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

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Date:	Fri, 10 Oct 2008 11:32:40 +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

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

---
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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