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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1779339172; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=N7qfxh/zfMOAlXTKCwcPQJBPsWRq4EyvbHE/YRBH0T8=; b=uARqszmDfkqT1jfFDWvswzOrjg5aZWiOcyLKMsZb86BzRZynvJdgn61Eez2N9NDFgZHj2B TIzgkZy5eO0Y+tthvD3YJwx+8+YWEUogOBXkfAlvp5o6qeyVp4Pqwoq7pwy2AKWqqzgOxy 8ga6myNIfl4HRMLojD2HcmOEn5tn5tg= From: Jiayuan Chen To: damon@lists.linux.dev Cc: Jiayuan Chen , SeongJae Park , Andrew Morton , Shu Anzai , Jiayuan Chen , Quanmin Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH 0/2] mm/damon/core: detect internal variation above max_nr_regions/2 Date: Thu, 21 May 2026 12:52:22 +0800 Message-ID: <20260521045236.115749-1-jiayuan.chen@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: sgocfcrrm13ntf8gmaxpan11dckzcu8f X-Rspamd-Queue-Id: 44EFDC0009 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1779339175-467571 X-HE-Meta: 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 YoFAg4XT 1DPq06phSoP6WDK5E5vkx81UAo6o6tP4QG9gFEvrp5azX/32McqOomHjomy4Xxkekn1U/6Ut3q6qmR/YwPMsREQP9ILGqXZC3+y+3KCTLFD8NiQYQo0azaHUVB924unkRIufdEmDQIfbqxbkV1Ugx7nQvDh7eRPkGXs7c3VJvWJg9T2Jg/zGRWsmlpZWEof0ztIUaxiIZWBfOs/IQDz/TLopoV5/fixNoVcKRwoqp+FKplhS6IQdVO0tckg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: kdamond_split_regions() bails out early when nr_regions is already above max_nr_regions / 2. A large region that picks up new internal variation after that point never gets split, so we lose visibility into its hot/cold structure. We hit this with damon-paddr on hugepage workloads and damon-vaddr on processes that mmap a large anonymous range. On our production tree we added a current_nr_regions counter (no good upstream home for it yet, so it's not in this series). We saw nr_regions never getting close to max_nr_regions, and the picture of the access pattern was too coarse. Example with max_nr_regions == 1500. A target ends up with 799 small hot/cold regions plus one big region (an earlier merge collapsed a uniformly-accessed range into a single piece): H:hot C:cold r1 r2 r3 r800 HHHHHH|CCCCCC|HHHHHH|...|HHHHHH..........................| nr_regions = 800 > max_nr_regions / 2 = 750 Now a cold subarea shows up inside r800: r1 r2 r3 r800 HHHHHH|CCCCCC|HHHHHH|...|HHHHHH........CCCCCC.............| The small regions can't merge with each other (their access counts differ), so budget never frees up. r800 can't be split because nr_regions > max_nr_regions / 2 returns early. The cold subarea stays invisible. Patch 1 lets this path still split regions that just changed (age == 0), up to whatever budget is left under max_nr_regions. If a split turns out useless, the next merge cycle undoes it. Patch 2 adds a KUnit test for the case where nr_regions is already above max_nr_regions / 2. Jiayuan Chen (2): mm/damon/core: split age==0 regions when nr_regions exceeds max/2 mm/damon/tests/core-kunit: test split above max_nr_regions/2 mm/damon/core.c | 68 ++++++++++++++++++++++++++++------- mm/damon/tests/core-kunit.h | 70 +++++++++++++++++++++++++++++++++++++ 2 files changed, 126 insertions(+), 12 deletions(-) -- 2.43.0