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[2003:cb:c74b:e00:9c12:cdf2:bf1c:b8ed]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429ed7945cesm144065035e9.40.2024.08.20.07.00.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 20 Aug 2024 07:00:20 -0700 (PDT) Message-ID: <9cb7a475-d87f-4205-969b-f4c623f78626@redhat.com> Date: Tue, 20 Aug 2024 16:00:18 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm: fix refcount check in mapping_evict_folio From: David Hildenbrand To: Boris Burkov , linux-mm@kvack.org Cc: willy@infradead.org, shakeel.butt@linux.dev References: 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: 99D2E120045 X-Rspamd-Server: rspam01 X-Stat-Signature: 799rahowbfikqbjibyu198d4oomh6fsq X-HE-Tag: 1724162426-343906 X-HE-Meta: U2FsdGVkX1/TbFgEVqboeWQNAGi1o4p3qLJzRL7QFkEU2b0V4uwseOiIu0zPW/67tQhP67w54/inQ/8Y0/KpSCoGBSoEfdmpisKUPxqm6ADXhAg3SRQ2rGogdI019ZhoXHJU0XUOMlI+Tlr5o1x0vuMOHYO5/QUMI4iIen45QRRIdmWr18S56KoiCpMtg79RvB+8UAp9qjird5DK0CG3cKlVwfus56Qy/uENqu6wN5yLSN8O8N2NhJO59jqYcP+34DmsihoQQ08PcbVdH5PRVlp/q3S4jA0bQahQ4T31MfMtO81eRp0zT+jW4Jpseq9Pk7/Ely7UqxzhDpGuGRXGeYaD/kw2SFHeXnLtI5nu90y1SE5FiZ3wQSCCdCxaK5P9iVnzJiPDnHZKyNukl5POl//hDThmQnGfLgQInZofRirVHwiTIrQ/3lLNKFacbeV+EXalKbT43M3OpulJG6YWlOwUoopQtUw9I6b55wLPfRQSZNu5JA9epM96KW3D5Srx69OAnLWmD2nfAPT3QUlDH23fA/6C4DVcxB3Xr4AybZMY9bqB2NRQVe0+O46FYR2Dc+OMBIi+6zXtfuyrV3Sri8BhcLJgvZc5rRdbuithrp6AKN27GM2x47iq2vIg+A3Sldpk5ENm3jLFs9vKE7ZV9PeUbB2csjDUmkU+1ysbdu7iobkMnBcNNAFoZf9mSdzJgIcoq3LFis0GsWB1fXdxhoa6zlEObDaC5cMHUGkIY+nV2OEf5bOUshooArMfb8WAvul/yFDR6blsbjZLiGL8fT0OtjfkyUO1Yg+LfUaBi+kpbQc1QJC9sMjioc9cV9ejRgSp2mtWJaHphLU7MonPdn38aUDwwVLOZ0+Dp7eNXzediLfcIXhWL1avomHKv9cR48JaqjSoFQ0gYiyMiQL9IE9bFSOIaJS8GjBxfUO50uy3+NJRBaQKsRe8ZExY6wX15u+9wrHsYpAQsnjP1UH TeqjSjUv MwMXJS39D1ahVRrveHlWXy9uHCUZgSEhaYLFdHx3KdJkQe7z3mCAdzQjfoG8UPvptfY6xo0TdnCBYsKzKcxRWbetcLwwY/OA35aQJRSTe9Nwa6uhr+e7aUuKtfVOOB26UwHU5lFkdw8rsFh3HQXTU6rANgoWJNoRrT5kQqk5Vmp/ciUSS1XccocImugUgW/oNWEsA59HVaZ2mRlsssLvS/1dRc1R8rg2qth4Nrh7tVPNFLVfuYXj0NzXS74nIYZAxDi+0oyGGUYsJx+KEeOgOZI4iq+mBtsKjUGaS4TAYuDz2245NRZr1YuBsVYxmYb4vxHlfwNk8Q3o5WBawX0JbZtjgP8y5WGuGeCNQj0L8+6WlhtnlbH5M+WmJNYDuM0LkPUFgAmCEtfWkqCw= 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 20.08.24 10:00, David Hildenbrand wrote: > 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"? ... and now I spotted that Willy had the same comment already, good :) -- Cheers, David / dhildenb