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 5FC7BEC111A for ; Mon, 23 Feb 2026 19:17:04 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 4716F6B0005; Mon, 23 Feb 2026 14:17:03 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 41BA16B0089; Mon, 23 Feb 2026 14:17:03 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 300D06B008A; Mon, 23 Feb 2026 14:17:03 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id 18BB66B0005 for ; Mon, 23 Feb 2026 14:17:03 -0500 (EST) Received: from smtpin12.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 9B9821B5BF1 for ; Mon, 23 Feb 2026 19:17:02 +0000 (UTC) X-FDA: 84476679084.12.E5CDE45 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf02.hostedemail.com (Postfix) with ESMTP id D3C4C8000A for ; Mon, 23 Feb 2026 19:17:00 +0000 (UTC) Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b="VKH56jk/"; spf=pass (imf02.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1771874220; 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=pW9/rztysEsKQP4BwEVSRhDgWZqG1r5zaI+0B7PY8bc=; b=OTRlC7xj4h/jIes6wr0OKq5/+e+Y1KDrySJ8zWPpDonTtGg4IELCI8EjNj1PjsWjMJO80Q sxbTdkC14B6id7pu8lGCv7u8HBAdSiIKyhnFn/O2FLhJE3JZho089fobuF7HJlWKDZJtYB ZF6EkAXp/gT51O/kY2doxkjfXrlFmsQ= ARC-Authentication-Results: i=1; imf02.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b="VKH56jk/"; spf=pass (imf02.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1771874220; a=rsa-sha256; cv=none; b=z7pflKfDKfGcHOQdptN0J0eDLCPi3VqRPZNEyUXufNOidiRzPUcYiLnvbt5ix4/9WaMg/n gBrFRohESj6qEZ8eGx8EtZUSIVL4aglsOMe2/1aeZnGYO1VYKMypsWi675bkBFZDr//lW4 yzs6s4jGNpEk/8/MLRjPMyIUi7aRpEY= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id EDF1460141; Mon, 23 Feb 2026 19:16:59 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 52399C116C6; Mon, 23 Feb 2026 19:16:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1771874219; bh=QoLi5Ji30jg6+9EIkK83WuAw9m8Ump5BIlaU2i6pUmM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=VKH56jk/9sGgHun+FC5oPBfjXO+gF2v1895vNaw2i5skZ+VXV82v2KHOh6B8wUVWq wT1SFhGyC4SDVnFsSFOcTCCjg+uzfw0qzYs3Nx/tkTy5RpRx0sd3DC+uBMGyroG9Uz M/0ITMM7q6lm/eF6y434ojIJ2d4fuAOXuIaNHdKWuHR0t6efZMBfuQVeyXIag35ZDS ovgVd37o931o+qzt1N2PwrJXMFrCOBpBD/sQHf15fhaPN/T/cO5kHg7DP/fZ3fw5pO oW+6viPFd+2/wMkU/+4eiK83yMDY8UHVKp1FdnigKgAvbTYPLOlBGvLWm/Wx1qc3r/ yXZH7M2dWVajg== Message-ID: <45b78153-262c-43b4-acf6-7e3f153fbcc0@kernel.org> Date: Mon, 23 Feb 2026 20:16:53 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] mm: centralize+fix comments about compound_mapcount() in new sync_with_folio_pmd_zap() To: Matthew Wilcox Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Rik van Riel , Harry Yoo , Jann Horn References: <20260223163920.287720-1-david@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: 7bit X-Stat-Signature: o8tdphgh8n93mb6dz98hkiux8ga8yg5i X-Rspam-User: X-Rspamd-Queue-Id: D3C4C8000A X-Rspamd-Server: rspam01 X-HE-Tag: 1771874220-18138 X-HE-Meta: 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 Z1T8GIj1 +2TiETiV9lyrmZzrLs5n26nUsWJAEmiwNdDG0/htYxxb1Nn55zRvteaFk6CyWGpa68ZFMe9scUv9rLcLkjmXSVb+F+BIYPWM1HBzPitBx5RbMF6ev7BMK5H7994F+y11T5y5hp9hMxe4WMEyFCtiq5yIwovei6czpki+rsWASwmpprMIWMItn+ZpcZXbfKgp6GEegVRbkcp61bjgqWJzYmTtNsBlJc0BJd9tuS9guxqmciDKO/YYkLM0kARWqRi8CgIqFNIxSmiGqzuTylJfw5wL96WQJbXiwqqSith6J71BrpWoq6696DTc6bQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/23/26 18:58, Matthew Wilcox wrote: > On Mon, Feb 23, 2026 at 05:39:20PM +0100, David Hildenbrand (Arm) wrote: >> void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte); >> >> +/** >> + * sync_with_folio_pmd_zap - sync with concurrent zapping of a folio PMD >> + * @mm: The mm_struct. >> + * @pmdp: Pointer to the pmd that was found to be pmd_none(). >> + * >> + * When we stumble over a pmd_none() without holding the PTL while unmapping a >> + * folio that could have been mapped at that PMD, it could be that concurrent >> + * zapping of the PMD is not complete yet. While the PMD might be pmd_none() >> + * already, the folio might still appear to be mapped (folio_mapped()). >> + * >> + * Wait for concurrent zapping to complete by grabbing the PTL. >> + */ > > I like this. The one thing we've lost is the name of the function which > does the zapping, which I think was a helpful detail. I dropped it as we seem to have a talent to rename functions but to forget about updating comments. This very patch is an example of that :) But well, I used folio_mapped() ... so ... > Perhaps not to > someone who's deep in "how page tables work", but I wouldn't know where > to look for the counterpart to this. So how about: > > Option A: > + * When we stumble over a pmd_none() without holding the PTL while > + * unmapping a folio that could have been mapped at that PMD, > + * zap_huge_pmd() may not be complete yet. While the PMD might be pmd_none() > + * already, the folio might still appear to be mapped (folio_mapped()). > > Option B: > + * When we find a pmd_none() while unmapping a folio without holding > + * the PTL, zap_huge_pmd() may have cleared the PMD but not yet > + * modified the folio to indicate that it's unmapped. Let's do a slightly longer B: * When we find a pmd_none() while unmapping a folio without holding * the PTL, zap_huge_pmd() may have cleared the PMD but not yet * modified the folio to indicate that it's unmapped. Skipping the PMD * without synchronization could make folio unmapping code assume * that unmapping failed. Good? If so, Andrew can you squash or should I resend? Thanks for the review! -- Cheers, David