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)

[Michal Hocko <mhocko@suse.com>]

On Tue 28-11-17 15:52:50, Yafang Shao wrote:
> 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.

You should discuss this with IO people (make sure to CC Tejun). I do not
see _why_ this is unreasonable to be honest. Anyway I still maintain
my position on the warning which is just calling for false positives
without any great advantage. So whatever you decide to change in the
ratio calculation I do not think we should add the warning back.
-- 
Michal Hocko
SUSE Labs

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