From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792950415; b=4aQ17taHsCcvi/uQv5zsxWdb2J1VVZjRroOkfAD/WD4AelXZHkIRra+iTdit3EY6J1yq 5wdpdZopgH3HcYq1+w5dLWgHTAmAMx567iXOI66k/LuvILT4Q1Dtc7zfRAZBDGSaC/fse 6YbVaW1sy7s1WUi+JKVRUoAOoAvrvpgTsGAKj3ZqBxl5w76s8qldJEPtnmW/xRdb7v8L0 FK68SceqlRC5s72hXmdsJk/9mf9L/b+gNb4EX6tsWlnnyFQyhsuxr+MF04iv9S1iEYlPW R4U0N8R4bV5DvGK3/VHW6hkWo/lx+5aCSESCr7jh2vaEbd9nC75UJGgw2eGv5G7k0JQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792950415; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=bAufoNrYjBpv8soDMVz4gw6z9qVyFbxX6lvDmVDSc8U=; b=Oqs6tam3diX0TNKDsrHgfQNLbvonwyf69aSl3ZlGnGDqJe0XKmM46psFa9fUkBFSPY5x oknw0+V3eaCgdHpolQERjhEh4tb1p9jqlDN+cS7iGkzBf0xMW7gICUgSOngilAbRQKS7h CYkuiTtRkf+GxJq1/oUkoL2/jIpdwKicbvoyCr7ysBlZOfWEZKC6lm0SXWgb9yIV+7RyC oTe/V0Vf4XcRQx4zhk9zb3uCkSp6ZD7YYAZHw2mF0xzKSuGvC5y+xbd4DXZTmPc61QtQ8 eHUbzs3MnoBoszv6fi2AbUL+nmpxH2PCL7fvttRzb4bfRISXGjHFV7+sX95okDsra/A== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Orn4yYUx; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3166E606A31A; Sun, 25 Oct 2026 17:46:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Orn4yYUx Received: by smtp.kernel.org (Postfix) with ESMTPSA id CF4A51F01825; Sun, 25 Oct 2026 17:46:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792950415; bh=bAufoNrYjBpv8soDMVz4gw6z9qVyFbxX6lvDmVDSc8U=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Orn4yYUxVQAf+rb3mcBcf9FcX+InyQ9gvp6Wym8xXBBOuurayefwVEqnLgDHb7liz 4nnLLaOv9GgQDJ9+XSn9N8DQAtCrYMSF7uTC8uhu7hPvTihAavVIhULWn1grlUZuvG dyqg2ZfvVCH0bwVD5qg6zb7wDQPlOvuqixyZ62z85o84JNJrzvVcPwdFB+KQX6pzDH mwkdqcL6OHbkefO1yO0NmzN79/tm7pGW7B/UBwcsPquQavTMH3CBWotUDDyd8DwXo2 MaYP95jK7Cg9uAoWnJpKbG6syPVtTHXx4d54IKh6PA6IHhkERtzk8lLAtUw5zAH7kD exfgHHU/1Flnw== From: SJ Park To: SJ Park Cc: Andrew Morton , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org Subject: Re: [RFC PATCH 7.2.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value Date: Sun, 25 Oct 2026 10:46:46 -0700 Message-ID: <20261025174647.165255-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20261025172434.161597-1-sj@kernel.org> References: X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Sun, 25 Oct 2026 10:24:33 -0700 SJ Park wrote: > Patch series "mm/damon: move zero damos quota target_value handling to the > core layer". > > Having zero DAMOS quota target value can cause division by zero. DAMON > API callers are checking the target value parameters to avoid that. It is > easy to make mistakes in some of the multiple API callers. Move that to > the core layer. > > Patch 1 adds the corner case handling into the core layer DAMON parameters > validation logic. Patches 2 and 3 remove no more needed DAMON API callers > side handling of the corner case in DAMON_LRU_SORT and DMON_SAMPLIE_MTIER, > respectively. > > This patch (of 3): > > If a DAMOS scheme has a damos_quota_goal of zero target_value, > damos_quota_goal() could trigger division-by-zero error. Hence each DAMON > API callers should do the zero target_value validation. It is easy to > make mistakes. Actually such bugs in DAMON_LRU_SORT and > DAMON_SAMPLE_MTIER were found and fixed [1]. > > It is better to handle the corner case only once in the core layer, > instead of multiple places in all DAMON API callers. One straightforward > option is using an alternative denominator for the corner case in the > damos_quota_goal(). However, the zero target_value is meaningless. In > this case, the quota goal is always evaluated as achieved or > over-achieved. The quota will only keep being reduced. > > Simply avoid using zero target_value by adding a check in the core layer > DAMOS quota goal parameters validation/commit path, > damos_commit_quota_goal(). Update it to return an error in the case. > Also update its caller to propagate the error. > > Notes for stable kernel porting. The upstream commit doesn't have > Fixes: and therefore not ported to stable kernels. It turned out it > fixes commit 8b549a4fd3c5 ("mm/damon/sysfs-schemes: commit damos quota > goals user input to DAMOS") which was merged into v6.8. The user impact > is a race condition that can result in divide-by-zero. Hence backport. > Reproducing the issue requires sysfs write permission, certain DAMON ABI > usages that are supposed to be uncommon, and a rare race timing, though. > The issue was discovered [2] by Sashiko as a part [3] of its bug report. > > [2] https://sashiko.dev/#/bug/linux-7f814c9a-986f-4103-a55a-ef9537dcf62b > [3] https://lore.kernel.org/20261021005234.94107-1-sj@kernel.org On v7.2, DAMON sysfs interface calls damon_commit_ctx() for validation purpose, for parameters commit but initial DAMON start. Hence this patch fixes the issue in parmeters commit but initial DAMON start. I will post a new version to cover the start case. Thanks, SJ [...]