Re: [PATCH linux-block/for-linus] writeback: fix possible underflow in write bandwidth calculation

[Jens Axboe <axboe@kernel.dk>]

On 03/22/2015 10:18 PM, Tejun Heo wrote:
>  From 1ebf33901ecc75d9496862dceb1ef0377980587c Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@kernel.org>
> Date: Mon, 23 Mar 2015 00:08:19 -0400
>
> 2f800fbd777b ("writeback: fix dirtied pages accounting on redirty")
> introduced account_page_redirty() which reverts stat updates for a
> redirtied page, making BDI_DIRTIED no longer monotonically increasing.
>
> bdi_update_write_bandwidth() uses the delta in BDI_DIRTIED as the
> basis for bandwidth calculation.  While unlikely, since the above
> patch, the newer value may be lower than the recorded past value and
> underflow the bandwidth calculation leading to a wild result.
>
> Fix it by subtracing min of the old and new values when calculating
> delta.  AFAIK, there hasn't been any report of it happening but the
> resulting erratic behavior would be non-critical and temporary, so
> it's possible that the issue is happening without being reported.  The
> risk of the fix is very low, so tagged for -stable.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Wu Fengguang <fengguang.wu@intel.com>
> Cc: Greg Thelen <gthelen@google.com>
> Fixes: 2f800fbd777b ("writeback: fix dirtied pages accounting on redirty")
> Cc: stable@vger.kernel.org

Added for this round, thanks Tejun.

-- 
Jens Axboe