linux-kernel - Re: [PATCH] mm: raise MemFree by reverting percpu_pagelist

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Date:	Mon, 14 May 2012 10:51:51 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Sasha Levin <levinsasha928@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: raise MemFree by reverting percpu_pagelist_fraction
 to 0

Hi Hugh,

On 05/11/2012 11:10 PM, Hugh Dickins wrote:

> On Fri, 11 May 2012, Minchan Kim wrote:
>> On 05/11/2012 05:30 PM, Sasha Levin wrote:
>>
>>>> Commit 93278814d359 "mm: fix division by 0 in percpu_pagelist_fraction()"
>>>> mistakenly initialized percpu_pagelist_fraction to the sysctl's minimum 8,
>>>> which leaves 1/8th of memory on percpu lists (on each cpu??); but most of
>>>> us expect it to be left unset at 0 (and it's not then used as a divisor).
>>>
>>> I'm a bit confused about this, does it mean that once you set
>>> percpu_pagelist_fraction to a value above the minimum, you can no
>>> longer set it back to being 0?
>>
>>
>> Unfortunately, Yes. :(
>> It's rather awkward and need fix.
> 
> It's inelegant, but does that actually need a fix?  Has anybody asked
> for that option in the six years of percpu_pagelist_fraction?


I don't have heard about it but thing we can't reset to 0 again once we set some number to above 8 is
very strange. Sometime, someone may raise the value on /proc/sys/vm/percpu_pagelist_fraction to test it
and realized function of the knob so he want to reset it to 0 default value, again. But he couldn't.
It's very strange. :( 

> 
> Does setting percpu_pagelist_fraction to some large number perhaps
> approximate to the default behaviour of percpu_pagelist_fraction 0?


Yes. But it's not intuitive. 

> 
> I don't care very much either way - just don't want this discussion
> to divert from applying last night's fix to the default behaviour
> that most people expect.


Of course. 

It's totally from my careless review. 
Actually, I didn't find to change default value to 8 when I review the patch.
I just focused on proc_dointvec_minmax's err return value.


Shame on me. :(
Thanks for spot it.

> 
> Hugh
> 
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-- 
Kind regards,
Minchan Kim
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