From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 050844B44AF; Thu, 17 Sep 2026 15:19:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789658414; cv=none; b=IZqp9H19sQaSbLDPFFAOpJsEG0Lrcn34zhPEsxA++UpEvvzf9dBkh49T3ikwA+vK6hZnnAzKs28l0+0Ilkco0tStOj/xAhoVc/Z6n7ANMHb22rcUCjaem/N12fA8yKoX8LgKxyfzjfYesMtZjx9utFjPSvHu2w3bIuXFFb6KuZU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789658414; c=relaxed/simple; bh=az1VTLp2mW90IBsk7YsFGV8HLrZ235AscPATYyKld0E=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=NRbcKOYx/dkPDH4e6fhYyr0En6eV+7IXUWpU3bctrfZlhTPYdNCxmRSdipG1pV081L+FOK+sQBP76+3y4skK093mBI3qlKGKZ3c0gswsQZHyKjOBoZ6t16ILFmeMoP44hp4Sw2wfdzJEl+Eh7JtiIbl0HaoyZBoraF1wn9G6bNM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=WclPI9Sg; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="WclPI9Sg" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BD4891F00893; Thu, 17 Sep 2026 15:19:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1789658393; bh=OnectFlbgRNPnJ3Vzs/sq27gtZx5AxILj7+5SKWclUU=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=WclPI9Sg6oJvNak233xciDiJMcOl5d+IOkTaEuKwLfboPuU3t5SjWPbaxPjoxtd6N gssh6m/FXgfwX0RpEClazkHYrIpnY5FXW1U+adgrM/iozD/ZfxBjxgYq87Rh4Ma0uU tLv6f4L+LKCPnKXoiX+HUF+cMk/IVFQJLBPAikK8= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, SJ Park , Andrew Morton , Sasha Levin Subject: [PATCH 6.18 0002/1250] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop Date: Thu, 17 Sep 2026 15:56:37 +0100 Message-ID: <20260917151551.982317194@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260917151551.901433442@linuxfoundation.org> References: <20260917151551.901433442@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: SJ Park commit 123e4619ab6c8ab1c4cb1d7a58311a2af13929cd upstream. Patch series "mm/damon: unurgent fixes for infinite loop, NULL de-ref and races", v1.1. Sashiko found a few issues in DAMON that could cause infinite loop, NULL dereference and monitoring results degradation. The first two sounds scary but the infinite loop happens only under unreasonable user setup. The NULL dereference is only in a unit test. Monitoring results degradation is trivial since it is only best-effort, and those happens from only unlikely races. Still those are bugs that better to fix if possible. Fix those. This patch (of 6): Due to online parameter update like events, the number of DAMON regions could be higher than the user-set upper limit. kdamond_merge_regions() repeats merge regions until the number meets the limit, while doubling the merge threshold up to the theoretical maximum threshold. It is tried only up to the theoretical maximum threshold because even the aggressive merging can fail from reducing the number of regions under the user-defined upper limit. For example, there could be many user-defined non-contiguous regions that cannot be merged. The threshold based loop break condition is evaluated by comparing the threshold for the next merging try against the theoretical maximum threshold. If max_thres is larger than UINT_MAX / 2, doubling the threshold could make it overflow, and bypass the loop break condition. In the case, if the number of regions cannot be reduced under the upper limit like explained above, the loop will run infinitely. Prevent the case by doing the break condition check before doubling the threshold. Also, prevent the threshold exceeding the maximum threshold, as it could overflow and apply the wrong merge threshold. This issue is unlikely to occur in real world, since having the max_thres higher than UINT_MAX / 2 require unrealistically large aggregation intervals compared to the sampling interval. Also, it requires an unrealistically large number of uncontiguous regions setup. Nonetheless, the consequence is bad and the fix is simple. The issue was discovered [1] by Sashiko. Link: https://lore.kernel.org/20260715031002.108504-1-sj@kernel.org Link: https://lore.kernel.org/20260715031002.108504-2-sj@kernel.org Link: https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org [1] Fixes: 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet") Signed-off-by: SJ Park Cc: # 6.10.x Signed-off-by: Andrew Morton Signed-off-by: SJ Park Signed-off-by: Sasha Levin --- mm/damon/core.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index a9168a03b3402..6fa02025d2af1 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2415,7 +2415,7 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold, max_thres = c->attrs.aggr_interval / (c->attrs.sample_interval ? c->attrs.sample_interval : 1); - do { + while (true) { nr_regions = 0; damon_for_each_target(t, c) { damon_merge_regions_of(t, threshold, sz_limit, @@ -2423,9 +2423,14 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold, nr_regions += damon_nr_regions(t); } count_age = false; - threshold = max(1, threshold * 2); - } while (nr_regions > c->attrs.max_nr_regions && - threshold / 2 < max_thres); + if (nr_regions <= c->attrs.max_nr_regions || + max_thres <= threshold) + break; + if (threshold < max_thres / 2) + threshold = max(1, threshold * 2); + else + threshold = max_thres; + } } /* -- 2.53.0