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=1792950475; b=O228hLYEVRXfq9uj9WuRkbjPOh1G3OBvK24lmVwoBmKSwiuJhsDzliwd37C7BxVJPSuo luyGyU8zEmV4ZclHTT40Jj1omGt7852XHfzxYDFbFMKA1n9payMd/l/+9aKjflQ/ktJaP /i4DhrnTSNJ+cNheu3emDqPoJmdLCZEcCVs0doaolGncuHcTrsMp/yV69T3rT35jIR8yD rNh18EHtjxdUfrreOviv+akibt5Y5DnUHPFsMbEpq1F76sW8Vq7vGI/1JC+K75wUHL4V9 e1W/AUKoogbjwSlBiW4y7XWuTuB8PAU6wUZ2AKDhXjiAufsDKnpBhIG7GP2p9N3felQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792950475; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=umoLJmKZTlAf2lH0+QaiNGrYZlN8I8eaFHN45iIXEqE=; b=xltr3B++7L01Gyr9RaXnESE9PwwfYVEM+i+GDKnwHtL9ff4Zx1jiY22sHPT3Z5Mq09h3 /mYuqdCWtUtw86YOXk2E0EpXWIhpexFoY4Bzks0AnGA5I7VQpoXbYee8uY3+sKaXB7ZC0 cqG86nifkFiZyM6EklUTCZaAxtJgudblap77rr8USmVEbpO/TrpwkzyMWKgnirPU2MUt3 MK2c+m2CIHGsJSedHv56qIW7NmMcNm4zDCOMTXWVsG6Ydx9kYqedfhOdpjHBx1qOwzvak v8G4RSVYkRJqDvXP8rBupfqI+29m3PmF+4uVTLWM0LC9WOwXJQKw0/buZz4pwU23Msg== 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=KwfF9bpF; 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 8479F606A321; Sun, 25 Oct 2026 17:47:55 +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=KwfF9bpF Received: by smtp.kernel.org (Postfix) with ESMTPSA id 076971F01825; Sun, 25 Oct 2026 17:47:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792950475; bh=umoLJmKZTlAf2lH0+QaiNGrYZlN8I8eaFHN45iIXEqE=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=KwfF9bpF4oRYrf7S8breNvRCiE8XkSYmqI1+/LvQkoxystZZh0MvwQtj6Vk/nIonf I6th8/DcipruVViu8UHTwLAXW3xnDiHsL366t7jvLnaKIlrAUcEHblTAorINeIoryG s1KMtV28iqJ9WRhBmg50gEixPnMhIhl924uAx2fDRUyi9Tyq4qE5IyL1AqFV84Cz8X gWsdQ5EGjixn1svyOHna+qhsq9qFPiPlINt+1dkttT7fyfG3fErWhqrisjBdPB/w6w BI62ym0X6bNV2PZzEOCBYvLnw2xQfqU7UmKksyIa6RfW4O6frlip5qKGBgdQYDC+27 K61RhAXvh3obw== 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 6.18.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value Date: Sun, 25 Oct 2026 10:47:47 -0700 Message-ID: <20261025174748.165405-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20261025172712.164385-1-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 10:27:10 -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 v6.18, 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 [...]