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Date: Mon, 27 Nov 2017 09:21:12 +0100
From: Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH] mm: print a warning once the vm dirtiness settings is
 illogical
Message-ID: <20171127082112.b7elnzy24qiqze46@dhcp22.suse.cz>
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To: Yafang Shao <laoar.shao@gmail.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

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.
-- 
Michal Hocko
SUSE Labs

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