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a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1783875284; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-transfer-encoding:content-transfer-encoding: in-reply-to:references:dkim-signature; bh=8xnGzHVEhJzHmaj8zqg2Ec1CH067snxS1T5Ejp1k8Wo=; b=R/Ga4QDps286s0q/fK/laTYFQPPXB7jvN6a5LCcTSzPc1xYgxfyVdcT6ZmSJsRSu9Huror S/HiODgVKCJMJ3tvukwcDAtcmJYog0KVf3gCPDHtQ06rUxnbUrFlEhxtg/XxWNQIzRQm/P Ufq/Z8ZJPPhuXdLJD03Nz10eXapYQZU= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 66E396004E; Sun, 12 Jul 2026 16:54:43 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id AC8AB1F000E9; Sun, 12 Jul 2026 16:54:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783875283; bh=8xnGzHVEhJzHmaj8zqg2Ec1CH067snxS1T5Ejp1k8Wo=; h=From:To:Cc:Subject:Date; b=YA5LLlgiAG2b39oDhfv1a+J8X+VkVbiTa/cHAGwNCuP2QLWhqnoop/z6ghtSG9Iug p0tu/I0XAT1DnC8BV1uj0hNiQSTnsBqBrZaIR2cqNUlovy/+IiPEEa1866YGFLjTpo 51e8GWlPN5qoaKG4edMAxodxPgM6j7khzSwQpoHq80WbKxItFMVEpY3JA+bIFM2wJh bKG0N9r8y7IeQNfwjHTDjZr1x9YsPxjJznBRzOPThBmUfHI4hroqMrIaZECwuMJLKr em3Wd5vANQku8XZ2qCVcJp6OaXMD3qqxxRJoorDmHZGQ4zhEt4brhTSzbXC3uSrXyc 7etC0DI8d4l6Q== From: SJ Park To: Andrew Morton Cc: SJ Park , stable@vger.kernel.org, damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH] mm/damon/core: skip aging from repeated aggressive merging Date: Sun, 12 Jul 2026 09:54:30 -0700 Message-ID: <20260712165432.87609-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 16B80A0009 X-Rspam-User: X-Stat-Signature: eh7mfakzbmaf45dwo698nfh1gc4xswz3 X-Rspamd-Server: rspam03 X-HE-Tag: 1783875283-879914 X-HE-Meta: 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 0LBUKTOG AMzY/PaYQmNtea79wg5fxu5y5rM07KQxPoCxCEUNd9wUAwbCc/2TvQ9F0emSpsRgnfOUYpuPB69MopbEKuPSJumC+0cJ5r2DTXUxYc8tTqu+T7usLdKuLKCwrEG6sfqDY3RjVbbyXRr4HagxTfzyXxjWDYmEIxZvlHYcaG5HsCjM/WP1XdUuo1Vn+5EbscL4hcOkusnjikE/0iPowBs/FNMtNW5vFm8dgs7yA0yC/r+nwGGk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. [1] https://lore.kernel.org/20260621203548.10718-1-sj@kernel.org [2] https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org Fixes: 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet") Cc: # 6.10 Signed-off-by: SJ Park --- Changes from RFC - RFC: https://lore.kernel.org/20260711171537.75278-1-sj@kernel.org - Drop RFC tag. 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 806a67d02a6e9..6c4215cc809ec 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -3308,7 +3308,7 @@ static unsigned int damon_merge_score(struct damon_region *r, bool last, * 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, struct damon_ctx *ctx) + unsigned long sz_limit, struct damon_ctx *ctx, bool count_age) { struct damon_region *r, *prev = NULL, *next; bool use_probe_hits = damon_has_probe_weights(ctx); @@ -3319,12 +3319,14 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres, score = damon_merge_score(r, false, ctx, use_probe_hits); last_score = damon_merge_score(r, true, ctx, use_probe_hits); - if (abs_diff(score, last_score) > thres) - r->age = 0; - else if ((score == 0) != (last_score == 0)) - r->age = 0; - else - r->age++; + if (count_age) { + if (abs_diff(score, last_score) > thres) + r->age = 0; + else if ((score == 0) != (last_score == 0)) + r->age = 0; + else + r->age++; + } if (!prev) goto set_prev_continue; @@ -3366,15 +3368,18 @@ 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, c); + damon_merge_regions_of(t, threshold, sz_limit, c, + 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 6ad73559dd8ea..68d30648c612e 100644 --- a/mm/damon/tests/core-kunit.h +++ b/mm/damon/tests/core-kunit.h @@ -257,7 +257,7 @@ static void damon_test_merge_regions_of(struct kunit *test) damon_add_region(r, t); } - damon_merge_regions_of(t, 9, 9999, ctx); + damon_merge_regions_of(t, 9, 9999, ctx, true); /* 0-112, 114-130, 130-156, 156-170, 170-230, 230-10170 */ KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 6u); for (i = 0; i < 6; i++) { base-commit: 96e4e82548418bcd5b3518d3b4efed1f5631772e -- 2.47.3