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[2003:cb:c74b:e00:9c12:cdf2:bf1c:b8ed]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429ded7d5bfsm191181005e9.46.2024.08.20.01.00.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 20 Aug 2024 01:00:29 -0700 (PDT) Message-ID: Date: Tue, 20 Aug 2024 10:00:28 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm: fix refcount check in mapping_evict_folio To: Boris Burkov , linux-mm@kvack.org Cc: willy@infradead.org, shakeel.butt@linux.dev References: 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: 7bit X-Rspam-User: X-Rspamd-Queue-Id: C0BF8C0034 X-Rspamd-Server: rspam01 X-Stat-Signature: 9fjcc57shtqj6mph31bjneq8zfhf1kp7 X-HE-Tag: 1724140834-944662 X-HE-Meta: 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 jJrZrgdk X4JqrnWdvxiLjuPF+Ov9DC47FggK10BFyt4viB96uqgU5i9JzBa0d9qkhqCAV0qOPFiezg1G/kjl4GgupvP7enmmQrE5iz1XtTW9B3vpyMBE1c8hY+GmK+HgD97VRRYhCgY1+RRxiKdHnYzpEffsOBtZ4AROAXz40CrCGXRq/lbgLYgX7nD37Porskqon9/LbdDySXZm7M+WhJhNev/aOwukwr6q+BCw14fHIkf7Jtwyb7V6tTMuo7oxF6WiPfYK5mmXbL/+w7P4NmEWKbO4srXmVCCIXWy/bb+RGBbrzNf9t1WahXixjulM0Hw8DcefmrFPr4FTnokPLDpLmul27u7RjZWsBlJJ4nE3O9LaL25Zuiga5H7TIxnUxebHtDNfOSs8HWAGqOFSGbvy1OeRF87jLRmjFJKGUujGGtaD3DJx51uiqsJE3tYu8smt8T3WCzSWj 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 13.08.24 20:25, Boris Burkov wrote: > The commit e41c81d0d30e ("mm/truncate: Replace page_mapped() call in > invalidate_inode_page()") replaced the page_mapped(page) check with a > refcount check. However, this refcount check does not work as expected > with drop_caches, at least for btrfs's metadata pages. > > Btrfs has a per-sb metadata inode with cached pages, and when not in > active use by btrfs, they have a refcount of 3. One from the initial > call to alloc_pages, one (nr_pages == 1) from filemap_add_folio, and one > from folio_attach_private. We would expect such pages to get dropped by > drop_caches. However, drop_caches calls into mapping_evict_folio via > mapping_try_invalidate which gets a reference on the folio with > find_lock_entries(). As a result, these pages have a refcount of 4, and > fail this check. > > For what it's worth, such pages do get reclaimed under memory pressure, > and if I change this refcount check to `if folio_mapped(folio)` > > The following script produces such pages and uses drgn to further > analyze the state of the folios: > https://github.com/boryas/scripts/blob/main/sh/strand-meta/run.sh > It should at least outline the basic idea for producing some btrfs > metadata via creating inlined-extent files. > > My proposed fix for the issue is to add one more hardcoded refcount to > this check to account for the caller having a refcount on the page. > However, I am less familiar with the other caller into > mapping_evict_folio in the page fault path, so I am concerned this fix > will not work properly there, and would appreciate extra scrutiny there. > > Fixes: e41c81d0d30e ("mm/truncate: Replace page_mapped() call in invalidate_inode_page()") > Signed-off-by: Boris Burkov > --- > mm/truncate.c | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/mm/truncate.c b/mm/truncate.c > index 4d61fbdd4b2f..c710c84710b4 100644 > --- a/mm/truncate.c > +++ b/mm/truncate.c > @@ -267,9 +267,17 @@ long mapping_evict_folio(struct address_space *mapping, struct folio *folio) > return 0; > if (folio_test_dirty(folio) || folio_test_writeback(folio)) > return 0; > - /* The refcount will be elevated if any page in the folio is mapped */ > + /* > + * The refcount will be elevated if any page in the folio is mapped. > + * > + * The refcounts break down as follows: > + * 1 per mapped page Using "mapped" is confusing -- I would have expected a folio_mapcount() here. Did you mean "1 reference from the pagecache to each page"? > + * 1 from folio_attach_private, if private is set > + * 1 from allocating the page in the first place > + * 1 from the caller > + */ > if (folio_ref_count(folio) > > - folio_nr_pages(folio) + folio_has_private(folio) + 1) > + folio_nr_pages(folio) + folio_has_private(folio) + 1 + 1) > return 0; > if (!filemap_release_folio(folio, 0)) > return 0; -- Cheers, David / dhildenb