[PATCH block/for-4.0-fixes] writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

[PATCH block/for-4.0-fixes] writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

[Tejun Heo]

global_update_bandwidth() uses static variable update_time as the
timestamp for the last update but forgets to initialize it to
INITIALIZE_JIFFIES.

This means that global_dirty_limit will be 5 mins into the future on
32bit and some large amount jiffies into the past on 64bit.  This
isn't critical as the only effect is that global_dirty_limit won't be
updated for the first 5 mins after booting on 32bit machines,
especially given the auxiliary nature of global_dirty_limit's role -
protecting against global dirty threshold's sudden dips; however, it
does lead to unintended suboptimal behavior.  Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: stable@vger.kernel.org
---
 mm/page-writeback.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -922,7 +922,7 @@ static void global_update_bandwidth(unsi
 				    unsigned long now)
 {
 	static DEFINE_SPINLOCK(dirty_lock);
-	static unsigned long update_time;
+	static unsigned long update_time = INITIAL_JIFFIES;
 
 	/*
 	 * check locklessly first to optimize away locking for the most time

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Re: [PATCH block/for-4.0-fixes] writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

[Jan Kara]

On Wed 04-03-15 10:22:43, Tejun Heo wrote:
> global_update_bandwidth() uses static variable update_time as the
> timestamp for the last update but forgets to initialize it to
> INITIALIZE_JIFFIES.
> 
> This means that global_dirty_limit will be 5 mins into the future on
> 32bit and some large amount jiffies into the past on 64bit.  This
> isn't critical as the only effect is that global_dirty_limit won't be
> updated for the first 5 mins after booting on 32bit machines,
> especially given the auxiliary nature of global_dirty_limit's role -
> protecting against global dirty threshold's sudden dips; however, it
> does lead to unintended suboptimal behavior.  Fix it.
  Looks good. You can add:
Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Wu Fengguang <fengguang.wu@intel.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: stable@vger.kernel.org
> ---
>  mm/page-writeback.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -922,7 +922,7 @@ static void global_update_bandwidth(unsi
>  				    unsigned long now)
>  {
>  	static DEFINE_SPINLOCK(dirty_lock);
> -	static unsigned long update_time;
> +	static unsigned long update_time = INITIAL_JIFFIES;
>  
>  	/*
>  	 * check locklessly first to optimize away locking for the most time
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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Re: [PATCH block/for-4.0-fixes] writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

[Jens Axboe]

On 03/04/2015 08:30 AM, Jan Kara wrote:
> On Wed 04-03-15 10:22:43, Tejun Heo wrote:
>> global_update_bandwidth() uses static variable update_time as the
>> timestamp for the last update but forgets to initialize it to
>> INITIALIZE_JIFFIES.
>>
>> This means that global_dirty_limit will be 5 mins into the future on
>> 32bit and some large amount jiffies into the past on 64bit.  This
>> isn't critical as the only effect is that global_dirty_limit won't be
>> updated for the first 5 mins after booting on 32bit machines,
>> especially given the auxiliary nature of global_dirty_limit's role -
>> protecting against global dirty threshold's sudden dips; however, it
>> does lead to unintended suboptimal behavior.  Fix it.
>    Looks good. You can add:
> Reviewed-by: Jan Kara <jack@suse.cz>

We should add that it fixes c42843f2f0bbc (from 2011!) as well.

-- 
Jens Axboe

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[PATCH block/for-4.0-fixes] writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

[Tejun Heo]

Subject: writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

global_update_bandwidth() uses static variable update_time as the
timestamp for the last update but forgets to initialize it to
INITIALIZE_JIFFIES.

This means that global_dirty_limit will be 5 mins into the future on
32bit and some large amount jiffies into the past on 64bit.  This
isn't critical as the only effect is that global_dirty_limit won't be
updated for the first 5 mins after booting on 32bit machines,
especially given the auxiliary nature of global_dirty_limit's role -
protecting against global dirty threshold's sudden dips; however, it
does lead to unintended suboptimal behavior.  Fix it.

Fixes: c42843f2f0bb ("writeback: introduce smoothed global dirty limit")
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: stable@vger.kernel.org
---
Added the "fixes" tag.  Jens, can you please route this one?

Thanks.

 mm/page-writeback.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -922,7 +922,7 @@ static void global_update_bandwidth(unsi
 				    unsigned long now)
 {
 	static DEFINE_SPINLOCK(dirty_lock);
-	static unsigned long update_time;
+	static unsigned long update_time = INITIAL_JIFFIES;
 
 	/*
 	 * check locklessly first to optimize away locking for the most time

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Re: [PATCH block/for-4.0-fixes] writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

[Jens Axboe]

On 03/04/2015 08:37 AM, Tejun Heo wrote:
> Subject: writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()
>
> global_update_bandwidth() uses static variable update_time as the
> timestamp for the last update but forgets to initialize it to
> INITIALIZE_JIFFIES.
>
> This means that global_dirty_limit will be 5 mins into the future on
> 32bit and some large amount jiffies into the past on 64bit.  This
> isn't critical as the only effect is that global_dirty_limit won't be
> updated for the first 5 mins after booting on 32bit machines,
> especially given the auxiliary nature of global_dirty_limit's role -
> protecting against global dirty threshold's sudden dips; however, it
> does lead to unintended suboptimal behavior.  Fix it.
>
> Fixes: c42843f2f0bb ("writeback: introduce smoothed global dirty limit")
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Acked-by: Jan Kara <jack@suse.cz>
> Cc: Wu Fengguang <fengguang.wu@intel.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: stable@vger.kernel.org
> ---
> Added the "fixes" tag.  Jens, can you please route this one?

Yup will do, thanks Tejun.

-- 
Jens Axboe

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