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]) by smtp.lore.kernel.org (Postfix) with ESMTP id EBF16C71136 for ; Fri, 13 Jun 2025 19:57:34 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 70E746B007B; Fri, 13 Jun 2025 15:57:34 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 6BF5C6B0089; Fri, 13 Jun 2025 15:57:34 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5873C6B008A; Fri, 13 Jun 2025 15:57:34 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 395B26B007B for ; Fri, 13 Jun 2025 15:57:34 -0400 (EDT) Received: from smtpin24.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id D003FBEA57 for ; Fri, 13 Jun 2025 19:57:33 +0000 (UTC) X-FDA: 83551437186.24.7438DE2 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf05.hostedemail.com (Postfix) with ESMTP id 5302A10000A for ; Fri, 13 Jun 2025 19:57:31 +0000 (UTC) Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="b/zgs/RP"; spf=pass (imf05.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=quarantine) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1749844651; 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=i0TQKjESHxK6cTzcBrQME8IBFyrZ8jShVytZ++oPYrY=; b=4bCs46COQrBSQU7kv5KOBgXHEx6FrP3H+GXvWpuBCJw7wu5vvvn4WTWXakBbSMbRO25yZD kUJvVW7/6uBzAnQAI3NtQaQey5KpCKed8BjlrGSH9C+o3QTdytRYmGf4XRhKz5Ooc+sFUc y92gnb9MyAaVWdkdj1i+t8TFp+MjXqA= ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="b/zgs/RP"; spf=pass (imf05.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=quarantine) header.from=redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1749844651; a=rsa-sha256; cv=none; b=DPytu5+y6ueUClkxj3u1W+e10MfPv93pOl4dBvWgsPQUqxAkLz7ASs2S9ni6FPwN6fCPBg NjmfnLmy9Xu60p9fCAKixzq5l6b3JxXwjYKYu0YwCPzhv9BuKUKUzXO1vEoJgD8hxc5T61 IIPP8tNmKdYkGFbykJjQ9UL/ap8pDRU= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1749844650; h=from:from: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:autocrypt:autocrypt; bh=i0TQKjESHxK6cTzcBrQME8IBFyrZ8jShVytZ++oPYrY=; b=b/zgs/RP/a2Mcs/7mq8n4XWBhOiRUI3b9phImyFpxwkIniBNDGJPqm03Bwo2MV5UqCWz/T FNUwuOQ8OG4yks8sVoV+RPrC4VlHhXedxkf8VkbdTSbhDXC2NGwka2Q80R8bqhZVC+8UPd s0FclP9mRRJs9l8n486CKaFf4dYBtC0= Received: from mail-wm1-f72.google.com (mail-wm1-f72.google.com [209.85.128.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-21-YP-HSATgPXqKvB8X-WbOrA-1; Fri, 13 Jun 2025 15:57:27 -0400 X-MC-Unique: YP-HSATgPXqKvB8X-WbOrA-1 X-Mimecast-MFC-AGG-ID: YP-HSATgPXqKvB8X-WbOrA_1749844646 Received: by mail-wm1-f72.google.com with SMTP id 5b1f17b1804b1-45334219311so6955595e9.2 for ; Fri, 13 Jun 2025 12:57:27 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1749844646; x=1750449446; h=content-transfer-encoding:in-reply-to:organization:autocrypt :content-language:from:references:cc:to:subject:user-agent :mime-version:date:message-id:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=i0TQKjESHxK6cTzcBrQME8IBFyrZ8jShVytZ++oPYrY=; b=mk7PoGSVMCVdxJyDBPGXu2xWaVyeQAj9t+GLzBGjvGzc2lGiW43lrJKdiPsKyTuYMs /LunEgsQHkO551MvwgLHcjYGixC8R3faqzs+bQ16ZxDNVa+tyAm47ItVaAJ4Rzifcx2h rylqGjWLd0eqsmdj0V9IvAuPO2Y3Tv/DT0o9wQPOQcJoF0B9xHCZrMcS8CTnfnhRPrG4 UDsbMczB1fL5L1mcVukKQkFZZr5c22Onjd3fVSJtpvAcJkg8r+LrTBpGisz7npqeiadA M8gd765V9ym5L81/lIIXYNnbrtoC66HBg3KolYFerbBK8MnoSAklrYPZoU7VXPMZ5Q5r sGTQ== X-Forwarded-Encrypted: i=1; AJvYcCUGm2GrsnEJC16Xxp+lpQBbi6tnqqoqKjNudmcTQgvP97t5+5ZsjGLT6LfVHBTikQBpxRiZZL4E3Q==@kvack.org X-Gm-Message-State: AOJu0YxBDe4vFCmhRNYBlU0HIWy+lWZYcku7InY/kRcJVDmeBSlJEoha dZkLp8JIXBV3IRA0vDsyR4lhzLcOgppaB9xEVATz73b9jN3yTRlQdZILZ3CSmt7xKoIKYzGYxxW rBK2SUj4Lc7prAlA1eiVRmiAKvWWjZcwM1MiGEavSyo45GrHjyJeQ X-Gm-Gg: ASbGncvtGF7mtq/2Z08suOo4knJ/H3vaCla7zOrGrrH/nZ7nsUmdUjbGeBFHQ2obJA3 k/mACuROnlMb3DMTlkj8b1vQu80uvZ1Sys/0Qa5rXWsIrzmJmROz93EBjFBVZT+AD8ulue+Eifk 6iN73jxr+8My+MxwxDrDZ+mv6KJxOxJDeyK+nFujt5t2NeONVhzQixWeT/rzTcU0YfNWsAgjaKb ykU07x13bduvfPEAxLtH3/JBpg9piHHbosBWc1Qk5qnCRU313ZUL8ETLYSTyDYllP6Q/k2QNmXE YhgU0mjSHEdiEzjZk0Kms4pltX2Gp2sJkPrlBLqvXwymaTqjhuv+8MQlznIeo+GG87yonIdfIlh