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>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d42a8a83aaf11e49381f5096d8dd04a8554c3b43..b034f79deb0e837d011c94547d9b3b53b8fa45f3 100644 (file)
--- 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);