From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 70E02280A3B for ; Thu, 12 Feb 2026 08:54:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770886452; cv=none; b=VscuiZKZ9RFSsJsQWjpW3oWRWTfLb8xpzP8PtoU/Zr4xSaco8NldxYCERQvN9UpkOcdGoO/FUD6/oeWGyV+5KGLJ4w4bV/ouV3CHXakQ+NqMVc4/JUMc6LtjnhSJS/vmjVfBD5CIamg+pyj3oPvZs+1dUo0r3vzMsJ3D/qn9kGE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770886452; c=relaxed/simple; bh=k3MEodMonFKXjo9Ttxo2bEV50SdtUgQar2VKT32zHqw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PFEVnqa5+k7Q0ahWrGSXFLTE8TQYxZsgJ5jpPboPMO+v9SDryi4BW26n0AHItQjodG4Z1pKNJqUTFKrCr2JxdyjJ2T5gbSFl+hmoYfAbmMcZmBEmky3vCZpAtIJHjKoNLFrGyqrDuituNOeWLhYXwZp30h95/aB1RXwXRsAdPYA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=O4PqR7Ld; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="O4PqR7Ld" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DCE60C4CEF7; Thu, 12 Feb 2026 08:54:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770886452; bh=k3MEodMonFKXjo9Ttxo2bEV50SdtUgQar2VKT32zHqw=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=O4PqR7LdKInuOIyuh/x8pyWHgAKOhVqykx8Tf4TL51yRXXpbvxVQFwrhxP1KLJQgv JD6wWS8ouBUVc9ULNmWrKn3yLW4xqye8H5uF+F3/KCPdnwl8K9pvYXTPPpApcOHh2a 3y5LXC0q9gdg6Q1P/hF/OXb0pjkzKymFLx7CVRe/zFZByfpzfMYVOMbIqSJ2sX0MAr ZEgr2u8AE+lfYJ72cil1eI+8pnGcJepRjqURN18exLxopHyvv8qVLCb9mjQ2tfs7S5 pJXuKvJfptTIYxJnQ6BX2Zhcwqpm3QnvgR6OcDWh4+LmMp1b/2ufEJdmiUJTH1ox2U aGfdkI9IByHFg== Message-ID: Date: Thu, 12 Feb 2026 09:54:04 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm: only set fault addrsss' access bit in do_anonymous_page To: Wenchao Hao Cc: Andrew Morton , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260210043456.2137482-1-haowenchao22@gmail.com> <5c4d773c-e3e7-4a71-b250-91701cbdd4a2@kernel.org> <201fcbb4-c43c-406c-ad54-f2c9301c12b0@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; format=flowed Content-Transfer-Encoding: 8bit On 2/12/26 02:57, Wenchao Hao wrote: > On Wed, Feb 11, 2026 at 5:05 PM David Hildenbrand (Arm) > wrote: >> >> On 2/11/26 01:49, Wenchao Hao wrote: >>> On Tue, Feb 10, 2026 at 5:07 PM David Hildenbrand (Arm) >>> wrote: >>> >>> We have enabled 64KB large folios on Android devices, which may introduce >>> some memory waste. I want to figure out the proportion of memory waste >>> caused by large folios. Reading the "Referenced" field from /proc/pid/smaps >>> is a relatively low-cost method. >> >> Right. And that imprecision is to be expected when you opt-in into >> something that manages memory in other granularity and only has a single >> a/d bit: a large folio. >> >> Sure, individual PTEs *might* have independent a/d bits, but the >> underlying thing (folio) has only a single one. And optimizations that >> build on top (pte coalescing) reuse that principle that having a single >> logical a/d bit is fine. >> >>> >>> Additionally, considering future hot/cold page identification, we aim to >>> detect 64KB large folios where some pages are actually unaccessed and split >>> them into normal pages to avoid memory waste. >>> >>> However, the current large folio implementation sets the access bit for all >>> page table entries (PTEs) of the large folio in the do_anonymous_page >>> function, making it hard to distinguish whether pre-allocated pages were >>> truly accessed. >> >> The deferred shrinker uses a much simpler mechanism: if the page content >> is zero, likely it was over-allocated and never used later. >> >> It's not completely lightweight (scan pages for 0 content), but is >> reliable, independent of the mapping type (PMD, cont-pte, whatever) and >> independent of any access/dirty bits, leaving performance unharmed. >> >> When you say "I want to figure out the proportion of memory waste", are >> we talking about a debug feature? >> > > Thanks for your explanation. I now understand the design logic. > > What I’m proposing is mainly for debugging. After enabling 64K large folio > on Android, we observed increased application memory footprint, especially > for anonymous pages. > > Since Android app memory usage depends on runtime scenarios, we cannot > confirm if the growth is directly caused by large folio. We want to > analyze memory > usage via the `Referenced` field in `/proc/[pid]/smaps`. Scanning for zero-filled pages will be much easier and more reliable. For a debug feature good enough. I'm wondering what the best interface for something like that could be: we don't want to make "/proc/[pid]/smaps" slower for all users. Maybe we could for debug kernels. For example, adding with CONFIG_DEBUG_KERNEL a new entry Anon_Zero: counter that just tests whether the page content of an anonymous page is all zeroes could be doable. -- Cheers, David