This is a note to let you know that I've just added the patch titled
writeback: fix possible underflow in write bandwidth calculation
to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
writeback-fix-possible-underflow-in-write-bandwidth-calculation.patch
and it can be found in the queue-3.14 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From c72efb658f7c8b27ca3d0efb5cfd5ded9fcac89e Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@xxxxxxxxxx>
Date: Mon, 23 Mar 2015 00:18:48 -0400
Subject: writeback: fix possible underflow in write bandwidth calculation
From: Tejun Heo <tj@xxxxxxxxxx>
commit c72efb658f7c8b27ca3d0efb5cfd5ded9fcac89e upstream.
>From 1ebf33901ecc75d9496862dceb1ef0377980587c Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@xxxxxxxxxx>
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@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Cc: Greg Thelen <gthelen@xxxxxxxxxx>
Fixes: 2f800fbd777b ("writeback: fix dirtied pages accounting on redirty")
Signed-off-by: Jens Axboe <axboe@xxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
mm/page-writeback.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -878,8 +878,11 @@ static void bdi_update_write_bandwidth(s
* bw * elapsed + write_bandwidth * (period - elapsed)
* write_bandwidth = ---------------------------------------------------
* period
+ *
+ * @written may have decreased due to account_page_redirty().
+ * Avoid underflowing @bw calculation.
*/
- bw = written - bdi->written_stamp;
+ bw = written - min(written, bdi->written_stamp);
bw *= HZ;
if (unlikely(elapsed > period)) {
do_div(bw, elapsed);
Patches currently in stable-queue which might be from tj@xxxxxxxxxx are
queue-3.14/writeback-add-missing-initial_jiffies-init-in-global_update_bandwidth.patch
queue-3.14/writeback-fix-possible-underflow-in-write-bandwidth-calculation.patch
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