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 97E523D524F; Mon, 20 Jul 2026 09:44:25 +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=1784540667; cv=none; b=NZqckkHIS5j5ysv2iyhF1Jgx2OWnPDt3PdLWFkRw/b71Piq9vCGdg1oIWse3JyZGpLHILaXc81aHg9jOLFIOZovq5cNVPmxF+RakVwxZh7EuZsjVI9l+78LTWV7DZmuuI0NRWAgobStPWEOjdTS8/NqYTEZ2sXabGk6F6jBA250= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784540667; c=relaxed/simple; bh=nqKUuzyvscmYfC9dZYbKViVv+cBrJxRQB+fC8eDYLP8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MuFWhbiXHzjeyyKGwTlJfoOVZB5lsm+HuHLXfoI0tGPhoqzhpdzoxYffTJJClJXKCjHd26KR/wugnRmw1TF4MVLIuuKY9TseoqT+b6mONL4WdbtgM6TngOU13XaF2NON7x0DdC+OMh0/LSgO6HqLI1fgIuNwOtoL7CbryUsdPQo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Uoks8aMO; 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="Uoks8aMO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6C8F01F00A3A; Mon, 20 Jul 2026 09:44:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784540665; bh=LJdgOgKGAT1CSJsO4REplq4ch5FATYh7SfBJqMlWH0A=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Uoks8aMONAGVPbnyVCd43r//Ov68jjgFFZfnckuXJWQKk+2WEYy+MXXY5LyYPsBJ1 b5unPiWEPRZYHOa0u8Y+WMzSiOIK+Vv+KHyk39Z5rUrF0sq4ZIHuA4+pzogLDyx0fC /tkzWLKSa+ZTN5mYXfapsS8x1NxtY6FQmFElOGOApOnTTGpQSF159Pq/jwurPBNN8G 1zwVqE4nhoTa5l6qyC/JHowXQmGppLu7ADKFqQlZNASZbHv1KB2cu8dXEiUgXiiNO5 j3J/xn5vXlU1vy6OT+cDbi2rZyavnadEjZn0HkiigvZuqs2F6GZ8hW81NfPcGVMWBg Ty03Zwiy7ZnCg== Message-ID: <59292ba2-e0cc-4e50-bb26-be9c15843a41@kernel.org> Date: Mon, 20 Jul 2026 10:44:17 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v3 0/3] mm/damon: introduce DAMOS_SPLIT action To: Lian Wang , damon@lists.linux.dev, linux-mm@kvack.org Cc: sj@kernel.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, npache@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, ryan.roberts@arm.com, daichaobing@sangfor.com.cn, wangkefeng.wang@huawei.com, gutierrez.asier@huawei-partners.com, zengheng4@huawei.com, kasong@tencent.com, corbet@lwn.net, skhan@linuxfoundation.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, lianux.wang@processmission.com, kunwu.chan@linux.dev References: <20260720030327.80153-1-lianux.mm@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/20/26 04:03, Lian Wang wrote: > DAMOS_SPLIT splits large folios in a target region down to a > configured target order, using the existing split_folio_to_order(). > No new core-mm code or exported symbols are introduced. > > Based on mm-unstable at 61cccb8363fc ("mm/swap, PM: hibernate: > atomically replace hibernation pin"). > > Different addresses within a PMD-mapped folio resolve to the same > PMD Accessed bit. Accesses to a small part of the folio can > therefore coarsen DAMON's observed hot set relative to the actual > working set. > > DAMOS already provides promotion actions (HUGEPAGE, COLLAPSE) but > has no corresponding demotion action. DAMOS_SPLIT fills this gap. > It is a mechanism, not a policy -- it does not decide which folios > to split. Selection is left to DAMON's existing access patterns, > filters, and future probe/PMU signals. > > target_order selects the split target: 0 for order-0 base pages, > or a supported smaller mTHP order. Both anonymous and file-backed > folios are supported. The locking follows split_huge_pages_in_pid() > in mm/huge_memory.c. > > Tests > ===== > > damos_split.py (VM + Kunpeng 920): > anon THP -> order-0 split: PASS > > sangfor_exp.py (Kunpeng 920, tmpfs, 4096 MiB): > > - Created a 4 GiB PMD-mapped tmpfs workload. > - Applied DAMOS_SPLIT with target_order=0. > - ShmemPmdMapped dropped from 4194304 KiB to 0 in every round. > - Repeated for five rounds without functional failures. > > The functional selftest (damos_split.py) is included in this series. > Additional experiment scripts and raw results are available on > request. Performance characterization using masim [1] and KMB [2] > is in progress. > > [1] https://github.com/sjp38/masim > [2] https://gitee.com/OpenCloudOS/kernel-multi-bench Hi, you give no real motivation and evaluation why this is required or why this gives the user any benefit. HUGEPAGE + COLLAPSE is clear: give me THPs in a size not controlled by user space, because the expectation is that this memory will be performance sensitive. A SPLIT with an explicit order is not really want we want and it does not fit the existing primitives. -- Cheers, David