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 27105C02182 for ; Tue, 21 Jan 2025 07:49:14 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 8ADF96B0083; Tue, 21 Jan 2025 02:49:13 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 8371A6B0088; Tue, 21 Jan 2025 02:49:13 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6619E280001; Tue, 21 Jan 2025 02:49:13 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 455706B0083 for ; Tue, 21 Jan 2025 02:49:13 -0500 (EST) Received: from smtpin13.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id E7BAFC078B for ; Tue, 21 Jan 2025 07:49:12 +0000 (UTC) X-FDA: 83030683344.13.44D244B Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf01.hostedemail.com (Postfix) with ESMTP id 5807640008 for ; Tue, 21 Jan 2025 07:49:10 +0000 (UTC) Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Nmp2a7rT; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf01.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1737445750; a=rsa-sha256; cv=none; b=XTANoNk71lRccFXk8wKe5XGJ8hCyOKh91iPCjNVPsxg1EIVLkhzlqgOMhfFQ7a9tfwNpS7 6Poq6TlYH2OpiU3Bb8SgOmov/4kjgbDElJ84ehcRLnaBFYZd1BI0Un2nSZP263y5QRdBm+ dE1up45zr4au6PMoy81/pE3Qpf6pYAg= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Nmp2a7rT; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf01.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1737445750; 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=pEWTOBE0YfA5NLzP4KdNTpkr07FskrCb/W5+adJSTLI=; b=YcChMHwp/richdxgEpxagbYNhuRFuH6GNBWd0ApRhiilDMNrKBbvMb3og0CcErwS5TQFqC hr+ko8BtQEMQuMRqxQBOL6AxGXs+ge68+e+XlDdVrreiN3Jzc/G3zXJY092ttrYBQJJQft B+j1CAE707RgUcAuvXXnqSjHuRU44Jc= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1737445749; 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=pEWTOBE0YfA5NLzP4KdNTpkr07FskrCb/W5+adJSTLI=; b=Nmp2a7rTGfXWrbiHgnvyPkcBbKn5gmaYOAMHF9AivPuNgrVYVCZSQt5SO7X6RJn5R7t8th aQXhpY8ta2+p6GbMb1kESP8J6fYtRcj2pHjipEfJqIkLfQnoYEZVoh9vT9kYJfc2CjHrFM bM0uy9jpNMSj2Yu0g7VQAXc7PSTDO5o= Received: from mail-wr1-f71.google.com (mail-wr1-f71.google.com [209.85.221.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-425-ouexpmTBOXaFM1IcflfVEw-1; Tue, 21 Jan 2025 02:49:08 -0500 X-MC-Unique: ouexpmTBOXaFM1IcflfVEw-1 X-Mimecast-MFC-AGG-ID: ouexpmTBOXaFM1IcflfVEw Received: by mail-wr1-f71.google.com with SMTP id ffacd0b85a97d-3862be3bfc9so3251402f8f.3 for ; Mon, 20 Jan 2025 23:49:08 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1737445747; x=1738050547; 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=pEWTOBE0YfA5NLzP4KdNTpkr07FskrCb/W5+adJSTLI=; b=dUaeCrOVE/VBnjRM+idrGwbHWAgzCniebQZVjda+ANnsW9mHXwOKMr+sxCJIVawgVI tCxGYVloGxDXpL/ykC/0MNl+wFPdnJn8Flxb4EE0yJObJrRUPPYPRFYZZ/aZYqZULawe WCLxbFx5Wj26ZdAZn7fTE/OYq4XKah2vWBM2ybRfLYI0beQk+umm29MIT7UhbHPzAm6C IhIw7nyGSECpMY0+TmQI504V+gY6dE67j+l1DosF1tAS59kz1OP/EFR2d/hQxxyZkY8C 0PIFawYyMn+PX15H9NastOAgO8wtfvMwOAV5JRBlIivvwZdX9xzpfXaJRlPxAPd1+Gey 0DBQ== X-Forwarded-Encrypted: i=1; AJvYcCWO5nQyIt5qfmVPM5eRnqcSe6m5F+gPZ2bIyKqGTPsTXgjuqMStJflrbI2YJP/DPG5CJathK8iYMQ==@kvack.org X-Gm-Message-State: AOJu0YyrZF0p8pcfioppOg8TToRiPIwfeSCkT0eO5x/wcEuM6nwcp7+x 0OcSpejyH17zznNMccp3939ZVXts2GPN5L9BLluY/boG9TYIgk0nOVnPOoythYELGjeaf5KSTMT IOBf50AKRB5jM/GA21rrSFacc8yIMbbKUFXKEuwi2FaBfXDc1 X-Gm-Gg: ASbGncsj6x7S68lafpHyOQ73tq8vmJow66zzm7+y6vNMvjD165t6Dy22SMzlG5qhuwP MBOJ1sG6VjMr/ZxW0hhg/VJvPh+c4g6CC1mL5i408Yc4il5DZjt+tF+jIxEr2SwT8/ePt4l71U1 InRCafRGuo+Xo2NBbwzkzmpLDEMe8YC5X/tFboDt0U1ys3eYtDVXnY1892ZqUZf8lnuFEpxzBIx Ch5ZTM8V1walqVsynHNrpgGA00qbdaqq6OZXJkcn/PQiNtngirDKnfokwf7bq7ykQZOw8AU20eI IIYkps6UgB3Bbj3hu9UL3xXBYbRLZw0cA5sfuOeGXdY1JlqN4LfAtfojMaykvu8xYGW6RuILmbC tktaRjyyWkGJo7mnWr0+jmg== X-Received: by 2002:adf:ef04:0:b0:385:f23a:2fe1 with SMTP id ffacd0b85a97d-38bf566f7c9mr11016947f8f.26.1737445747047; Mon, 20 Jan 2025 23:49:07 -0800 (PST) X-Google-Smtp-Source: AGHT+IGzHEhQMANKQac8Qho0ClYfJCOl0n1h+CUlYADqWE59ZH5VVRLVP0S8q9br897Namh1jCaz0A== X-Received: by 2002:adf:ef04:0:b0:385:f23a:2fe1 with SMTP id ffacd0b85a97d-38bf566f7c9mr11016928f8f.26.1737445746685; Mon, 20 Jan 2025 23:49:06 -0800 (PST) Received: from ?IPV6:2003:cb:c709:6200:16ba:af70:999d:6a1a? (p200300cbc709620016baaf70999d6a1a.dip0.t-ipconnect.de. [2003:cb:c709:6200:16ba:af70:999d:6a1a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38bf3221db2sm12368688f8f.29.2025.01.20.23.49.