2017-11-28 15:45 GMT+08:00 Michal Hocko <mhocko@xxxxxxxx>:
> On Tue 28-11-17 14:12:15, Yafang Shao wrote:
>> 2017-11-28 11:11 GMT+08:00 Yafang Shao <laoar.shao@xxxxxxxxx>:
>> > Hi Michal,
>> >
>> > What about bellow change ?
>> > It makes the function domain_dirty_limits() more clear.
>> > And the result will have a higher precision.
>> >
>> >
>> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> > index 8a15511..2b5e507 100644
>> > --- a/mm/page-writeback.c
>> > +++ b/mm/page-writeback.c
>> > @@ -397,8 +397,8 @@ static void domain_dirty_limits(struct
>> > dirty_throttle_control *dtc)
>> > unsigned long bytes = vm_dirty_bytes;
>> > unsigned long bg_bytes = dirty_background_bytes;
>> > /* convert ratios to per-PAGE_SIZE for higher precision */
>> > - unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
>> > - unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
>> > + unsigned long ratio = vm_dirty_ratio;
>> > + unsigned long bg_ratio = dirty_background_ratio;
>> > unsigned long thresh;
>> > unsigned long bg_thresh;
>> > struct task_struct *tsk;
>> > @@ -416,28 +416,33 @@ static void domain_dirty_limits(struct
>> > dirty_throttle_control *dtc)
>> > */
>> > if (bytes)
>> > ratio = min(DIV_ROUND_UP(bytes, global_avail),
>> > - PAGE_SIZE);
>> > + 100);
>> > if (bg_bytes)
>> > bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
>> > - PAGE_SIZE);
>> > + 99); /* bg_ratio should less than ratio */
>> > bytes = bg_bytes = 0;
>> > }
>>
>>
>> Errata:
>>
>> if (bytes)
>> - ratio = min(DIV_ROUND_UP(bytes, global_avail),
>> - PAGE_SIZE);
>> + ratio = min(DIV_ROUND_UP(bytes / PAGE_SIZE, global_avail),
>> + 100);
>> if (bg_bytes)
>> - bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
>> - PAGE_SIZE);
>> + bg_ratio = min(DIV_ROUND_UP(bg_bytes / PAGE_SIZE, global_avail),
>> + 100 - 1); /* bg_ratio should be less than ratio */
>> bytes = bg_bytes = 0;
>
> And you really think this makes code easier to follow? I am somehow not
> conviced...
>
There's hidden bug in the original code, because it is too complex to
clearly understand.
See bellow,
ratio = min(DIV_ROUND_UP(bytes, global_avail),
PAGE_SIZE)
Suppose the vm_dirty_bytes is set to 512M (this is a reasonable
value), and the global_avail is only 10000 pages (this is not low),
then DIV_ROUND_UP(bytes, global_avail) is 53688, which is bigger than
4096, so the ratio will be 4096.
That's unreasonable.
Thanks
Yafang
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