Re: [PATCH] mm: print a warning once the vm dirtiness settings is illogical

[Michal Hocko <mhocko@suse.com>]

On Mon 27-11-17 16:32:42, Yafang Shao wrote:
> 2017-11-27 16:29 GMT+08:00 Yafang Shao <laoar.shao@gmail.com>:
> > 2017-11-27 16:21 GMT+08:00 Michal Hocko <mhocko@suse.com>:
> >> 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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