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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-44fcd135f6bsm12749905e9.3.2025.05.27.01.40.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 May 2025 01:40:47 -0700 (PDT) Message-ID: Date: Tue, 27 May 2025 10:40:46 +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> <0dc0fef0-ec1d-4971-81e6-0add808a8551@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: Pgs3lCr6N4TZjFp8Qma_1nq0QLEOwm2Jlu3d0CPEv7Q_1748335249 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: D4162A0008 X-Rspamd-Server: rspam09 X-Stat-Signature: 8861b46yw36auth61j9n9nc5qjmhfkgp X-HE-Tag: 1748335251-543407 X-HE-Meta: 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 CNcUWigV 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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 10:06, Dev Jain wrote: > > On 27/05/25 1:18 pm, David Hildenbrand wrote: >> 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()). > > > You are correct that any temp ref can block migration, however, that > reference has to come after the folios have been isolated in the > migration path. > > So the probability of someone taking a reference on the folio is quite > low since it has been isolated. The problem with filemap_get_entry() is > that it finds > > the folio in the pagecache, so isolation is useless, then takes a > reference and then shmem_get_folio_gfp() does a folio_lock() instead of > folio_try_lock(). > > This was the race which I talked about an year back at [1]. My concern > is that we are adding another candidate to that race; just wondering if > there is > > a better solution to fix the race mentioned in Shivank's patchset. Note that in this code here, we're not locking the folio just yet, we're only grabbing a reference for a very short time. We will isolate+lock the folios in collapse_file(), where we also lock all slots in the pagecache. The alternative would be to also lock all slots here, which is arguably worse. -- Cheers, David / dhildenb