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[2003:d8:2f30:ec00:8f7e:58a4:ebf0:6a36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45006498d40sm21401805e9.6.2025.05.28.05.57.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 May 2025 05:57:25 -0700 (PDT) Message-ID: Date: Wed, 28 May 2025 14:57:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: reply: [RFC] pin_user_pages_fast failure count increased To: Zhaoyang Huang Cc: Hyesoo Yu , jaewon31.kim@samsung.com, John Hubbard , "zhaoyang.huang@unisoc.com" , "surenb@google.com" , "Steve.Kang@unisoc.com" , Jaewon Kim , "linux-mm@kvack.org" , Jang-Hyuck Kim References: <4e2305d6-b067-4963-b16a-367a254d22c1@nvidia.com> <20250526074845.GA2848800@tiffany> <20250526093258.GA3489925@tiffany> <20250526111744epcms1p89d664f5cebd1e690730f32b66c24e3c0@epcms1p8> <20250528012329.GA1545287@tiffany> <20250528033626.GA1607193@tiffany> 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: Pep0uq_RzYy-4Cs2kQLX1G91I_OK3Bc_0gQwB5YR-0k_1748437047 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 36FC114000E X-Stat-Signature: fka5n8dk76mtpfjc4tra4idujsfjyp8w X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1748437050-63214 X-HE-Meta: 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 +e7gpDzw 1D7xEMzJzAZ3zey9hJ/98RjaayfsK/In00UEn8DcTxVxd4ZoFpdWfctTOltaNhf8LAuMpUobv7R2EkpCsugnstFiuEkJ71DTtz18rwTUGx2rams13vI/MvgUqExFOGxHk88eFbe4m/dfv2uL6D0x5cSjYFSPgZBN6XRp9mWkm0GQ6hn/ykMg6mn47pB/6QiFur4dmDKbJFI+mjamL/kCjs5GmrLsR/t/X/DuS+e/Lhnr5djJ9DUHAiDk8CX1emo/7UwTlJCLkXYLUwX0fUZjAe8ZCC/hvocwfz6hlkN/bXJrGk7Jd/Lbg4uSpLDpztnNHPdnmVzMEV2GW7ATiEn8cj3gZULJL51HdpCdsGVlRV5xAoEQL/ZCy+jr2jGJdjSKmNruqhZ8oRkG6K8V4p55lBMookO0k2E7qXUN0/J6c/zhZA3UC36hohXUe2u6mscScsuUIp9Yjf1t9D5BPOIDa0Phwb3S+fDZON+gWlghAhiPQ9lxX8fDiraNDmd1VqoeDim3LF/DmChDf5+8dKJiivPQ8y7opzS9r4S413qzHDLUrXGJ6McoxT1CE1wXn59CfVXQA+0jPYOeL04KTzCOxvV6IRNosPJdmTD3fUdzlPG+ylMNeCPni3PW3my1eFlaDTOQF 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 28.05.25 12:59, Zhaoyang Huang wrote: > On Wed, May 28, 2025 at 3:55 PM David Hildenbrand wrote: >> >> On 28.05.25 05:36, Hyesoo Yu wrote: >>> On Wed, May 28, 2025 at 10:49:36AM +0800, Zhaoyang Huang wrote: >>>> On Wed, May 28, 2025 at 9:25 AM Hyesoo Yu wrote: >>>>> >>>>> On Mon, May 26, 2025 at 07:49:57PM +0800, Zhaoyang Huang wrote: >>>>> >>>>> Hello, Zhaoyang. >>>>> >>>>> I don't believe commit 1aaf8c was just intended to prevent an infinite loop. >>>>> The commit was introduced to allow pinning CMA memory in the pKVM on AOSP. >>>>> >>>>> That leads me to question whether the assumption that CMA can be long-term pinned is actually valid. >>>> That depends on the user of CMA, yes for my scenario since it worked >>>> for the guest os. For common scenario such as the file/anon mapping, >>>> the page will be judged as unpinnable for long-term and be migrated >>>> out of CMA area. >>> >>> Your scenario and the common scenarios can not be distinguished from the kernel API's perspective. >>> Even in common cases, the page may be in a non-LRU state temporiarily, and in such situations, >>> pinning CMA can lead to bugs - we've encountered multiple issues because of this. >>> >> >> Right. We just disallow long-term pinning CMA pages, because we don't >> know who the real owner is that would be okay with long-term pinning them. >> >>>>> >>>>> In my opinion, it might be more appropriate to revert that commit 1aaf8c and instead ensure >>>>> that pKVM avoids using CMA for memory that requires long-term pinning through GUP ? >>>> It is not a pkvm issue but a defect of applying FOLL_LONGTERM over >>>> non-LRU CMA pages. >>> >>> In include/linux/mm_types.h, the CMA should be migrated when FOLL_LONGTERM. >>> >>> * In the CMA case: long term pins in a CMA region would unnecessarily fragment >>> * that region. And so, CMA attempts to migrate the page before pinning, when >>> * FOLL_LONGTERM is specified. >>> >>> Given this, would it make sense to avoid using FOLL_LONGTERM in this code path ? >> >> If something is unbounded in time, FOLL_LONGTERM is the right thing to use. >> >>>>> >>>>> Alternatively, instead of changing the current logic that prevents longterm GUP from pinning CMA, >>>>> it would be better to propose a new patch that specifically addresses the pKVM scenario like adding new FOLL_flags ? >>>> I don't think so. pin_user_pages is an exported API which can't make >>>> assumptions over the caller. >>> >>> My point is not to base the patch on assumptions about the caller, >>> but to define a clear mechanism that ensures safe behavior in the intended scenario. >>> >>> For example, you can add FOLL_NO_MIGRATION and skip to migrate unpinnable pages. >> >> Not sure which exact semantics you have in mind. But failing if we would >> have to migrate might be ok. Not sure if the caller should worry about >> that, though: the caller should not have to worry about page placement >> in general. > With going over the whole thread, I think the root cause is > collect_longterm_unpinnable_folios() hit the race window between > lru_add_drain_all() and folio_isolate_lru() by chance and returned > with ret=0 which finally have the CMA page pinned, right? However, I > find the proposed patch below will fail the PKVM > scenario(FOLL_LONGTERM set with non-LRU CMA pages) again as the CMA > pages never go to LRU which will have the __gup_longterm_locked loop > in do while(ret == -EAGAIN) as it did before 1aaf8c. For upstream that is the exact right thing to do. CMA pages must not get longterm pinned. -- Cheers, David / dhildenb