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=1794080896; b=e+X8zRjHSZ4/YUiiy6toyq5/iY/lluENolgZEojaMG8LavQZqrgzIFYDJ7jITJNu6Fjo y3t+mcG7OJV/iUgjqCOqw8qabVuxGbxMPqgbxSQFLUMfFjoR67sRnRbqGrGmKfS/kCSnM J2xiP5wWDyXob+/5H/UbKQXPkyyX8EW4Ye6nUJShamLIu7NZDqB6tS72B2B3NVLNfXYl4 Fql3WJhrZWMpmNAzAyVm5bLaEAnTp0fBsfJ9+ByRa3eey2kWIyS+GZm9lMtJoSBu8BmGJ DjCDIb8+Z7fbBEPn0J8gKnOzBlSfb3utiSmndIJ1DrQ835rmzGAROMwlG3Df05RCRhg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794080896; 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=PgfupnOI9y63nTPBwUuVBVA9ftyhpGgpL4SadbS98oM=; b=vESpW8MByV/roQP+nP5rLIpnGFqhBkrWUdCj1rC3arNAk+LAuks4X1gkjNsz8rxtGzI/ kgHu7yQRZmuyQ1EISHPILQJfdCaPrHGQLb5vIvckYXuKr5gsFVExH5AQZLlb+XMGswBC/ kAGNSwENxCPeDNzyxHzywzruBlcCUT0u9RxLpAKii1ttTJL9WwUUra/MIBGRNLJ8ePflC 6SLr2P6eqN5oVMEHbL8QIR2nKwOOn+pRpX8DI9dwF5kBtu5l+7Lbw6E7GB98HDf6JK2k0 k+Rd245C276ZV2J49ngpaD/VdqKsN2vlYkl5/uu2TsofNBA3/OnmUpERrFc9qMdWskA== 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="A97LRtV/"; 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 83DE9606A32D; Sat, 7 Nov 2026 19:48:16 +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=A97LRtV/ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5808D1F0089F; Sat, 7 Nov 2026 19:48:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794080896; bh=PgfupnOI9y63nTPBwUuVBVA9ftyhpGgpL4SadbS98oM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=A97LRtV/UUHCQnkUrvGqtIgzlx3uxP7kbRD8h5sUFLlcyZ7oYisz4YMCY6RSdd6Sm aGMqDDuK6P+5x9Seobd4qsGICa4HESW+Cz/F1qLcDQzVfMsYE6L5a7c8cXt5TqjdlB 4dhzoAL1Y52RPCY0ERC7LzQSxATD+urOPRwyUDyWq3EdlK73K/izVvC8jaUQ3IJSpR +0HtFI4/RzJ8QAlpH5i5TJbibWEyZoUuLHW3ITklZGXyjWFSogroDc/g9fHJqQeoAZ ZVGRJlud74lr1UcNyWAVexAmqn9fxBB6cWjp08cpvKEPWyNrZ3ikNWMFnA5vG+z/3y PVLi6hgK3gFzw== Message-ID: Date: Sat, 7 Nov 2026 20:48:06 +0100 X-Mailing-List: linux-kernel@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 v3 2/4] docs/mm: ksm: describe how smart scan picks the scans to skip To: Qiliang Yuan , Andrew Morton , Xu Xin , Chengming Zhou , Stefan Roesch , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , Jonathan Corbet , Shuah Khan , Randy Dunlap Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org References: <20261107-bug-mm-ksm-smart-scan-align-skips-v3-0-16d5ce8b5fd6@gmail.com> <20261107-bug-mm-ksm-smart-scan-align-skips-v3-2-16d5ce8b5fd6@gmail.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: <20261107-bug-mm-ksm-smart-scan-align-skips-v3-2-16d5ce8b5fd6@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/7/26 17:57, Qiliang Yuan wrote: > Documentation/mm/ksm.rst covers the stable tree and its reverse mapping, > and Documentation/admin-guide/mm/ksm.rst only says that smart scan skips > pages KSM has failed to merge before. > > Neither says how often an unmerged page is looked at, or that the scans > are picked by the global scan number so that identical pages of > different ages still meet in some scan, which merging through the > unstable tree depends on. > > Add a section on smart scan that describes both. > > Signed-off-by: Qiliang Yuan > --- > Documentation/mm/ksm.rst | 16 ++++++++++++++++ > 1 file changed, 16 insertions(+) > > diff --git a/Documentation/mm/ksm.rst b/Documentation/mm/ksm.rst > index 5a80573162def..f954d221e8017 100644 > --- a/Documentation/mm/ksm.rst > +++ b/Documentation/mm/ksm.rst > @@ -75,6 +75,22 @@ The whole list of stable_node "dups" linked in the stable_node > The frequency of such scans is defined by > ``stable_node_chains_prune_millisecs`` sysfs tunable. > > +Smart scan > +---------- > +With ``smart_scan`` enabled, ksmd skips pages that it has already tried > +to merge without success. The ``age`` of a page's ksm_rmap_item counts > +the scans that went by while the page stayed unmerged. A page younger > +than three scans is looked at on every scan; after that it is looked at > +every second scan up to an age of five, every fourth scan up to an age > +of eight, and every eighth scan from then on. > + > +The scans that look at a page are picked by the global scan number > +``ksm_scan.seqnr``, not counted per page. The unstable tree is rebuilt > +on every scan, so two identical pages can only be merged in a scan that > +looks at both. As the periods are powers of two, every page is looked > +at on each scan whose number is a multiple of eight, so two identical > +pages meet at least every eighth scan whatever their ages. Yeah, much clearer, thanks Reviewed-by: David Hildenbrand (Arm) -- Cheers, David