linux-kernel - Re: [patch] mm, writeback: prevent race when calculating dirty limits

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Message-ID: <20140718065444.GA21453@dhcp22.suse.cz>
Date:	Fri, 18 Jul 2014 08:54:44 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>, stable@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, writeback: prevent race when calculating dirty limits

On Wed 16-07-14 17:36:49, David Rientjes wrote:
> Setting vm_dirty_bytes and dirty_background_bytes is not protected by any 
> serialization.
> 
> Therefore, it's possible for either variable to change value after the 
> test in global_dirty_limits() to determine whether available_memory needs 
> to be initialized or not.
> 
> Always ensure that available_memory is properly initialized.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: David Rientjes <rientjes@...gle.com>

Makes sense to me
Acked-by: Michal Hocko <mhocko@...e.cz>

> ---
>  mm/page-writeback.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -261,14 +261,11 @@ static unsigned long global_dirtyable_memory(void)
>   */
>  void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
>  {
> +	const unsigned long available_memory = global_dirtyable_memory();
>  	unsigned long background;
>  	unsigned long dirty;
> -	unsigned long uninitialized_var(available_memory);
>  	struct task_struct *tsk;
>  
> -	if (!vm_dirty_bytes || !dirty_background_bytes)
> -		available_memory = global_dirtyable_memory();
> -
>  	if (vm_dirty_bytes)
>  		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
>  	else

-- 
Michal Hocko
SUSE Labs
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