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=1793022468; b=hW3wWKdlasX6NE2orefOV35zdJjWZyBxbdLacMWLoBEBdKuWGIIKynTh5hK5JU6KvxZx FaI+9gYe+rd70JJdasueJiXjeoG4wP44+HzHa3U/H7IOiD+XgV5vt9QHdKiZMHjgSBWnM Z//2YngWEges8xxtMxYDEDtkxd5Zw8DjYCDmI3oMSUMSejZS4geBv4SYrqOGKKoiyL007 v0ag0Y0Yo/RFOCZZMXBx4BTER5PF9KB/HHR+RIdc9MzAhZubnsx4mt0VmhvLhbzzIWTl9 PQFilFhVjCHqOo6IZOuxi1M6KvEZ28/gQgP97o6wAYrXFQ+AMlqedc+Ph+h58RbNX6w== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793022468; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: MIME-Version:Content-Transfer-Encoding; bh=aXB9q+SjCodJprte8ZqvjzyulFgl828dew1/IFtc6cc=; b=G2i77zt+SGDGmp3uNwISOIYKKiCT49g57trnX7ozVQAq3sTo1MCRkYHA4jj6eseXWZIi L2p2XMVxBhnppsB334RQyOcRf+2Sb+DV3+pFGuFwxovH/fV6mcEu7ZpaOfUxBuc3K+/tl u2ysrUllbO4JinzzgCg75fCxdrRH/kZCCDRbGx+X3wm6DLk8QdcHDFW/fZqV1pt7T0VDa F1g6Y75vSNqrP8LpKaIoCsDNXMN4m714mlwjGzs563kJnjRdHtVX3f+t7hufw3yBF8PLX Yt4dqWzZPeL1cGkXSWYNRGUfuCYfCR1SyIe5+cE2auSct+6J8DpIev1/EMbfRxGzazQ== 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=FuknI81y; 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 91272606A31A; Mon, 26 Oct 2026 13:47:48 +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=FuknI81y Received: by smtp.kernel.org (Postfix) with ESMTPSA id E19021F01825; Mon, 26 Oct 2026 13:47:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793022468; bh=aXB9q+SjCodJprte8ZqvjzyulFgl828dew1/IFtc6cc=; h=From:To:Cc:Subject:Date; b=FuknI81ysijUZpiN3I8AI1drSsxf4I4W+69dTEHZKpuflQISUwVDqE39PD3fIJYCw lhJUtaM9OZMr4yWHWyglUOxl003BdzsrZFGsMysr63GezpUhcuCEdMmGkkyLI/luFq OakdRhMcb/Fo2hOJeqL6PNUgBaj1IPUQX6IUmZ2LlU09PqoAPms4rObvneHX3GY8zH VETwiEnGGb++JIFUc5a6iKPBjfqnUWKPmpnun4abliYzyJk5A3Sy3LNBMKHuSRz3U2 HfTOIQRGc1e1n5T0cz1tSraKe9M6VEgAOX2K9DetLQzZjHp0l4qHJqm4/zyZtbVImM Gb6ezA3sehgfw== 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 6.18.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value Date: Mon, 26 Oct 2026 06:47:39 -0700 Message-ID: <20261026134740.149027-1-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. Also, the upstream commit on this stable kernel can fix the issue for parameters commit paths but DAMON start path. That's because DAMON start path does not invoke damon_commit_quota_goals(). To fix the path with minimum change, add a new small validation of quota goal target value before damon_start() call. [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 --- Changes from RFC - RFC: https://lore.kernel.org/20261025172712.164385-1-sj@kernel.org - Add a validation for damon_start() path. mm/damon/core.c | 22 ++++++++++++++++------ mm/damon/sysfs.c | 19 +++++++++++++++++++ 2 files changed, 35 insertions(+), 6 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 4e6e3d7d0a5c0..8fa788b0d709c 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -807,15 +807,18 @@ 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; } /** @@ -833,14 +836,17 @@ 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) @@ -849,7 +855,11 @@ 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; diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c index 7f52470be36e3..3a79d8c89b5bb 100644 --- a/mm/damon/sysfs.c +++ b/mm/damon/sysfs.c @@ -1592,6 +1592,20 @@ static int damon_sysfs_repeat_call_fn(void *data) return 0; } +static bool damon_sysfs_valid_start_ctx(struct damon_ctx *ctx) +{ + struct damos *s; + struct damos_quota_goal *g; + + damon_for_each_scheme(s, ctx) { + damos_for_each_quota_goal(g, (&s->quota)) { + if (!g->target_value) + return false; + } + } + return true; +} + static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond) { struct damon_ctx *ctx; @@ -1618,6 +1632,11 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond) kfree(repeat_call_control); return PTR_ERR(ctx); } + if (!damon_sysfs_valid_start_ctx(ctx)) { + kfree(repeat_call_control); + damon_destroy_ctx(ctx); + return -EINVAL; + } err = damon_start(&ctx, 1, false); if (err) { kfree(repeat_call_control); base-commit: 2926d88bb7fc4608fd942453d929c39e4e5ec006 -- 2.47.3