49/54] writeback: fix negative bdi max pause
    Date
    3.10-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Fengguang Wu <fengguang.wu@intel.com>

    commit e3b6c655b91e01a1dade056cfa358581b47a5351 upstream.

    Toralf runs trinity on UML/i386. After some time it hangs and the last
    message line is

    BUG: soft lockup - CPU#0 stuck for 22s! [trinity-child0:1521]

    It's found that pages_dirtied becomes very large. More than 1000000000
    pages in this case:

    period = HZ * pages_dirtied / task_ratelimit;
    BUG_ON(pages_dirtied > 2000000000);
    BUG_ON(pages_dirtied > 1000000000); <---------

    UML debug printf shows that we got negative pause here:

    ick: pause : -984
    ick: pages_dirtied : 0
    ick: task_ratelimit: 0

    pause:
    + if (pause < 0) {
    + extern int printf(char *, ...);
    + printf("ick : pause : %li\n", pause);
    + printf("ick: pages_dirtied : %lu\n", pages_dirtied);
    + printf("ick: task_ratelimit: %lu\n", task_ratelimit);
    + BUG_ON(1);
    + }
    trace_balance_dirty_pages(bdi,

    Since pause is bounded by [min_pause, max_pause] where min_pause is also
    bounded by max_pause. It's suspected and demonstrated that the
    max_pause calculation goes wrong:

    ick: pause : -717
    ick: min_pause : -177
    ick: max_pause : -717
    ick: pages_dirtied : 14
    ick: task_ratelimit: 0

    The problem lies in the two "long = unsigned long" assignments in
    bdi_max_pause() which might go negative if the highest bit is 1, and the
    min_t(long, ...) check failed to protect it falling under 0. Fix all of
    them by using "unsigned long" throughout the function.

    Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
    Reported-by: Toralf Förster <toralf.foerster@gmx.de>
    Tested-by: Toralf Förster <toralf.foerster@gmx.de>
    Reviewed-by: Jan Kara <jack@suse.cz>
    Cc: Richard Weinberger <richard@nod.at>
    Cc: Geert Uytterhoeven <geert@linux-m68k.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/page-writeback.c | 10 +++++-----
    1 file changed, 5 insertions(+), 5 deletions(-)

    --- a/mm/page-writeback.c
    +++ b/mm/page-writeback.c
    @@ -1104,11 +1104,11 @@ static unsigned long dirty_poll_interval
    return 1;
    }

    -static long bdi_max_pause(struct backing_dev_info *bdi,
    - unsigned long bdi_dirty)
    +static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
    + unsigned long bdi_dirty)
    {
    - long bw = bdi->avg_write_bandwidth;
    - long t;
    + unsigned long bw = bdi->avg_write_bandwidth;
    + unsigned long t;

    /*
    * Limit pause time for small memory systems. If sleeping for too long
    @@ -1120,7 +1120,7 @@ static long bdi_max_pause(struct backing
    t = bdi_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
    t++;

    - return min_t(long, t, MAX_PAUSE);
    + return min_t(unsigned long, t, MAX_PAUSE);
    }

    static long bdi_min_pause(struct backing_dev_info *bdi,

    --
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