linux-kernel - Re: [PATCH block/for-4.0-fixes] writeback: add missing INITIAL_JIFFIES init in global_update

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Message-ID: <20150304153050.GA1249@quack.suse.cz>
Date:	Wed, 4 Mar 2015 16:30:50 +0100
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH block/for-4.0-fixes] writeback: add missing
 INITIAL_JIFFIES init in global_update_bandwidth()

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@...e.cz>

								Honza

> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Wu Fengguang <fengguang.wu@...el.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: stable@...r.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@...e.cz>
SUSE Labs, CR
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