Re: [PATCH v5] mm,writeback: fix divide by zero in pos_ratio_polynom

Masayoshi Mizuma
2014-05-08T10:17:55Z

Hi Rik,

On Wed, 30 Apr 2014 16:42:55 -0400 Rik van Riel wrote:
> On Wed, 30 Apr 2014 13:13:53 -0700
> Andrew Morton <akpm@linux-foundation.org> wrote:
>
>> This was a consequence of 64->32 truncation and it can't happen any
>> more, can it?
>
> Andrew, this is cleaner indeed :)
>
> Masayoshi-san, does the bug still happen with this version, or does
> this fix the problem?

I applied the v5 patch and the divide error did not happen when I ran the test.
Thank you for fixing it!

Thanks,
Masayoshi Mizuma

>
> ---8<---
>
> Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
>
> It is possible for "limit - setpoint + 1" to equal zero, after
> getting truncated to a 32 bit variable, and resulting in a divide
> by zero error.
>
> Using the fully 64 bit divide functions avoids this problem.
>
> Also uninline pos_ratio_polynom, at Andrew's request.
>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
> mm/page-writeback.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ef41349..a4317da 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -593,14 +593,14 @@ unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
> * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
> * => fast response on large errors; small oscillation near setpoint
> */
> -static inline long long pos_ratio_polynom(unsigned long setpoint,
> +static long long pos_ratio_polynom(unsigned long setpoint,
> unsigned long dirty,
> unsigned long limit)
> {
> long long pos_ratio;
> long x;
>
> - x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
> + x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
> limit - setpoint + 1);
> pos_ratio = x;
> pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
> @@ -842,7 +842,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
> x_intercept = bdi_setpoint + span;
>
> if (bdi_dirty < x_intercept - span / 4) {
> - pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
> + pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
> x_intercept - bdi_setpoint + 1);
> } else
> pos_ratio /= 4;
>
> --
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