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[2003:cb:c73b:3100:2d28:e0b7:1254:b2f6]) by smtp.gmail.com with ESMTPSA id u22-20020a05600c139600b00414906f1ea1sm21196816wmf.17.2024.04.03.04.47.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Apr 2024 04:47:06 -0700 (PDT) Message-ID: <134b6c6d-5bcf-4394-85ad-2da2e8dec1d3@redhat.com> Date: Wed, 3 Apr 2024 13:47:05 +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> <2f8af9d1-8c8e-4e1c-a794-76f6bb287b08@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-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 4C88340009 X-Stat-Signature: 4mszcf9m9et875w8mk7pf5zx8nx37p5k X-Rspam-User: X-HE-Tag: 1712144831-826483 X-HE-Meta: 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 8mv1eBrm 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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 13:08, Zhaoyang Huang wrote: > On Wed, Apr 3, 2024 at 4:01 PM David Hildenbrand wrote: >> >> 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. >> >> [...] >> > Thanks for the reminder. But I don't find the spot where holding > "mapping->invalidate_lock" by check the callstack of > 'kill_bdev()->truncate_inode_pages()->truncate_inode_pages_range()', > or the lock is holded beyond this? Well, truncate_inode_pages() documents: "Called under (and serialised by) inode->i_rwsem and mapping->invalidate_lock." If that's not the case than that's either (a) a BUG or (b) an undocumented exception to the rule, whereby other mechanisms are in place to prevent any further pagecache magic. I mean, kill_bdev() documents " Kill _all_ buffers and pagecache , dirty or not..", so *something* has to be in place to guarantee that there cannot be something concurrently filling the pagecache again, otherwise kill_bdev() could not possibly do something reasonable. For example, blkdev_flush_mapping() is called when bd_openers goes to 0, and my best guess is that nobody should be able to make use of that device at that point. Similarly, changing the blocksize sounds like something that wouldn't be done at arbitrary points in time ... So kill_bdev() already has a "special" smell to it and I suspect (b) applies, where concurrent pagecache action is not really any concern. But I'm not an expert and I looked at most of that code right now for the first time ... >> >>>> >>>> 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. > Sorry, it is theoretically yet according to my understanding. Okay, if you find a reproducer, please share it and we can investigate if it's a locking problem or something else. As of now, I'm not convinced that there is an actual issue that needs fixing. -- Cheers, David / dhildenb