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.
Thanks
Yafang
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