[patch] mm, writeback: prevent race when calculating dirty limits

[patch] mm, writeback: prevent race when calculating dirty limits

[David Rientjes]

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@vger.kernel.org
Signed-off-by: David Rientjes <rientjes@google.com>
---
 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

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

[Johannes Weiner]

On Wed, Jul 16, 2014 at 05:36:49PM -0700, 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@vger.kernel.org
> Signed-off-by: David Rientjes <rientjes@google.com>

Any such race should be barely noticable to the user, so I assume you
realized this while looking at the code?  The patch looks good, but I
don't see that it's stable material.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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

[Michal Hocko]

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@vger.kernel.org
> Signed-off-by: David Rientjes <rientjes@google.com>

Makes sense to me
Acked-by: Michal Hocko <mhocko@suse.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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