On Mon 27-11-17 16:32:42, Yafang Shao wrote:
> 2017-11-27 16:29 GMT+08:00 Yafang Shao <laoar.shao@xxxxxxxxx>:
> > 2017-11-27 16:21 GMT+08:00 Michal Hocko <mhocko@xxxxxxxx>:
> >> On Mon 27-11-17 16:06:50, Yafang Shao wrote:
> >>> +cc fcicq
> >> [...]
> >>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> >>> index 8a15511..6c5c018 100644
> >>> --- a/mm/page-writeback.c
> >>> +++ b/mm/page-writeback.c
> >>> @@ -377,7 +377,16 @@ static unsigned long global_dirtyable_memory(void)
> >>> if (!vm_highmem_is_dirtyable)
> >>> x -= highmem_dirtyable_memory(x);
> >>>
> >>> - return x + 1; /* Ensure that we never return 0 */
> >>> + /*
> >>> + * - Why 100 ?
> >>> + * - Because the return value will be used by dirty ratio and
> >>> + * dirty background ratio to calculate dirty thresh and bg thresh,
> >>> + * so if the return value is two small, the thresh value maybe
> >>> + * calculated to 0.
> >>> + * As the max value of ratio is 100, so the return value is added
> >>> + * by 100 here.
> >>> + */
> >>> + return x + 100;
> >>
> >> No. We should just revert 0f6d24f87856 ("mm/page-writeback.c: print a
> >> warning if the vm dirtiness settings are illogical") because it is of a
> >> dubious value and it causes problems. I am not even sure why it got
> >> merged. It doesn't have any ack or review and I remember objecting to
> >> the patch previously as pointless.
> >> --
> >
> > It is reviewed and merged by Andrew.
> >
> > From Andrew:
> >> I think this means that a script which alters both dirty_bytes and
> > dirty_background_bytes must alter dirty_background_bytes first if they
> > are being decreased and must alter dirty_bytes first if they are being
> > increased. Or something like that.
> >
> > It will help us to find the error if we don't change these values like this.
> >
>
> And actually it help us find another issue that when availble_memroy
> is too small, the thresh and bg_thresh will be 0, that's absolutely
> wrong.
Why is it wrong?
--
Michal Hocko
SUSE Labs
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