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[2003:cb:c73b:3100:2d28:e0b7:1254:b2f6]) by smtp.gmail.com with ESMTPSA id dj13-20020a0560000b0d00b0033e9fca1e49sm16397012wrb.60.2024.04.03.01.01.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Apr 2024 01:01:32 -0700 (PDT) Message-ID: <2f8af9d1-8c8e-4e1c-a794-76f6bb287b08@redhat.com> Date: Wed, 3 Apr 2024 10:01:31 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv2 1/1] mm: fix unproperly folio_put by changing API in read_pages To: Zhaoyang Huang Cc: "zhaoyang.huang" , Andrew Morton , NeilBrown , linux-mm@kvack.org, linux-kernel@vger.kernel.org, steve.kang@unisoc.com, Matthew Wilcox , Christoph Hellwig References: <20240401081734.1433755-1-zhaoyang.huang@unisoc.com> <736b982a-57c9-441a-812c-87cdee2e096e@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-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 35868140013 X-Stat-Signature: echi3cyuughpj97wgedr9kj3zhc5cmu5 X-HE-Tag: 1712131299-254572 X-HE-Meta: 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 ecK9e7AW 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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 03.04.24 07:50, Zhaoyang Huang wrote: > On Tue, Apr 2, 2024 at 8:58 PM David Hildenbrand wrote: >> >> On 01.04.24 10:17, zhaoyang.huang wrote: >>> From: Zhaoyang Huang >>> >>> An VM_BUG_ON in step 9 of [1] could happen as the refcnt is dropped >>> unproperly during the procedure of read_pages()->readahead_folio->folio_put. >>> This is introduced by commit 9fd472af84ab ("mm: improve cleanup when >>> ->readpages doesn't process all pages")'. >>> >>> key steps of[1] in brief: >>> 2'. Thread_truncate get folio to its local fbatch by find_get_entry in step 2 >>> 7'. Last refcnt remained which is not as expect as from alloc_pages >>> but from thread_truncate's local fbatch in step 7 >>> 8'. Thread_reclaim succeed to isolate the folio by the wrong refcnt(not >>> the value but meaning) in step 8 >>> 9'. Thread_truncate hit the VM_BUG_ON in step 9 >>> >>> [1] >>> Thread_readahead: >>> 0. folio = filemap_alloc_folio(gfp_mask, 0); >>> (refcount 1: alloc_pages) >>> 1. ret = filemap_add_folio(mapping, folio, index + i, gfp_mask); >>> (refcount 2: alloc_pages, page_cache) [not going into all details, just a high-level remark] page_cache_ra_unbounded() does a filemap_invalidate_lock_shared(), which is a down_read_trylock(&mapping->invalidate_lock). That is, all read_pages() calls in mm/readahead.c happen under mapping->invalidate_lock in read mode. ... and ... >>> >>> Thread_truncate: >>> 2. folio = find_get_entries(&fbatch_truncate); >>> (refcount 3: alloc_pages, page_cache, fbatch_truncate)) truncation, such as truncate_inode_pages() must be called under mapping->invalidate_lock held in write mode. So naive me would have thought that readahead and truncate cannot race in that way. [...] >> >> Something that would help here is an actual reproducer that triggersthis >> issue. >> >> To me, it's unclear at this point if we are talking about an actual >> issue or a theoretical issue? > Thanks for feedback. Above callstack is a theoretical issue so far > which is arised from an ongoing analysis of a practical livelock issue > generated by folio_try_get_rcu which is related to abnormal folio > refcnt state. So do you think this callstack makes sense? I'm not an expert on that code, and only spent 5 min looking into the code. So my reasoning about invalidate_lock above might be completely wrong. It would be a very rare race that was not reported so far in practice. And it certainly wouldn't be the easiest one to explain, because the call chain above is a bit elaborate and does not explain which locks are involved and how they fail to protect us from any such race. For this case in particular, I think we really need a real reproducer to convince people that the actual issue does exist and the fix actually resolves the issue. -- Cheers, David / dhildenb