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Jan 2025 23:49:06 -0800 (PST) Message-ID: Date: Tue, 21 Jan 2025 08:49:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: gup: fix infinite loop within __get_longterm_locked To: Zhaoyang Huang Cc: "zhaoyang.huang" , Andrew Morton , Alistair Popple , John Hubbard , linux-mm@kvack.org, linux-kernel@vger.kernel.org, steve.kang@unisoc.com References: <20250120092604.3590285-1-zhaoyang.huang@unisoc.com> <4326b3f1-f99d-4be8-97fd-286c75358635@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: mfjl5XSVHvqnIv_0dVTl89cgodx4Kqy2kjzPbS-aK8A_1737445747 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 5807640008 X-Stat-Signature: dgkxpp3d6jc3n3ik8mu6jec6zr6jrn9q X-HE-Tag: 1737445750-876981 X-HE-Meta: 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 jJY+MSMR 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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 21.01.25 02:31, Zhaoyang Huang wrote: > On Tue, Jan 21, 2025 at 4:14 AM David Hildenbrand wrote: >> >> On 20.01.25 10:26, zhaoyang.huang wrote: >>> From: Zhaoyang Huang >>> >>> Infinite loop within __get_longterm_locked detected in an unique usage >>> of pin_user_pages where the VA's pages are all unpinnable(vm_ops->fault >>> function allocate pages via cma_alloc for hardware purpose and leave them >>> out of LRU) Fixing this by have 'collected' reflect the actual number> of pages in movable_folio_list. >> >> Maybe something like: >> >> " >> We can run into an infinite loop in __get_longterm_locked() when >> collect_longterm_unpinnable_folios() finds only folios that are isolated >> from the LRU or were never added to the LRU. This can happen when all >> folios to be pinned are never added to the LRU, for example when >> vm_ops->fault allocated pages using cma_alloc() and never added them to >> the LRU. >> >> We incorrectly update the "collected" variable even if nothing was >> collected. Fix it by incrementing "collected" only when we isolated a >> folio and added it to the list of folios to migrate. >> " >> >> I assume, long-term these things will not actually be folios, but pages, >> and we'll have to skip them in different code -- or assume they can be >> longterm pinned even on CMA because they are allocated by the CMA-owning >> driver. > Thanks for the commit message. will update them to v2 >> >>> >>> Signed-off-by: Zhaoyang Huang >>> --- >>> mm/gup.c | 4 ++-- >>> 1 file changed, 2 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/gup.c b/mm/gup.c >>> index 3b75e631f369..2231ce7221f9 100644 >>> --- a/mm/gup.c >>> +++ b/mm/gup.c >>> @@ -2341,8 +2341,6 @@ static unsigned long collect_longterm_unpinnable_folios( >>> if (folio_is_longterm_pinnable(folio)) >>> continue; >>> >>> - collected++; >>> - >>> if (folio_is_device_coherent(folio)) >>> continue; >>> >>> @@ -2359,6 +2357,8 @@ static unsigned long collect_longterm_unpinnable_folios( >>> if (!folio_isolate_lru(folio)) >>> continue; >>> >>> + collected++; >>> + >>> list_add_tail(&folio->lru, movable_folio_list); >>> node_stat_mod_folio(folio, >>> NR_ISOLATED_ANON + folio_is_file_lru(folio), >> >> What if folio_isolate_hugetlb() succeeded? The return value can tell us >> if it actually succeeded. > How about remove the variable 'collected' and change the criteria to > if(list_empty(&movable_folio_list)) That works if we know that the input list is empty, which is the case. So let's turn that function into a void function, ans simply check list_empty() in the caller that prepares the empty list. -- Cheers, David / dhildenb