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 5734A2941A; Wed, 9 Sep 2026 04:38:14 +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=1788928696; cv=none; b=M219LXNJNVxGfdc96ouMAtDwUSsaX5pPydsWZU37GrrzgAcoW6VU/Gjb2iSO1UtsFVkngNpaPPNrqKk3Rf4Yo747djT+V/5npKLYBABNBZX2IXECnMBb6VI8/hYrmPxGIJ+myH+RO/zfHynK3qH/UT9+lAFelgXMRJJjozMnKaU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788928696; c=relaxed/simple; bh=X8+y5mG7Y/3N05RLguqOqEBmX8OPFMHMVQwW1F0u3rQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=P6snl0Ozp8mqrsvKYd/iPIn/65vDaijKZVh7FVIZ6wYeqd3NBmj9Hesccc0hE7oX2fTj2T+z5K/RwbqKb/XouN6O5HiAH8mIbi9/9ZVJzbg5rQ8g9KPKcdf8UHpnAFQYvX0WGLFEW5zLREHmAtKtNcand364rHSWl9qnoecyY7Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Fl/Pbff9; 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="Fl/Pbff9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A7BB31F00A3A; Wed, 9 Sep 2026 04:38:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788928694; bh=gmTt40PH9U+H2i6TUDRV2HYdSRMNkVhmo0Fjh4Js9/Q=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Fl/Pbff9JMCphXKt4nzHn8RALnJBO87Hn5Zsb7CkkucmZeSti8Yiwx30zJnkEXDsE zxoExStRP5sCoIAQkpQv7oJdS0WFSC9Sv45I5Q8opBq9iq2VdC4bKg9hK33Gf7wu/4 XlF0yT99bYxEi0rIjB6XKHdVr7VOKkrI1T91hp+HgktSCyhm0RuUcBzLqBXoEHekvz reBVIsF9XJhWIzZRV2AREdy4T+6NCbzs6Nth5Q0N9W5mAPGwQumeTiNKc00YjfcIOw 0KRwfsNSy/4Hf9uO1BRYhePNFX99L9clMJbUrwSOZejJISO7LNG6QEJZHwOi7rikiE 7zMcFQrOgPcUw== From: SJ Park To: stable@vger.kernel.org Cc: damon@lists.linux.dev, SJ Park , Andrew Morton Subject: [PATCH 6.12.y] mm/damon/core: skip aging from repeated aggressive merging Date: Tue, 8 Sep 2026 21:38:07 -0700 Message-ID: <20260909043807.210326-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <2026090840-slapstick-viability-4dbd@gregkh> References: <2026090840-slapstick-viability-4dbd@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 | 17 +++++++++++------ mm/damon/tests/core-kunit.h | 2 +- 2 files changed, 12 insertions(+), 7 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 71264018cf2d1..8901a80c8a21f 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -1786,15 +1786,17 @@ 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 - r->age++; + if (count_age) { + if (abs(r->nr_accesses - r->last_nr_accesses) > thres) + r->age = 0; + else + r->age++; + } if (prev && prev->ar.end == r->ar.start && abs(prev->nr_accesses - r->nr_accesses) <= thres && @@ -1828,15 +1830,18 @@ 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 d7737f5df6688..0e1420b7b0fba 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