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V" , Zi Yan , shuah Khan , Dev Jain References: <20241219124717.4907-1-donettom@linux.ibm.com> <9bc000ab-1982-41f0-9ca0-2a4ead9aa982@linux.ibm.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: <9bc000ab-1982-41f0-9ca0-2a4ead9aa982@linux.ibm.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: zYEf8tZ4QIO-gPTd8b-8DKIbPEeg_l8av3r-X7KtRJg_1734689519 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: D23ED40005 X-Stat-Signature: cwi4dafp7ouhy4yk5c4fqqspcu6ucm5d X-Rspam-User: X-HE-Tag: 1734689484-603056 X-HE-Meta: U2FsdGVkX1+maDCFdgzlkrgOxoeW4N1DXkjCHwouX3dGlDpHtIasiHYpkgVGGSkhc+7y2GB1vXP2ZF9iXyC/iOgclRhPWHvLmYEDNHoSp7mZoosHRU8qL0F6LWEJHmnemhRUyFvwaEhrNwRP8KJojwh9raBCDrZvWD5468NwAElRFU3UXWwfFhPWkH3D5whk9Y7kqLsRUFHXMa0a27LI0HtiMIJ2wt2NxN7nIfH+wzqy8iVd8uONCBDHjLrQadfwgnRJtnbUhaEacPZyO+1WYZIfrLV9qqSReFIPIBgJ2BBhTSGYqZJpSgmKPvPUhK4c1rCdoqCdXPByQFlxDIsTxkETn4PweCxuVBx0mdzuF32g+hxNO/KMLAcYzBLW/n9r/qHlPAowCAl8UveynizviM+Nl7CjfWWPYdOt4838J4Cfn9bKtdaPGeF3kitXh3y067rUL9Mosx7BBZLUqE4Hz0zRo18Wdj7mR/EdLlwrjYMPnh3Ry9PxyLLsseE9iuBbSPKR+vTYtLtjokwY6HtbqbY3dfgLz8dShwqycgJNisPk0cfYIhuc2Z4jwHyr1POpSZql1wB/tL8I7kLdKVx/olyIJp/vaZj6i8alDtApOkFjdQSEAzPtWsx47Op48Z2+UGm5SiZ/slfVRJ1/3CC7H56R72gLAoOlqYe3fv9JlgFxrCQz62iUDlm7vqw/9gghzt7bxShYdpSidmu4sYZtMhAxy5O75VhPL29L8Jre7Wr91Qe0OWWNDtqXFmMZ4l1OU0mEOkRCGq8kU7YB8ULb9OcHzhJBAlMX+ol/e3KIkv0UD4bdA+lsHEwgnjzgV73FrLE/tnpEgK7V7ckUgAtkVfcoqv/zSzNJxivYtrX6vEFWlpopCYkOui5hNCjk4QBWcS0E/8DeH3zjRe68ejgFWZGWN2DJVL45h/47fI/ZtLsTnpcvhlLfcM0lM3ApsO/jgzVzMhmYzvf7QFqGw3z iqwZt9Ut VSrjL9F4YF9lt7q0at/HoQmzMtVYVo8+uU84mC7c7JvSog4ZydNIeQ6KLRhlRLjffeaX2LfHqDZedKEOikHwLz/nYau8yT/jO5eRsglVzh6/dMyjUcESLZBjVp+8881UIIgovYuHM+TJwWiHKuWY/634kIWeuySOefhRXnCb0yQzWjncX9iKiZ+WiBujPeT0PQMoDQs+qUg7vZ8B6QLwwAhlcN3LvQzKvyMW26DVCt3GLzqgG066Z3x+FNthAdKtnlj3DfXqtLaaimS9ZG3ZXinDvtcY5m7WovEpLSRP/rojHfkE7C5C2CO+el7FlMmWhQIpNJmYhl+V9GOtblQoYfvhu9++XGnDdwKBSXj6112xtx6XQl16f6b+ZSaABW5pr7CAZt2wVJuCoX1hCxIMoEddE7yK/RD0Y7xANdnMHynFebHJSexZDf16UtKnfP/GLibF9u5zzh2qwPuUlu8o/+wiI7g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.185188, 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 20.12.24 03:55, Donet Tom wrote: > > On 12/19/24 18:28, David Hildenbrand wrote: >> On 19.12.24 13:47, Donet Tom wrote: >>> The migration selftest is currently failing for shared anonymous >>> mappings due to a race condition. >>> >>> During migration, the source folio's PTE is unmapped by nuking the >>> PTE, flushing the TLB,and then marking the page for migration >>> (by creating the swap entries). The issue arises when, immediately >>> after the PTE is nuked and the TLB is flushed, but before the page >>> is marked for migration, another thread accesses the page. This >>> triggers a page fault, and the page fault handler invokes >>> do_pte_missing() instead of do_swap_page(), as the page is not yet >>> marked for migration. >>> >>> In the fault handling path, do_pte_missing() calls __do_fault() >>> ->shmem_fault() -> shmem_get_folio_gfp() -> filemap_get_entry(). >>> This eventually calls folio_try_get(), incrementing the reference >>> count of the folio undergoing migration. The thread then blocks >>> on folio_lock(), as the migration path holds the lock. This >>> results in the migration failing in __migrate_folio(), which expects >>> the folio's reference count to be 2. However, the reference count is >>> incremented by the fault handler, leading to the failure. >>> >>> The issue arises because, after nuking the PTE and before marking the >>> page for migration, the page is accessed. To address this, we have >>> updated the logic to first nuke the PTE, then mark the page for >>> migration, and only then flush the TLB. With this patch, If the page is >>> accessed immediately after nuking the PTE, the TLB entry is still >>> valid, so no fault occurs. >> >> But what about if the PTE is not in the TLB yet, and you get an access >> from another CPU just after clearing the PTE (but not flushing the >> TLB)? The other CPU will still observe PTE=none, trigger a fault etc. >> > Yes, in this scenario, the migration will fail. Do you think the > migration test > failure, even after a retry, should be considered a major issue that > must be fixed? I think it is something we should definitely improve, but I think our page migration should handle this in a better way, not the unmap logic. I recall we discussed with Dev some ideas on how to improve that? I'm pretty sure one can trigger similar case using a tmpfs file and using read/write in a loop instead of memory access -> page faults. So where racing with page faults is completely out of the picture. -- Cheers, David / dhildenb