Re: [PATCH] Revert "mm/page-writeback.c: print a warning if the vm dirtiness settings are illogical" (was: Re: [PATCH] mm: print a warning once the vm dirtiness settings is illogical)

[Yafang Shao <laoar.shao@gmail.com>]

2017-11-28 15:45 GMT+08:00 Michal Hocko <mhocko@suse.com>:
> On Tue 28-11-17 14:12:15, Yafang Shao wrote:
>> 2017-11-28 11:11 GMT+08:00 Yafang Shao <laoar.shao@gmail.com>:
>> > 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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