Re: [patch 2/2] mm: add dirty_background_bytes and dirty_bytes sysctls

[Randy Dunlap <randy.dunlap@oracle.com>]

Andrew Morton wrote:
> On Sun, 23 Nov 2008 10:55:16 -0800 (PST)
> David Rientjes <rientjes@google.com> wrote:
> 
>> This change introduces two new sysctls to /proc/sys/vm:
>> dirty_background_bytes and dirty_bytes.
>>
>>
>> ...
>>
>> @@ -365,23 +429,29 @@ void
>>  get_dirty_limits(unsigned long *pbackground, unsigned long *pdirty,
>>  		 unsigned long *pbdi_dirty, struct backing_dev_info *bdi)
>>  {
>> -	int background_ratio;		/* Percentages */
>> -	int dirty_ratio;
>>  	unsigned long background;
>>  	unsigned long dirty;
>>  	unsigned long available_memory = determine_dirtyable_memory();
>>  	struct task_struct *tsk;
>>  
>> -	dirty_ratio = vm_dirty_ratio;
>> -	if (dirty_ratio < 5)
>> -		dirty_ratio = 5;
>> +	if (vm_dirty_bytes)
>> +		dirty = (vm_dirty_bytes + PAGE_SIZE) / PAGE_SIZE;
> 
> It would be conventional to use DIV_ROUND_UP() here.
> 
>> +	else {
>> +		int dirty_ratio;
> 
> hm, I wonder why vm_dirty_ratio has a signed type.  
> 
>> -	background_ratio = dirty_background_ratio;
>> -	if (background_ratio >= dirty_ratio)
>> -		background_ratio = dirty_ratio / 2;
>> +		dirty_ratio = vm_dirty_ratio;
>> +		if (dirty_ratio < 5)
>> +			dirty_ratio = 5;
>> +		dirty = (dirty_ratio * available_memory) / 100;
>> +	}
>> +
>> +	if (dirty_background_bytes)
>> +		background = (dirty_background_bytes + PAGE_SIZE) / PAGE_SIZE;
> 
> DIV_ROUND_UP()?

Note that DIV_ROUND_UP() would make that

		background = (dirty_background_byte + PAGE_SIZE - 1) / PAGE_SIZE;

but that's usually what is needed/intended.

>> +	else
>> +		background = (dirty_background_ratio * available_memory) / 100;
>>  
>> -	background = (background_ratio * available_memory) / 100;
>> -	dirty = (dirty_ratio * available_memory) / 100;
>> +	if (background >= dirty)
>> +		background = dirty / 2;
>>  	tsk = current;
>>  	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
>>  		background += background / 4;

-- 
~Randy