| Message ID | 1428569906-29834-1-git-send-email-luis.henriques@canonical.com |
|---|---|
| State | New |
| Headers | show |
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 001d86441639..6a3348761648 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -860,8 +860,11 @@ 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);
This is a note to let you know that I have just added a patch titled writeback: fix possible underflow in write bandwidth calculation to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree which can be found at: http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue This patch is scheduled to be released in version 3.16.7-ckt10. If you, or anyone else, feels it should not be added to this tree, please reply to this email. For more information about the 3.16.y-ckt tree, see https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable Thanks. -Luis ------ From c6973024cb4ab326e1b16092e126b8c98551f521 Mon Sep 17 00:00:00 2001 From: Tejun Heo <tj@kernel.org> Date: Mon, 23 Mar 2015 00:18:48 -0400 Subject: 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 nunderflow 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: Luis Henriques <luis.henriques@canonical.com> --- mm/page-writeback.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)