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 9089638736F; Wed, 9 Sep 2026 04:41:03 +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=1788928866; cv=none; b=Qzxm27qiMJgeBD09XcsdaXEDELvFcK5Jlwb4ciDm8APCepUsd6JuuDyBUxsg7umwrLKH++NMgAOM9iqkX8gTLVl0HY5I9lo9Z3kErIbX7eqRynrYeGmtbocmeOlTu+EJprHAul7x45DUKstNpN/SinizXIdUpNeNZBcPOwQ43GE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788928866; c=relaxed/simple; bh=byZhMLfKxBGjxD0VBKwPC/GwsXnX9exJwA0EkIpf92Y=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=pn3clMu+zG0lO/OkOuYxTPrCIlIXgoLQEMiWrVnfBNoAgPmV+LDv2aS8RcFyfT7Ik4zCv5bmqYZmDifE88+KiMT6VEztW59u6cQNFTsmNb3EJWrpgZLmbNctZnPIgNeHBCr8feb0IxNbY3PqvyfxsU905VmQKvQ58XKr2eJ+WWk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YoDqkSuY; 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="YoDqkSuY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 57A151F00A3A; Wed, 9 Sep 2026 04:41:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788928862; bh=RppHdJ069L4RLGMx76xnjPu/gB00fphi6UNzE0iIqGM=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=YoDqkSuY+Xk1xkH/DAA7iRQE2ygIKWlvlE75a5TgM918YE43deUmGxr23dF2L3jqw QMaVScPyvhxjhQqhg579rzVfpFiYy3FpFkD0CeGU3fnd1j10LGh65H6l9i/8SLZ2YH h5IWzBiR40MsQQqpCtlH/a/RfiXEDU0NYvebzQj7lDlKhTT/uHOLNhZYre7R45QHu3 HZ5NHxzoO+BOpRogbQlxGP/YeUgv2viAWmvPj3W1toWTe9yJ6ROLVdc2B0K8iwVItg INvyPIMXkIvc+G47xOCG0gNWTgDFmlRLhHQExL/m+NmntjWzhc11siaBv16Ok/eR8m rC6CgcNW7xgBg== From: SJ Park To: stable@vger.kernel.org Cc: damon@lists.linux.dev, SJ Park , Andrew Morton Subject: [PATCH 6.18.y] mm/damon/core: skip aging from repeated aggressive merging Date: Tue, 8 Sep 2026 21:40:55 -0700 Message-ID: <20260909044055.213060-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <2026090840-shifter-galvanize-450f@gregkh> References: <2026090840-shifter-galvanize-450f@gregkh> Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The number of DAMON regions could temporarily exceed the user-defined maximum number of regions limit for corner cases. For example, users could lower the limit via runtime parameters update. For such a case, kdamond_merge_regions() repeats merging regions in the case doubling the merge threshold. The repeated merge operation could update the age of regions multiple times. This corrupts the monitoring results. Fix the issue by asking the merge operation to skip aging for the corner case. The user impact is degradation of the monitoring quality. The impact should be mild, since the degradation is only temporal, and it is not common to happen in realistic setups. The issue was discovered [1,2] by Sashiko. Link: https://lore.kernel.org/20260712165432.87609-1-sj@kernel.org Link: https://lore.kernel.org/20260621203548.10718-1-sj@kernel.org [1] Link: https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org [2] Fixes: 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet") Signed-off-by: SJ Park Cc: # 6.10 Signed-off-by: Andrew Morton (cherry picked from commit 0250dbe08c730d003ef9f484da56ae09a1ea0c4c) Signed-off-by: SJ Park --- mm/damon/core.c | 21 +++++++++++++-------- mm/damon/tests/core-kunit.h | 2 +- 2 files changed, 14 insertions(+), 9 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 70ac1f08753d1..c98539efd2a56 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2364,17 +2364,19 @@ static void damon_merge_two_regions(struct damon_target *t, * sz_limit size upper limit of each region */ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres, - unsigned long sz_limit) + unsigned long sz_limit, bool count_age) { struct damon_region *r, *prev = NULL, *next; damon_for_each_region_safe(r, next, t) { - if (abs(r->nr_accesses - r->last_nr_accesses) > thres) - r->age = 0; - else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0)) - r->age = 0; - else - r->age++; + if (count_age) { + if (abs(r->nr_accesses - r->last_nr_accesses) > thres) + r->age = 0; + else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0)) + r->age = 0; + else + r->age++; + } if (prev && prev->ar.end == r->ar.start && abs(prev->nr_accesses - r->nr_accesses) <= thres && @@ -2408,15 +2410,18 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold, struct damon_target *t; unsigned int nr_regions; unsigned int max_thres; + bool count_age = true; max_thres = c->attrs.aggr_interval / (c->attrs.sample_interval ? c->attrs.sample_interval : 1); do { nr_regions = 0; damon_for_each_target(t, c) { - damon_merge_regions_of(t, threshold, sz_limit); + damon_merge_regions_of(t, threshold, sz_limit, + count_age); 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); diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h index 6687778752728..80dcf1d641e07 100644 --- a/mm/damon/tests/core-kunit.h +++ b/mm/damon/tests/core-kunit.h @@ -261,7 +261,7 @@ static void damon_test_merge_regions_of(struct kunit *test) damon_add_region(r, t); } - damon_merge_regions_of(t, 9, 9999); + damon_merge_regions_of(t, 9, 9999, true); /* 0-112, 114-130, 130-156, 156-170 */ KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 5u); for (i = 0; i < 5; i++) { -- 2.47.3