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 E9F8428643C; Fri, 5 Jun 2026 18:03:20 +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=1780682603; cv=none; b=m2jbwarKUfs/IbxP16y3dXC+MMQ8ECMivyFYX4dqGOUyhERURFPAW1pwoAszJebuGN3QdebtoIeLa7a7F26X36oR1DokZFTU7yDhUJvB+Numi8cOu3Q2SlzCAmpcuftrmOtD6nofaNhu9oPUaAtLk8+WyoTKMGGLgax7k6soEu4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780682603; c=relaxed/simple; bh=iLUIFagN6XfRq77k6IUTip1085kPQXst1NkQQ75aMUs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QqYZGBNUg2YxnvuFltfAUr6rwnUWUZiB8q0IjdPyUw82ifFHl2fuDT1MmQAB5F2D4hSRsTV0VyT/nw48zqwkd7y0cZ9pTBQ9dnuSt2ZKMK7MD+1mDfQ+ZG5TITvvuE03qjgujwlFIrLbPLBw+ns9mFkptf3Qsx7Im2Yi9ACV+mA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Z+e1iPjf; 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="Z+e1iPjf" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BEDA81F00893; Fri, 5 Jun 2026 18:03:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780682600; bh=PGzu0aXe6HFW26JonkhpJzsdo5T0zMBSmF+3OrFmmlw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Z+e1iPjfg+9HqlR3h8DHfBgPQRE68RxUVv8D1XNuenBu9ZF2YKFA1VjR+VhNGmsjH GFEsnohYRLoqPHInWHvAv+Y1P65hv1c/MMrxJSbBN+yOfJqPCciWJB5eDfz41GF86T qMqtn3TPL6lAtshubXlzylKeK/mZnTMTuJ8OreQfyscHUNmR0JCN2NxoetsjDP9SNX z6t2qhgbRR6iVRlYl5H6U92DpJ60wb1xEgXi0Mh5HIK/ZHtPmOapC6wlHz+l+codie TgFnFCzdg4BQYDwbENybXouCX/1iPohb/lA5DoazHpV+yJs7xg5DN0jxXw7ub4HMGJ C0hR8pP7Fim6A== Message-ID: Date: Fri, 5 Jun 2026 20:03:05 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v19 11/14] mm/khugepaged: Introduce mTHP collapse support To: Nico Pache , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Cc: aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, liam@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com References: <20260605161422.213817-1-npache@redhat.com> <20260605161422.213817-12-npache@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260605161422.213817-12-npache@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/5/26 18:14, Nico Pache wrote: > Enable khugepaged to collapse to mTHP orders. This patch implements the > main scanning logic using a bitmap to track occupied pages and the > algorithm to find optimal collapse sizes. > > Previous to this patch, PMD collapse had 3 main phases, a light weight > scanning phase (mmap_read_lock) that determines a potential PMD > collapse, an alloc phase (mmap unlocked), then finally heavier collapse > phase (mmap_write_lock). > > To enabled mTHP collapse we make the following changes: > > During PMD scan phase, track occupied pages in a bitmap. When mTHP > orders are enabled, we remove the restriction of max_ptes_none during the > scan phase to avoid missing potential mTHP collapse candidates. Once we > have scanned the full PMD range and updated the bitmap to track occupied > pages, we use the bitmap to find the optimal mTHP size. > > Implement mthp_collapse() to walk forward through the bitmap and > determine the best eligible order for each naturally-aligned region. The > algorithm starts at the beginning of the PMD range and, for each offset, > tries the highest order that fits the alignment. If the number of > occupied PTEs in that region satisfies the max_ptes_none threshold for > that order, a collapse is attempted. On failure, the order is > decremented and the same offset is retried at the next smaller size. Once > the smallest enabled order is exhausted (or a collapse succeeds), the > offset advances past the region just processed, and the next attempt > starts at the highest order permitted by the new offset's natural > alignment. > > The algorithm works as follows: > 1) set offset=0 and order=HPAGE_PMD_ORDER > 2) if the order is not enabled, go to step (5) > 3) count occupied PTEs in the (offset, order) range using > bitmap_weight_from() > 4) if the count satisfies the max_ptes_none threshold, attempt > collapse; on success, advance to step (6) > 5) if a smaller enabled order exists, decrement order and retry > from step (2) at the same offset > 6) advance offset past the current region and compute the next > order from the new offset's natural alignment via __ffs(offset), > capped at HPAGE_PMD_ORDER > 7) repeat from step (2) until the full PMD range is covered > > mTHP collapses reject regions containing swapped out or shared pages. > This is because adding new entries can lead to new none pages, and these > may lead to constant promotion into a higher order mTHP. A similar > issue can occur with "max_ptes_none > HPAGE_PMD_NR/2" due to a collapse > introducing at least 2x the number of pages, and on a future scan will > satisfy the promotion condition once again. This issue is prevented via > the collapse_max_ptes_none() function which imposes the max_ptes_none > restrictions above. > > We currently only support mTHP collapse for max_ptes_none values of 0 > and HPAGE_PMD_NR - 1. resulting in the following behavior: > > - max_ptes_none=0: Never introduce new empty pages during collapse > - max_ptes_none=HPAGE_PMD_NR-1: Always try collapse to the highest > available mTHP order > > Any other max_ptes_none value will emit a warning and default mTHP > collapse to max_ptes_none=0. There should be no behavior change for PMD > collapse. > > Once we determine what mTHP sizes fits best in that PMD range a collapse > is attempted. A minimum collapse order of 2 is used as this is the lowest > order supported by anon memory as defined by THP_ORDERS_ALL_ANON. > > Currently madv_collapse is not supported and will only attempt PMD > collapse. > > We can also remove the check for is_khugepaged inside the PMD scan as > the collapse_max_ptes_none() function handles this logic now. > > Signed-off-by: Nico Pache > --- Yeah, overall much simpler and much easier to get. As discussed, we can optimize this later to traverse enabled orders more efficiently. > + bitmap_zero(cc->mthp_present_ptes, MAX_PTRS_PER_PTE); > memset(cc->node_load, 0, sizeof(cc->node_load)); > nodes_clear(cc->alloc_nmask); > + > + enabled_orders = collapse_possible_orders(vma, vma->vm_flags, tva_flags); > + > + /* > + * If PMD is the only enabled order, enforce max_ptes_none, otherwise > + * scan all pages to populate the bitmap for mTHP collapse. > + */ I think it would have been good to mention where the check is performed for mTHP collapse. Can be added later. Acked-by: David Hildenbrand (Arm) -- Cheers, David