From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794311666; b=ywFtNeOFKRmtZVv0c64YB0BO0JpojJmIIY4exNz4AoQtdeeswBX/BvFwlVyoIrq0bBrc l7cLwarIzxdi1OqYgxiAGNE3ZDPDgGQy0k1rOwi3w07EEvmNlzbaj4NzHmJcXuLE1ovIT mDA4S/TY0ZZOm8U6TrQFo0rVZFARHvUGKqaH8zBZyMs9wHJ6lDpNG1ji9h0Lro0Kg2Ckz 6w/ojPUUz7liBjHlZ7HSfOOjiXnHCMSqwLrDail47VRlvx7fniyXAEynFwVmpBOZPxBG1 fGipO6uthUUTBkn3A/SIYW29O26BBRJM3fiRdlaohIlJXlYS1XLxdoWs3vXVDhYtXIw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794311666; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=7WXfVCVuf8uXYy2DZQnojC5ljvRy+PP3sdHEmJdjQAk=; b=a79VB7n0EOvOgVi7VgmPNkafs7/TQyuiI53s5ltp14G7/G/K4HMCHpxFR++N7m2LwuTv a9T6g6EnRajzA+TPoyU6lcXu9zQoJjvKQaMZ20Pkym/vMYwZHogkpdodusks5D5s4sfU/ azdnxS4iXNrnWwn2VGC7lqmwd20YcBKfXPopUEke0+yJnEkPyRkFi3FUJat54jyZbRw/1 /QftIlJguAx7JwqFXgcuymDz3vOpXquLgSrVgWD7nDlKCf8mvjitclsVvTeUt0Nc549K2 nQ/jyE1tlbE60m9Mvl2fXSnPNaBq/Prt32uz5SyjLu4bo14asRUs/+wwAxVrMFbU1ZA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PvPHTv4u; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0BDBD606A9A9 for ; Tue, 10 Nov 2026 11:54:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PvPHTv4u Received: by smtp.kernel.org (Postfix) with ESMTPSA id EC03D1F008A1; Tue, 10 Nov 2026 11:54:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794311665; bh=7WXfVCVuf8uXYy2DZQnojC5ljvRy+PP3sdHEmJdjQAk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=PvPHTv4u5XDMIkz+tNdsAkpxlI2GSaLdB22KQ+CkbqmoSCOTtaX4li1Civgvbq8Da 5H3WjEDe60cW/mOlTXgGA+4yAPTafLe/cRIi7Qh3X13H3heCRD8AMdPo68Dh60az+Z NAYSda3bbR01apCSJ8seyXFtpysD41xR0nnbz4luv02FolbQOPXoR9jPsBYPpuujY6 n5B0C5fCF3nlI1iLxaQLsLH6ViDBFWsAJX1VaZVAbhDD1+ULOBrrIwHSz6HNYboTZv BX5keLxP55jUMWXuESmgjQV1fame0xkhVbDp/iL+5WeYpPiYzWyunb5LynRubfkCpQ 2uLi4apjTLjRw== Message-ID: Date: Tue, 10 Nov 2026 12:54:19 +0100 X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH resend] ksm: add merge_only_zero_pages to merge only empty anonymous pages To: "Xu Xin (vivo)" Cc: g@dgvxl46587.localhost.localdomain, Andrew Morton , Jonathan Corbet , Chengming Zhou , Lorenzo Stoakes , "Liam R . Howlett" , linux-mm@kvack.org, linux-doc@vger.kernel.org, Huan Yang , Wang Xiao References: <20261106034841.233085-1-xu.xin@linux.dev> <899b1b2b-4f6c-45e2-abd6-a333ac77b23b@kernel.org> <3e126b7a-d7d9-4a8c-9c83-b6fbf5bf1539@kernel.org> 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 11/10/26 02:34, Xu Xin (vivo) wrote: > On Mon, Nov 09, 2026 at 06:41:39PM +0100, David Hildenbrand (Arm) wrote: >> On 11/9/26 02:39, Xu Xin (vivo) wrote: >>> >>> Ha, fair question :) This came from our own investigation. We were looking >>> into whether KSM can actually provide benefits on Android phones, and this >>> observation came from real-world application workloads. To be fully honest: >>> I did use an LLM to help polish the English wording of the commit description, >>> but I carefully rechecked and verified every sentence myself.. >> >> The reason why I ask is that this effectively disables most deduplication where >> KSM was originally invented for: VMs running the same kernels, libaries, >> executables etc. > > Yes, it closes the original scenarios like KVM, so I wrote a Note to say that in > commit description and Documentations. But it opens up some new application scenarios > — scenarios with high security and low cpu-consuming requirements, where merging > zero pages alone can already bring benefits. > >> >> So yeah, disabling most deduplication shrinks side-channels ... by disabling the >> core functionality of KSM, to dedupicate pages with the same content :) >> >>> >>> >>> We still keep the stable/unstable tree data structures, but zero-page merging >>> does not participate in the stable/unstable tree search path. One more reason >>> for keeping them is that merge_only_zero_page is designed as a dynamic knob >>> that can be toggled at runtime, so when it is disabled KSM can fall back to >>> the traditional full KSM behavior. >> >> >> What's the memory overhead from keeping these datastructures in the >> merge_only_zero_page-only path? > > In the merge_only_zero_page-only path of current implementation, the main memory > overhead comes from the ksm_rmap_items for non-zero pages, RB trees of stable and > unstable, and mm_slots. > > If the allocation of rmap_item can be avoided (following Tu Jinjiang’s idea), then > only the stable_tree and unstable_tree remain, which are basically empty, and these > overheads become so low that they are negligible. The mm_slots are essential so can't > be avoid, but won't take up much memory. Okay, so that's the right approach then. -- Cheers, David