Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932987AbbFPJG3 (ORCPT ); Tue, 16 Jun 2015 05:06:29 -0400 Received: from mail.kernel.org ([198.145.29.136]:56554 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932705AbbFPIrp (ORCPT ); Tue, 16 Jun 2015 04:47:45 -0400 From: lizf@kernel.org To: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Tejun Heo , Jens Axboe , Jan Kara , Wu Fengguang , Greg Thelen , Jens Axboe , Zefan Li Subject: [PATCH 3.4 128/172] writeback: fix possible underflow in write bandwidth calculation Date: Tue, 16 Jun 2015 16:36:22 +0800 Message-Id: <1434443826-4929-128-git-send-email-lizf@kernel.org> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1434443587-4599-1-git-send-email-lizf@kernel.org> References: <1434443587-4599-1-git-send-email-lizf@kernel.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2520 Lines: 65 From: Tejun Heo 3.4.108-rc1 review patch. If anyone has any objections, please let me know. ------------------ commit c72efb658f7c8b27ca3d0efb5cfd5ded9fcac89e upstream. >From 1ebf33901ecc75d9496862dceb1ef0377980587c Mon Sep 17 00:00:00 2001 From: Tejun Heo 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 Cc: Jens Axboe Cc: Jan Kara Cc: Wu Fengguang Cc: Greg Thelen Fixes: 2f800fbd777b ("writeback: fix dirtied pages accounting on redirty") Signed-off-by: Jens Axboe Signed-off-by: Zefan Li --- mm/page-writeback.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index b747dcd..25aaa31 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -766,8 +766,11 @@ 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); -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/