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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4c591c044sm11822337f8f.52.2025.05.27.00.48.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 May 2025 00:48:23 -0700 (PDT) Message-ID: <0dc0fef0-ec1d-4971-81e6-0add808a8551@redhat.com> Date: Tue, 27 May 2025 09:48:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V3 1/2] mm/khugepaged: fix race with folio split/free using temporary reference To: Dev Jain , Shivank Garg , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, fengwei.yin@intel.com, bharata@amd.com, syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com References: <20250526182818.37978-1-shivankg@amd.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: rXBnjeJ9tVbe6lpTq2amiWRDrnvowvCG2BNipLOm_Zw_1748332105 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: rspam06 X-Rspamd-Queue-Id: 4A26D80006 X-Stat-Signature: 8umixxbqzbf3tonn4z5kbad1wgb13ksn X-Rspam-User: X-HE-Tag: 1748332108-734500 X-HE-Meta: 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 WEmX2/Ov 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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 27.05.25 05:20, Dev Jain wrote: > > On 26/05/25 11:58 pm, Shivank Garg wrote: >> hpage_collapse_scan_file() calls is_refcount_suitable(), which in turn >> calls folio_mapcount(). folio_mapcount() checks folio_test_large() before >> proceeding to folio_large_mapcount(), but there is a race window where the >> folio may get split/freed between these checks, triggering: >> >> VM_WARN_ON_FOLIO(!folio_test_large(folio), folio) >> >> Take a temporary reference to the folio in hpage_collapse_scan_file(). >> This stabilizes the folio during refcount check and prevents incorrect >> large folio detection due to concurrent split/free. Use helper >> folio_expected_ref_count() + 1 to compare with folio_ref_count() >> instead of using is_refcount_suitable(). >> >> Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single value") >> Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com >> Closes: https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com >> Suggested-by: David Hildenbrand >> Acked-by: David Hildenbrand >> Signed-off-by: Shivank Garg >> --- > > The patch looks fine. > > I was just wondering about the implications of this on migration. Earlier > we had a refcount race between migration and shmem page fault via filemap_get_entry() > taking a reference and not releasing it till we take the folio lock, which was held > by the migration path. I would like to *think* that real workloads, when migrating > pages, will *not* be faulting on those pages simultaneously, just guessing. But now > we have a kernel thread (khugepaged) racing against migration. I may just be over-speculating. I'm not quite sure I understand the concern you have. Any temporary reference can temporarily block migration, however, the retry logic should be able to handle that just fine -- and this code is not really special (see filemap_get_entry()). -- Cheers, David / dhildenb