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From: Yafang Shao <laoar.shao@gmail.com>
Date: Mon, 27 Nov 2017 16:32:42 +0800
Message-ID: <CALOAHbCH1JG=BmpgOwq+7W3wXuHqhXkisj+p-rPXeivTdXa7-w@mail.gmail.com>
Subject: Re: [PATCH] mm: print a warning once the vm dirtiness settings is illogical
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Sender: owner-linux-mm@kvack.org
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To: Michal Hocko <mhocko@suse.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>, Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>, Linux MM <linux-mm@kvack.org>, fcicq@fcicq.net

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.

Thanks
Yafang

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