On Tue 28-11-17 15:52:50, Yafang Shao wrote: > 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. 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
