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[2003:cb:c710:2b00:9c9f:9357:3f4c:5358]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3502b8969fbsm3791380f8f.37.2024.05.10.00.54.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 10 May 2024 00:54:21 -0700 (PDT) Message-ID: Date: Fri, 10 May 2024 09:54:20 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to migration glitches To: John Hubbard , Alistair Popple , Jason Gunthorpe Cc: Christoph Hellwig , Andrew Morton , LKML , linux-rdma@vger.kernel.org, linux-mm@kvack.org, Mike Marciniszyn , Leon Romanovsky , Artemy Kovalyov , Michael Guralnik , Pak Markthub , Vivek Kasireddy References: <20240501003117.257735-1-jhubbard@nvidia.com> <20240501121032.GA941030@nvidia.com> <87r0el3tfi.fsf@nvdebian.thelocal> <92289167-5655-4c51-8dfc-df7ae53fdb7b@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-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: xq51z7wt656zt8rk8ktx11aac1sw9b8m X-Rspam-User: X-Rspamd-Queue-Id: B5A1D100009 X-Rspamd-Server: rspam05 X-HE-Tag: 1715327665-729448 X-HE-Meta: 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 EoViHIT6 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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 02.05.24 20:10, John Hubbard wrote: > On 5/1/24 11:56 PM, David Hildenbrand wrote: >> On 02.05.24 03:05, Alistair Popple wrote: >>> Jason Gunthorpe writes: > ... >>>>> This doesn't make sense.  IFF a blind retry is all that is needed it >>>>> should be done in the core functionality.  I fear it's not that easy, >>>>> though. >>>> >>>> +1 >>>> >>>> This migration retry weirdness is a GUP issue, it needs to be solved >>>> in the mm not exposed to every pin_user_pages caller. >>>> >>>> If it turns out ZONE_MOVEABLE pages can't actually be reliably moved >>>> then it is pretty broken.. >>> >>> I wonder if we should remove the arbitrary retry limit in >>> migrate_pages() entirely for ZONE_MOVEABLE pages and just loop until >>> they migrate? By definition there should only be transient references on >>> these pages so why do we need to limit the number of retries in the >>> first place? >> >> There are some weird things that still needs fixing: vmsplice() is the >> example that doesn't use FOLL_LONGTERM. >> > > Hi David! > Sorry for the late reply! > Do you have any other call sites in mind? It sounds like one way forward > is to fix each call site... We also have udmabuf, that is currently getting fixed [1] similarly to how we handle GUP. Could you and/or Jason also have a look at the GUP-related bits? I acked it but the patch set does not seem to make progress. https://lkml.kernel.org/r/20240411070157.3318425-1-vivek.kasireddy@intel.com The sad story is: (a) vmsplice() is harder to fix (identify all put_page() and replace them by unpin_user_page()), but will get fixed at some point. (b) even !longterm DMA can take double-digit seconds (c) other wrong code is likely to exist or to appear again and it's rather hard to identify+prevent reliably IMHO we should expect migration to take a longer time and maybe never happening in some cases. Memory offlining (e.g., echo "offline" > sys/devices/system/memory/memory0/state) currently tries forever to migrate pages and can be killed if started from user space using a fatal signal. If memory offlining happens from kernel context (unplugging DIMM, ACPI code triggers offlining), we'd much rather want to fail at some point instead of looping forever, but it hasn't really popped up as a problem so far. virtio-mem uses alloc_contig_range() for best-effort allocation and will skip such temporarily unmovable ranges to try again later. Here, we really don't want to loop forever in migration code but rather fail earlier and try unplug of another memory block. So as long as page pinning is triggered from user context where the user can simply abort the process (e.g., kill the process), sleep+retry on ZONE_MOVABLE + MIGRATE_CMA sounds reasonable. > > This is an unhappy story right now: the pin_user_pages*() APIs are > significantly worse than before the "migrate pages away automatically" > upgrade, from a user point of view. Because now, the APIs fail > intermittently for callers who follow the rules--because > pin_user_pages() is not fully working yet, basically. > > Other ideas, large or small, about how to approach a fix? What Jason says makes sense to me: sleep+retry. My only concern is when pin_user_pages_*() is called from non-killable context where failing at some point might be more reasonable. But maybe that use case doesn't really exist? -- Cheers, David / dhildenb