From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0D33BCA5FB1 for ; Wed, 30 Sep 2026 11:38:51 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 135006B008C; Wed, 30 Sep 2026 07:38:50 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0E64C6B0092; Wed, 30 Sep 2026 07:38:50 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F3D046B0093; Wed, 30 Sep 2026 07:38:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id C62646B008C for ; Wed, 30 Sep 2026 07:38:49 -0400 (EDT) Received: from smtpin30.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay03.hostedemail.com (Postfix) with ESMTP id D2D03A0744 for ; Wed, 30 Sep 2026 11:38:47 +0000 (UTC) X-FDA: 85270231494.30.35EAC6F Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf30.hostedemail.com (Postfix) with ESMTP id 16C0A80006 for ; Wed, 30 Sep 2026 11:38:45 +0000 (UTC) Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=hlXqgclW; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf30.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1790768326; b=S5wwSu31mouK4nC6VfVdTAqH5rb+rz1BUqG0VLQUjMtokuByuVNvbZQQaqutz5sJIf9WVI RS06BTpRjZlZkezofA40lG9xeWIxq17edui7yCHa44UsDIBZOXRJ8W3c2aU/GCUvSQRMJJ GrLQpbzUM5bn8mPG4x6Z6/SSmgTH7Vo= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=hlXqgclW; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf30.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1790768326; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=Wz/bhxjtBi7K7HV8B4o74LbjtU5sEbHi12rmGSrEnCo=; b=Kbn8y4+Jk6rI5H584BdAfcII1YHk0MF3A5C5hAlCCZufdnRaZSdAnxAkJj33rMUXKnfTPK J8na2HFiBvS668kbuPXPWbmC7us65sN123seroYoFVAZ1cafHD+eQ9Xa2m8mzA03ZjU+Gw NTtX2guspGnNETANkdv4TRGfBTNG7+8= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id E94FE41815; Wed, 30 Sep 2026 11:38:44 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 01D321F000FF; Wed, 30 Sep 2026 11:38:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790768324; bh=Wz/bhxjtBi7K7HV8B4o74LbjtU5sEbHi12rmGSrEnCo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hlXqgclWy+RKfo1IVb2/Lhnu8JTiq/kzASCVvfz+5uw2hJoEuS+yzXluIarqaXv0l Tfh2Y9vvfC38nBPizbuXgNfVgBkbKCtg4rBPAAkvrfNYYqEdAzUMx+WgWijY2jw5f0 SkEL5tePsyaJMBG9alVE2MokNkkd683JUEgy1eCiCnqakNhyCZ5TkkDgolpBTQWMG4 P8WxUFHSS45NEIsTQS9U6Rdnt/14/iaq6kYsi4dwHs+PD2Mjo8c4FUAFcJsZC88i/c /GzlVOokskwUvLEALhYyAt5dUSp0WAEYx0hELPCjB9Zea9+O996J5Gov0AzAvObyqH D5wobrpPTONqA== Message-ID: Date: Wed, 30 Sep 2026 13:38:39 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] madvise: best-effort deferred writeback for shared file mappings To: =?UTF-8?B?5aSp54u8?= , "Lorenzo Stoakes (ARM)" Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Matthew Wilcox , Jan Kara References: <7ebe4591-2306-4b5c-b83e-6a2e34b52bd8@kernel.org> <85aff663-2131-47da-ac04-8f0799a49b91@kernel.org> <1ce9cf3a-15dc-4815-ba29-2e22b05206fb@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 X-Rspamd-Server: rspam06 X-Stat-Signature: z148xziyqnmo33hmnskmuh6i3ubo4wko X-Rspam-User: X-Rspamd-Queue-Id: 16C0A80006 X-HE-Tag: 1790768325-720484 X-HE-Meta: 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 jNyG9q1N A56yy0YhXl6zTKGb4E2ThpdtOCrB/3Z6kZAP1S+n87UAyGqu53c6ozwCrXBMENH/+jfA+a3Qv1eexl9C6RbjXSM6I4Owl+zumkrdQBJiAGk+VWXF5v+C+oSUuLEOtaLvfgO6KsF7xWFIJgmB5aLLHbYXUrGMvN5ZJ5sbTWtEt/l0OstwQfYpdim2E6ldRkcVOVjI3nJKS2C4ppRbjMSLF+hROCdYAafBUO+CxwkPIQSTgMOb44vupiq8Yn7EE9NAxM+QtUEyeKpTA70JFJI8td9Q+kw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/30/26 12:27, 天狼 wrote: > Hi David, Lorenzo, > Hi, >> You really need an actual file on disk. Shared memory / shmem / memfd is >> not sufficient? > > Yes, an ordinary disk-backed file is the intended output, and subsequent > consumers access it through mmap. > > Shared memory can preserve state across a process crash if the shared > object remains alive. However, using shmem/memfd would introduce a separate > staging and transfer step to put the data into the final file. Building > directly in a file-backed MAP_SHARED mapping lets us preserve the latest > userspace writes across process crashes and retain the same file-cache > pages for subsequent consumers. Why do you really need the data on a file in disk? What you could do is, load it once from the file into shmem, then let everybody work on shmem, and have some background thread that periodically writes the shmem content out to the real file on disk. Then, you could completely control when I/O would happen. [...] >> It'd be problematic to effectively corrupt the page cache by saying "hey >> this is dirty but just clear dirty state and pretend this is what the >> disk has". > > To clarify, the proposal does not involve clearing dirty state or > pretending that memory and disk contents match. The pages would remain > dirty and continue to count toward the existing dirty-page limits. > IIUC, vmscan would still write out the folio if dirty, so I'd assume that at least memory reclaim would not be affected, only background writeback. > The requested hint only expresses that the application expects to modify > the data again soon and would prefer background writeback to happen later. > >> Delaying writeback could interfere with the writeback balancing logic. > > I understand that concern. The hint should not exempt the application from > dirty-page limits or throttling. If writeback is needed for balancing, > memory pressure, or explicit synchronization, the kernel should override > the hint. > > The intended benefit is to avoid writing intermediate versions when the > kernel has room to defer that work. There is no requirement to prevent > writeback until construction finishes. > > madvise is not the only option, deferring writeback for the entire file > would also be acceptable. > > Could such an advisory preference be accommodated within the existing > writeback balancing policy while retaining its limits and fairness? When using MAP_SHARED with disk-based files for VM memory, people ran into similar problems: the VM will constantly dirty many folios and background writeback will just permanently write these out and wear storage. For VMs you really only want the memory persisted in the file once you e.g., migrate the VM. Essentially, once the VM is paused. One of the reasons why we tell people to not use that combination and use shmem/hugetlb instead. I do wonder whether there is some similarity ... we don't want brackground writeback as long as there is heavy activity on these files. -- Cheers, David