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 C829748C406 for ; Fri, 11 Sep 2026 13:49:16 +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=1789134560; cv=none; b=pBMDhT9/g37hSVkLTAlGo9owfyQbm1R9r2ZYcgj55TBJc89HPrHgUD/Xka6bmp6LxLnUHNtVhbhSE89uqu86o3xSa81/HBdkwmlbdCJyfX6avXKGufH2e2BfnXaYp6TFG4XHcIG3rmCXbrfk52WDGK6xfJNrXxV//PM0JL4oHS4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789134560; c=relaxed/simple; bh=kdkmqAatzIjtZKxl1W2e6MjWNUcq5Ih9uYD3TIIG4dI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=r530moNykGmNsuoNN1i3hcty30wtfSbZpkV2J6kf5HuCEizF8mJu7jeDL9eV3Z0TBkASSKDJy9q/arZoRkpZIR+sMlEVJymtO6FkEZ6+waaHX0U4xgkxA6LEBytNII3DuqbrmOvbKNvDyD6lFfpaM56OnCFkGoqu/MMm0NXQ1GE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ihuZCf9I; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ihuZCf9I" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 31E631F000FF; Fri, 11 Sep 2026 13:49:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789134552; bh=G0uYWeyT4fK92rPfsbD17JlqulCmFAAmuH19nLfAWkg=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=ihuZCf9IxaqozGUp/LTyz0wafslJ8VOHAtndmM4tKFT4N9A9+/0eoGhRh8cvSUXdG S7pkHBT7C4rl4HYYNWZRiBUFqjF4RgRTHco+hvHpFvKjtqgbmR6koU0hhXcGpMbTW8 W1DKzUjlkX/mj+XwUCFihI+qlwYkOehvXU1yxvX0Tdycanss3G4IkiKkKeFFOsAVVM mgyLtBwg2nsF3DkQHl96IcICPDc2nrNHvObK7R35crqgbEiR/0nBLWBBzZVfkSYLqy CSbVHJrSfKKU8qy6Dciv4D3O0PbZzdNeZmft7S4iJlg2hKNJFSzCisfLposLeCmQL7 n7z6z8KxpV3tw== From: SJ Park To: stable@vger.kernel.org Cc: SJ Park , Andrew Morton Subject: [PATCH 6.18.y v2] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop Date: Fri, 11 Sep 2026 06:49:04 -0700 Message-ID: <20260911134904.96369-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <2026090831-drinkable-maybe-5e01@gregkh> References: <2026090831-drinkable-maybe-5e01@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 (cherry picked from commit 123e4619ab6c8ab1c4cb1d7a58311a2af13929cd) Signed-off-by: SJ Park --- 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; + } } /* -- 2.47.3