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Date: Mon, 24 Nov 2008 15:38:43 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: David Rientjes <rientjes@google.com>
Cc: peterz@infradead.org, david@fromorbit.com, cl@linux-foundation.org,
       linux-kernel@vger.kernel.org
Subject: Re: [patch 2/2] mm: add dirty_background_bytes and dirty_bytes
 sysctls
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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()?

> +	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;