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    Subject[PATCH 3.4 170/172] writeback: use |1 instead of +1 to protect against div by zero
    Date
    From: Tejun Heo <tj@kernel.org>

    3.4.108-rc1 review patch. If anyone has any objections, please let me know.

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


    commit 464d1387acb94dc43ba772b35242345e3d2ead1b upstream.

    mm/page-writeback.c has several places where 1 is added to the divisor
    to prevent division by zero exceptions; however, if the original
    divisor is equivalent to -1, adding 1 leads to division by zero.

    There are three places where +1 is used for this purpose - one in
    pos_ratio_polynom() and two in bdi_position_ratio(). The second one
    in bdi_position_ratio() actually triggered div-by-zero oops on a
    machine running a 3.10 kernel. The divisor is

    x_intercept - bdi_setpoint + 1 == span + 1

    span is confirmed to be (u32)-1. It isn't clear how it ended up that
    but it could be from write bandwidth calculation underflow fixed by
    c72efb658f7c ("writeback: fix possible underflow in write bandwidth
    calculation").

    At any rate, +1 isn't a proper protection against div-by-zero. This
    patch converts all +1 protections to |1. Note that
    bdi_update_dirty_ratelimit() was already using |1 before this patch.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Reviewed-by: Jan Kara <jack@suse.cz>
    Signed-off-by: Jens Axboe <axboe@fb.com>
    [lizf: Backported to 3.4: drop other two changes as there's only one
    such statment in 3.4]
    Signed-off-by: Zefan Li <lizefan@huawei.com>
    ---
    mm/page-writeback.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/mm/page-writeback.c b/mm/page-writeback.c
    index 25aaa31..ecd1c4c 100644
    --- a/mm/page-writeback.c
    +++ b/mm/page-writeback.c
    @@ -716,7 +716,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
    * scale global setpoint to bdi's:
    * bdi_setpoint = setpoint * bdi_thresh / thresh
    */
    - x = div_u64((u64)bdi_thresh << 16, thresh + 1);
    + x = div_u64((u64)bdi_thresh << 16, thresh | 1);
    bdi_setpoint = setpoint * (u64)x >> 16;
    /*
    * Use span=(8*write_bw) in single bdi case as indicated by
    --
    1.9.1


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