cOTCkYUECzIZxowDCBVTXHK/IV4/afa3pvljQUadS6UKSmtE9Cw== X-Received: by 2002:a05:600c:358b:b0:450:6b55:cf91 with SMTP id 5b1f17b1804b1-4533ca4681amr9623055e9.6.1749844646120; Fri, 13 Jun 2025 12:57:26 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEvApC5lDtccKglB9U88TXnFqonrOhZhGCcvfW7NYtyiyeauiD9kcVAxL15TvPCzlvjDstuXQ== X-Received: by 2002:a05:600c:358b:b0:450:6b55:cf91 with SMTP id 5b1f17b1804b1-4533ca4681amr9622825e9.6.1749844645704; Fri, 13 Jun 2025 12:57:25 -0700 (PDT) Received: from ?IPV6:2003:d8:2f1a:3700:2982:b5f7:a04e:4cb4? (p200300d82f1a37002982b5f7a04e4cb4.dip0.t-ipconnect.de. [2003:d8:2f1a:3700:2982:b5f7:a04e:4cb4]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4532dea1cc5sm63859665e9.16.2025.06.13.12.57.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 13 Jun 2025 12:57:24 -0700 (PDT) Message-ID: Date: Fri, 13 Jun 2025 21:57:23 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/5] mm,hugetlb: Document the reason to lock the folio in the faulting path To: Oscar Salvador Cc: Andrew Morton , Muchun Song , James Houghton , Peter Xu , Gavin Guo , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250612134701.377855-1-osalvador@suse.de> <20250612134701.377855-3-osalvador@suse.de> <44f0f1cc-307a-46e3-9e73-8b2061e4e938@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: tzGx-rom7QTtrduxPJAWb0xd5ID6dlTnRRSAq-tEDAQ_1749844646 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 5302A10000A X-Rspamd-Server: rspam07 X-Stat-Signature: tzf4gp4b3stbhxfpe5ghn5u1giuou9rk X-Rspam-User: X-HE-Tag: 1749844651-388183 X-HE-Meta: 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 x3fjU3T0 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 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 13.06.25 16:23, Oscar Salvador wrote: > On Fri, Jun 13, 2025 at 03:56:15PM +0200, David Hildenbrand wrote: >> On 12.06.25 15:46, Oscar Salvador wrote: >>> - /* hugetlb_wp() requires page locks of pte_page(vmf.orig_pte) */ >>> + /* >>> + * We need to lock the folio before calling hugetlb_wp(). >>> + * Either the folio is in the pagecache and we need to copy it over >>> + * to another file, so it must remain stable throughout the operation, >> >> But as discussed, why is that the case? We don't need that for ordinary >> pages, and existing folio mappings can already concurrently modify the page? > > Normal faulting path takes the lock when we fault in a file read-only or to > to map it privately. > That is done via __do_fault or cow_fault, in __do_fault()->vma->vm_ops_>fault(). > E.g. filemap_fault() will locate the page and lock it. > And it will hold it during the entire operation, note that we unlock it > after we have called finish_fault(). > > The page can't go away because filemap_fault also gets a reference on > it, so I guess it's to hold it stable. > What I meant is: Assume we have a pagecache page mapped into our page tables R/O (MAP_PRIVATE mapping). During a write fault on such a pagecache page, we end up in do_wp_page()->wp_page_copy() we perform the copy via __wp_page_copy_user() without the folio lock. In wp_page_copy(), we retake the pt lock, to make sure that the page is still mapped (pte_same). If the page is no longer mapped, we retry the fault. In that case, we only want to make sure that the folio is still mapped after possibly dropping the page table lock in between. As we are holding an additional folio reference in do_wp_page()->wp_page_copy(), the folio cannot get freed concurrently. There is indeed the do_cow_fault() path where we avoid faulting in the pagecache page in the first place. So no page table reference, an I can understand why we would need the folio lock there. Regarding hugetlb_no_page(): I think we could drop the folio lock for a pagecache folio after inserting the folio into the page table. Just like do_wp_page()->wp_page_copy(), we would have to verify again under PTL if the folio is still mapped ... which we already do through pte_same() checks? -- Cheers, David / dhildenb