From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: [PATCH v2 block/for-next] blk-iocost: fix divide-by-zero in transfer_surpluses() Date: Fri, 11 Sep 2020 18:40:49 -0400 Message-ID: <20200911224049.GA865564@mtj.thefacebook.com> References: <20200911170746.GG4295@mtj.thefacebook.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=YYtbWxsMj/6n0uZf9WI97Rz4VWxjIwP6xqNBdneeUNo=; b=JkeVh8VK3P/IsY0yYuXRdbFDfSKwWuZBsdVeYQRNJXfK2KGyYQA/AQG1w6u3J0hR27 xeKsvGfP60teeylfKsR7sGVXFMV23UrkcYS7IDmwba1mQ6wUDlymmXWC2YHbSd29ru18 pt2KX5E/s3IOIPopfD1+EeR9VSvm3ykan7xzxtVrzfWo4ez/vOxR+5JrJjTOwyjKrpFu TuMTwzTuMnCHWaDumPkGYbWJTCWoxq4zR3eSIPh7oON1BRWWNFipreIaAwRz8hYzfHqT 1o5fPvv/NulyfvabHbVdfiQCigpSGk1nVtsUJ2oyfjGmIk7eh233XREeAOk/JxjqdGzp 66JQ== Content-Disposition: inline In-Reply-To: Sender: linux-block-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Jens Axboe Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, cgroups@vger.kernel.org Conceptually, root_iocg->hweight_donating must be less than WEIGHT_ONE but all hweight calculations round up and thus it may end up >= WEIGHT_ONE triggering divide-by-zero and other issues. Bound the value to avoid surprises. Signed-off-by: Tejun Heo Fixes: e08d02aa5fc9 ("blk-iocost: implement Andy's method for donation weight updates") Signed-off-by: Tejun Heo --- Jens, I was flipping between doing max_t(, 1) over the whole divider and doing min_t(, WEIGHT_ONE - 1) for hweight_donating. I thought that I as testing after the last change but it obviously wasn't and the previous patch doesn't compile due to missing type argument. Can you please apply this patch instead? I can send an incremental patch if that'd be better. My apologies. Thanks. block/blk-iocost.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) diff --git a/block/blk-iocost.c b/block/blk-iocost.c index fc897bb142bcd..6e29b4dcf3566 100644 --- a/block/blk-iocost.c +++ b/block/blk-iocost.c @@ -1881,15 +1881,21 @@ static void transfer_surpluses(struct list_head *surpluses, struct ioc_now *now) /* * Calculate the global donation rate (gamma) - the rate to adjust - * non-donating budgets by. No need to use 64bit multiplication here as - * the first operand is guaranteed to be smaller than WEIGHT_ONE - * (1<<16). + * non-donating budgets by. + * + * No need to use 64bit multiplication here as the first operand is + * guaranteed to be smaller than WEIGHT_ONE (1<<16). + * + * We know that there are beneficiary nodes and the sum of the donating + * hweights can't be whole; however, due to the round-ups during hweight + * calculations, root_iocg->hweight_donating might still end up equal to + * or greater than whole. Limit the range when calculating the divider. * * gamma = (1 - t_r') / (1 - t_r) */ gamma = DIV_ROUND_UP( (WEIGHT_ONE - root_iocg->hweight_after_donation) * WEIGHT_ONE, - WEIGHT_ONE - root_iocg->hweight_donating); + WEIGHT_ONE - min_t(u32, root_iocg->hweight_donating, WEIGHT_ONE - 1)); /* * Calculate adjusted hwi, child_adjusted_sum and inuse for the inner