path: root/mm
diff options
authorTejun Heo <tj@kernel.org>2015-03-23 00:18:48 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2015-04-19 10:10:48 +0200
commite58126f57083a3160c2883d7cf22c38ed1a75f58 (patch)
tree8e54546f917710a72bb4c8cead4d5382b2cf6dd9 /mm
parentf16678367dae3cd3da3fd0c93b13bad7ad8d301b (diff)
writeback: fix possible underflow in write bandwidth calculation
commit c72efb658f7c8b27ca3d0efb5cfd5ded9fcac89e upstream. 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") Signed-off-by: Jens Axboe <axboe@fb.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'mm')
1 files changed, 4 insertions, 1 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d42a8a83aaf1..b034f79deb0e 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -793,8 +793,11 @@ static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
* 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);