On 04/30/2014 04:13 PM, Andrew Morton wrote:
On Wed, 30 Apr 2014 16:02:18 -0400 Rik van Riel <r...@redhat.com> wrote:
I believe this should do the trick.
---8<---
Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
It is possible for "limit - setpoint + 1" to equal zero, leading to a
divide by zero error. Blindly adding 1 to "limit - setpoint" is not
working, so we need to actually test the divisor before calling div64.
Changelog is a bit stale.
Will update.
-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)
{
+ unsigned long divisor;
long long pos_ratio;
long x;
- x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
- limit - setpoint + 1);
+ divisor = limit - setpoint;
+ if (!divisor)
+ divisor = 1; /* Avoid div-by-zero */
This was a consequence of 64->32 truncation and it can't happen any
more, can it?
That is a good question. Looking at the code some more,
I guess it may indeed be exclusively due to the truncation,
and we can go back to the older code, just with the fully
64 bit divide functions...
Good thing Masayoshi-san has a reproducer :)
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