linux-kernel - Re: [PATCH] mm: prevent divide error for small values of vm_dirty

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Date:	Wed, 29 Apr 2009 10:44:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrea Righi <righi.andrea@...il.com>
Cc:	David Rientjes <rientjes@...gle.com>, akpm@...ux-foundation.org,
	Dave Chinner <david@...morbit.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: prevent divide error for small values of
 vm_dirty_bytes

On Wed, 2009-04-29 at 10:29 +0200, Andrea Righi wrote:
> Avoid to set less than two pages for vm_dirty_byte: this is necessary to avoid
> potential division by 0 (like the following) in get_dirty_limits().

isn't changing the .extra1 in the sysctl table a better fix?

> Signed-off-by: Andrea Righi <righi.andrea@...il.com>
> ---
>  mm/page-writeback.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 30351f0..9d6c427 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -213,6 +213,12 @@ int dirty_bytes_handler(struct ctl_table *table, int write,
>  	int ret;
>  
>  	ret = proc_doulongvec_minmax(table, write, filp, buffer, lenp, ppos);
> +	/*
> +	 * Do not allow to set less than two pages for vm_dirty_byte: this is
> +	 * necessary to avoid division by 0 in get_dirty_limits().
> +	 */
> +	vm_dirty_bytes = max_t(typeof(vm_dirty_bytes),
> +				vm_dirty_bytes, 2 * PAGE_SIZE);
>  	if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
>  		update_completion_period();
>  		vm_dirty_ratio = 0;

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