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authorTejun Heo <tj@kernel.org>2015-03-23 00:18:48 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2015-04-19 10:11:07 +0200
commitfbcec5461485e43b7a3324aa06b5a22e1b872e41 (patch)
tree3b62e991a089d993a16bb28bc7f433cfd7e9a6ba
parentd8b274f40cb5cf5ef5cd18f9830a60233011b631 (diff)
downloadlwn-fbcec5461485e43b7a3324aa06b5a22e1b872e41.tar.gz
lwn-fbcec5461485e43b7a3324aa06b5a22e1b872e41.zip
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>
-rw-r--r--mm/page-writeback.c5
1 files changed, 4 insertions, 1 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d3653325a255..51d8d15f48d7 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -878,8 +878,11 @@ 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);