From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B546A1422AB for ; Sun, 22 Jun 2025 16:04:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750608296; cv=none; b=pdA4v4IdefslwZny4ZRJ3TH3d0Y2MJNZ4/5J+DBgrv4NFfsluENnGfwuMx+TMKnQpgKYpZ4v3UlO0UUhnKWcu6eGaDJdK69BTFuOl8gQjW3nichVLjTaonYb0G/OiacW29RC9dp8JoxIw9TnoCWXR5fgl98LA9xtkdjoQEC+qro= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750608296; c=relaxed/simple; bh=GwerDBqWOjiSScKzaXyqY2GyXlariUNJbjLoOZJdSiY=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=HRGXtoWjPg6LmaV7x73arsZZ6mOK+78CpwSvOROYCl1bIWiYiUk5lnkN5wVuTV0lPbY4VkAl41d7s0wc/OmQAkjrsA65DmE7tbZo2IhUN2VFHLdgF/dsCE8O1LDVaUNHck4KhMDfqTOY0mrFwMQCx0MvwJ9eEB3zR48t2Gl5bDU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=uFsz/XzA; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="uFsz/XzA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 075A8C4CEE3; Sun, 22 Jun 2025 16:04:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1750608296; bh=GwerDBqWOjiSScKzaXyqY2GyXlariUNJbjLoOZJdSiY=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=uFsz/XzAtqVvI344+B8Hl5ee7p4MwnG2OvMjZusp1xiq2U4nKH7HUerXjJIJ49nTH UBD44jOyppgmpYaAZJSDXUs36NVRlS1rFddNBJ3r6cJE8gQ50D4xG6gz0ShaTJOfs2 +TFuFuZuLK/6/C7J3nvmaX/Ki5/gqzrCBwOcb20IUooMhYMHjlKwQia+f3CvdcScaV 6x7aIvGeZn0Es2k91Kv+YmiiXZN8BKC3RLs0/ad3PlEH9F/nwsNu6bcgw18WSHkk9b y7u1Y3XUcM/wop9TDgqrwr0RBJYbtN8iInVdwRoFLBjQQRc1TdsFiOkhKp0wRPmlXS ZiDqAzTs1BJmA== From: SeongJae Park To: Honggyu Kim Cc: SeongJae Park , damon@lists.linux.dev, Andrew Morton , linux-mm@kvack.org, kernel_team@skhynix.com, Yunjeong Mun Subject: Re: [PATCH 1/3] mm/damon: do not allow creating zero size region Date: Sun, 22 Jun 2025 09:04:53 -0700 Message-Id: <20250622160453.47161-1-sj@kernel.org> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20250622120926.1712-2-honggyu.kim@sk.com> References: Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Sun, 22 Jun 2025 21:09:23 +0900 Honggyu Kim wrote: > Creating zero size region leads a divide by zero error inside > damon_get_intervals_score() as follows. > > static unsigned long damon_get_intervals_score(struct damon_ctx *c) > { > struct damon_target *t; > struct damon_region *r; > unsigned long sz_region, max_access_events = 0, access_events = 0; > unsigned long target_access_events; > unsigned long goal_bp = c->attrs.intervals_goal.access_bp; > > damon_for_each_target(t, c) { > damon_for_each_region(r, t) { > sz_region = damon_sz_region(r); > max_access_events += sz_region * c->attrs.aggr_samples; > access_events += sz_region * r->nr_accesses; > } > } > target_access_events = max_access_events * goal_bp / 10000; > return access_events * 10000 / target_access_events; /* divide by zero! */ > } Thank you for finding this issue! Coudl you please further share how zero size region can be made, and if user-space can make the situation? > > This patch makes a NULL return for such cases when creating a region > inside damon_new_region(). I agree zero size region could look silly. But I don't really think it should be prohibited. What about modifying damon_get_intervals_score() instead? Maybe we can set target_access_events as 1 in this case. > > Signed-off-by: Honggyu Kim > Cc: Yunjeong Mun > --- > mm/damon/core.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/damon/core.c b/mm/damon/core.c > index b217e0120e09..44740da337fd 100644 > --- a/mm/damon/core.c > +++ b/mm/damon/core.c > @@ -126,8 +126,11 @@ struct damon_region *damon_new_region(unsigned long start, unsigned long end) > if (!region) > return NULL; > > + if (start == end) > + return NULL; > region->ar.start = start; > region->ar.end = end; > + Above new line is unnecessary in my opinion. > region->nr_accesses = 0; > region->nr_accesses_bp = 0; > INIT_LIST_HEAD(®ion->list); > -- > 2.34.1 Thanks, SJ