linux-kernel - Re: [PATCH -mm] page-writeback: fine-grained dirty_ratio and dirty_background

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Date:	Fri, 10 Oct 2008 15:13:19 +0200
From:	Andrea Righi <righi.andrea@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	balbir@...ux.vnet.ibm.com, Michael Rubin <mrubin@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, menage@...gle.com,
	dave@...ux.vnet.ibm.com, chlunde@...g.uio.no, dpshah@...gle.com,
	eric.rannaud@...il.com, fernando@....ntt.co.jp, agk@...rceware.org,
	m.innocenti@...eca.it, s-uchida@...jp.nec.com, ryov@...inux.co.jp,
	matt@...ehost.com, dradford@...ehost.com,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm] page-writeback: fine-grained dirty_ratio and dirty_background_ratio

Andrea Righi wrote:
> KAMEZAWA Hiroyuki wrote:
>> <snip>
>>
>>> -int dirty_background_ratio = 5;
>>> +int dirty_background_ratio = 5 * PERCENT_PCM;
>>>  
>>>  /*
>>>   * free highmem will not be subtracted from the total free memory
>>> @@ -77,7 +77,7 @@ int vm_highmem_is_dirtyable;
>>>  /*
>>>   * The generator of dirty data starts writeback at this percentage
>>>   */
>>> -int vm_dirty_ratio = 10;
>>> +int vm_dirty_ratio = 10 * PERCENT_PCM;
>>>  
>>>  /*
>>>   * The interval between `kupdate'-style writebacks, in jiffies
>>> @@ -135,7 +135,8 @@ static int calc_period_shift(void)
>>>  {
>>>  	unsigned long dirty_total;
>>>  
>>> -	dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) / 100;
>>> +	dirty_total = (vm_dirty_ratio * determine_dirtyable_memory())
>>> +			/ ONE_HUNDRED_PCM;
>>>  	return 2 + ilog2(dirty_total - 1);
>>>  }
>>>  
>> I wonder...isn't this overflow in 32bit system ?
> 
> Correct! the worst case is (in pages):
> 
> 4GB = 100,000 * determine_dirtyable_memory()
> 
> that means 42950 pages (~168MB) of dirtyable memory is enough to overflow :(.
> Using an u64 for dirty_total should resolve.
> 
> Delta patch is below.
> 
> Unfortunately I have all 64-bit machines right now. Maybe tomorrow I'll
> be able to get a 32-bit box, if someone doesn't test this before.
> 
> Thanks!
> -Andrea

I've been able to quickly resolve creating a 1GB mem i386 VM with kvm. :)

Everything seems to work fine and with the following fix it doesn't overflow.

-Andrea


> 
> ---
> Subject: fix overflow in 32-bit systems using fine-grained dirty_ratio
> 
> Signed-off-by: Andrea Righi <righi.andrea@...il.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  mm/page-writeback.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 6bc8c9b..29913e5 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -133,7 +133,7 @@ static struct prop_descriptor vm_dirties;
>   */
>  static int calc_period_shift(void)
>  {
> -	unsigned long dirty_total;
> +	u64 dirty_total;
>  
>  	dirty_total = (vm_dirty_ratio * determine_dirtyable_memory())
>  			/ ONE_HUNDRED_PCM;
--
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