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[2003:d8:2f1b:b800:6fdb:1af2:4fbd:1fdf]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4efe6cd15sm21362771f8f.39.2025.06.04.02.12.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Jun 2025 02:12:29 -0700 (PDT) Message-ID: Date: Wed, 4 Jun 2025 11:12:28 +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> <223cf8fc-7743-497f-893c-37ac689af002@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: u_j6bkQHCXvOm_fwIUzmxCVOJpaI9frx3pOOMMASkZk_1749028351 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: CA1CF8000D X-Stat-Signature: h9mm6kqgajcco3y4gtdujxzpffneeisk X-Rspam-User: X-HE-Tag: 1749028355-441298 X-HE-Meta: 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 Uqmlg8qh 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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 04.06.25 03:04, Zhaoyang Huang wrote: > On Tue, Jun 3, 2025 at 9:12 PM David Hildenbrand wrote: >> >> 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. I think the key >>> point is to find a way to distinguish the temporary(on the way to LRU) >>> and permanent CMA pages within collect_longterm_unpinnable_folios. >>> >>> static long >>> check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs) >>> { >>> + bool any_unpinnable; >>> LIST_HEAD(movable_folio_list); >>> >>> - collect_longterm_unpinnable_folios(&movable_folio_list, pofs); >>> - if (list_empty(&movable_folio_list)) >>> - return 0; >>> + any_unpinnable = >>> collect_longterm_unpinnable_folios(&movable_folio_list, pofs); >>> + if (list_empty(&movable_folio_list)) { >>> + if (any_unpinnable) >>> + pofs_unpin(pofs); >>> + return any_unpinnable ? -EAGAIN : 0; >>> + } >>> >> So, what's the status of that? We should fix it upstream (*not* caring >> about controversial out-of-tree pkvm issues). > Leaving aside the pkvm issue, we should also care about the CMA pages > mapping to VM by special driver which are intended to be long term > pinned (actually they are fetched by cma_alloc and then mapped to VM > instead of alloc_pages during normal page fault). Is there any such "special driver" in the tree? > Could we distinguish > them by the patch below based on 1aaf8c122, that is, this kind of > pages is not on page cache and have equaled refcnt to mapcount No, not like that. We'd need some proper indication that this page was allocated by the CMA area owner, and that owner agrees that the folio can be long-term pinned (maybe that agreement is by mapping it into user space, tbd). Will you send the fix or should I do it? Discussing about broken use cases that do no apply upstream is not particularly helpful when we're dealing with a real upstream bug. -- Cheers, David / dhildenb