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=1792948534; b=r5dA0ycwZ8tn8th79YYB362AVMw31FWEyDLShGxrWimU96CSeOfWCdFeyjyhTjFX1cne gMGJ7NgDPGDOFrhCQ2p3X9/l1JJEXvlDZTFtCm5YKxm/r+JvhdlI10U2EsVWG/3UuOAEP WFuqIAdwmp7uuz7Iq1Y6IXlEFLdStSNqD8VPZcxppfVLE7qE1oLzsaft8fj3c2EzScSpt VfBxNn1JBH5rwalZlLjfLYQIz48Rm9tgR6gwi8wF5kTI4q4eS/vZ4lEUT6LfHgTssFATL /no3g8knWh9vqCqCv0BAzUsZ9HosfqM63MHBX5Qjl7k5t2jTJY1Qz9e5PDlzeNoaiBQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792948534; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: MIME-Version:Content-Transfer-Encoding; bh=DAENk+cctPe93c75V7p9sIsmyuSg6UDiR9dUUMQ5Phs=; b=lFpmeinbhWWAxUr8n9jcQU1zOo6LubZyO/DWZ6uovkvZFwL+cZET15M55V22+KpXosO6 g1qCPtTjSCV+8700bZ6yPgWI47MsKvLfN1WqBM8ygHeLvc0/Tkzvrs/1dPhus0sMpj0E7 WRsQSW8QkHncOsdCVKWjdQtQAfJQkwEHuevZqdYWFLxO2qi/PGWiZ1qZNueRjfJ2H7WkX OhrOog6eDr788ySAX4MoS2bq8yz1jTxzGHYb4JfBett9gbkDT3nKrokPPDpw2UgNZ7ksB 0SMwQSR7VeJfkg7S3BfKDVLVdDSDvxDlEBv/6L2CVyfeYbbiC3fM84ueu+vaanj6nOg== 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=Om0gVv64; 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 5656D606A31A; Sun, 25 Oct 2026 17:15:34 +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=Om0gVv64 Received: by smtp.kernel.org (Postfix) with ESMTPSA id AD02B1F01825; Sun, 25 Oct 2026 17:15:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792948533; bh=DAENk+cctPe93c75V7p9sIsmyuSg6UDiR9dUUMQ5Phs=; h=From:To:Cc:Subject:Date; b=Om0gVv64HOumU1N1ER+f2HJvdZV0lCn0JRKPEOIP+1IQOo0UeiQmqUD8Mjl2pRelz CkrGFIoXOIZdKKV/88tLEji6ZFWSDyBiF2id7AyEqg/WEVkj1lOmILnz0fOApi6y+e lNS/QcDesWoWg5w+877A4O88K9Trtpw86nXHEf0LigmOlwF526qOlrnTLtjIvv2fJf 82OV9+gBwT5sqGGV0EKg7AHX4ANPfAB3JjmtLFPAf4fTA/u051P/V2B1fIwooRAvzp svI/j+pYPYQ0MRIrMR0qM4KT285f6VeYXemULvHZH416QcwjTKCBtQBQjBejRzpiX6 tVOpdA5qld8Bg== From: SJ Park To: Cc: SJ Park , Andrew Morton , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org Subject: [RFC PATCH 7.3.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value Date: Sun, 25 Oct 2026 10:15:25 -0700 Message-ID: <20261025171526.157414-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 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 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 Link: https://lore.kernel.org/20260903010722.94244-1-sj@kernel.org Link: https://lore.kernel.org/20260903010722.94244-2-sj@kernel.org Link: https://lore.kernel.org/20260803134034.15217-1-sj@kernel.org [1] Signed-off-by: SJ Park Signed-off-by: Andrew Morton (cherry picked from commit 610c4438fb66e06d9e693c0ad3b2c1be54c0d61f) Signed-off-by: SJ Park --- mm/damon/core.c | 22 ++++++++++++++++------ 1 file changed, 16 insertions(+), 6 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index ab3c4d7549644..74d22a596fc13 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -1136,15 +1136,18 @@ static void damos_commit_quota_goal_union( } } -static void damos_commit_quota_goal( +static int damos_commit_quota_goal( struct damos_quota_goal *dst, struct damos_quota_goal *src) { + if (!src->target_value) + return -EINVAL; dst->metric = src->metric; dst->target_value = src->target_value; if (dst->metric == DAMOS_QUOTA_USER_INPUT) dst->current_value = src->current_value; /* keep last_psi_total as is, since it will be updated in next cycle */ damos_commit_quota_goal_union(dst, src); + return 0; } /** @@ -1162,14 +1165,17 @@ static void damos_commit_quota_goal( int damos_commit_quota_goals(struct damos_quota *dst, struct damos_quota *src) { struct damos_quota_goal *dst_goal, *next, *src_goal, *new_goal; - int i = 0, j = 0; + int i = 0, j = 0, err; damos_for_each_quota_goal_safe(dst_goal, next, dst) { src_goal = damos_nth_quota_goal(i++, src); - if (src_goal) - damos_commit_quota_goal(dst_goal, src_goal); - else + if (src_goal) { + err = damos_commit_quota_goal(dst_goal, src_goal); + if (err) + return err; + } else { damos_destroy_quota_goal(dst_goal); + } } damos_for_each_quota_goal_safe(src_goal, next, src) { if (j++ < i) @@ -1178,7 +1184,11 @@ int damos_commit_quota_goals(struct damos_quota *dst, struct damos_quota *src) src_goal->metric, src_goal->target_value); if (!new_goal) return -ENOMEM; - damos_commit_quota_goal(new_goal, src_goal); + err = damos_commit_quota_goal(new_goal, src_goal); + if (err) { + damos_free_quota_goal(new_goal); + return err; + } damos_add_quota_goal(dst, new_goal); } return 0; base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222 -- 2.47.3