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b=lV9/dT8FHtdDoJpmXQ0Sy1v8sQyBKBrYr/BK878gTWYi52sLwjNxUBCbjV8eZKIL3 sagrUd9iT3OSHGfcxTTqoO0fABAXkOK+KwnjfUcI/tmNdjThAiet+pKWRb9Wcu9XBK CDzvHY2EcQ4GQG4JZM4jywaYIiwgB/YRNQMAF9GDxge6c0TOjiHNEybfJoFW+gpSLO t3meqPrxAtEvpoZgfGoLzSSKBuTOnOXfx7k8vXSAKsADCmHwSnBAKcEke11OWlQ+hD 0Z5LR3bBkbC66NJ1hKtDexbWuNF082TWoI0VeULw9rkGzrFU2mxUd+++7x/CbD95Si R1yVYngZ1Onsg== Message-ID: <9575e79a-4cbf-48fe-bd3f-ff89e2ffdb92@kernel.org> Date: Wed, 11 Nov 2026 13:52:20 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm/filemap: fix lseek() skipping a new large folio To: Jan Kara Cc: "Matthew Wilcox (Oracle)" , Andrew Morton , William Kucharski , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chris Mason References: <20261110-lseek_hole-v1-1-c089c960b846@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 983DA1C0005 X-Rspamd-Server: rspam12 X-Stat-Signature: g6bd3suy9n1tfujd5tkmibhni1f73bs4 X-Rspam-User: X-Spam-Status: No, score=-12.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1794401548-921244 X-HE-Meta: U2FsdGVkX182b/4VSzbCbPq9pIennHeSgDD9JqBD9snEQFYMvQ6b+2mKoPhNqDe8HPqOhqXXaJikir9o6XX/Ttv4Q2ChBUxdZ6/6farhv52JAQqQ7TFpvrdmcY+AgKMTgiN1FSa0VubDcDDxwD1bphTJAs7PS3XwI52n+kGgv4okYY3WPlxKcUKfRBmhTLEwf9V60WB33Ah44yLrN+DGLZtLFB4PuqwMJIB+9MYTuZHL+qnzdbnl1d9v1wOAmqDDZYdLUuqfx0zBTfrO8JsOA8GbjsR+MLiqAtG7IE3kGTzldPO6XJ/f/Z6Omar1yIORSst1vTQ+01RxZU33tg1JTVrLu8tJUe1u1str1tJSCr+0qQxtiDNEGCal5crupjRGB9ZCCwuk4J6OOcUWnEePzKdd2Y2J0Vttl8ZOpG1Ru1FzMnsI1Tdw7rwgx1lXoCz88CBRm4BDfrSzZQhFyL5TKU6qKLG+36pYmTLs0mxEM9wmWTIq5NQrTda83NZTChORF/I1AqlmZJ+/mKnMI0DltWpdx5fh9i7rLToVJnCKNZ72Js+j0JfDk7VyZ6jynQB95GZjF+eDYJ+OcFQKPyL4eKwwEEOm/Iw0Fjp9iEXIJ5Y71vT+yUabnpjpNJNF7tM6gRcs0R9FHHhMH+GI/7GRFV/ICG33YviHD/D7ikNbHkMZ3TbneuTJUDS8gTEQpP+RrZxiCsyzkIG5wA5fO/NM1hTC+tB2NxOlKrqdXHkjx6lm5ihhJkKV1XIPLSEiVUc2rFyPyLFhqqLXJKiNzWABLr1/emhK0qgVD450wX1du/UxO0/3D1eZSNh0gpJgJuZr3LeZLW4oLP6fd49ou5nJ9amo6062CuU1OP/ShrGCrUZsz4taUYxQVa32H6WFF00g4dq1wa07a1eYtdocpQINhy5J8yfKkxeadvdZZ9vsnOdVUcn9orvZVN35PnGGCoGT9zympuIAcaNDZx+pFuD qJq2s6aO xAJl0qBXUXsw17u6+dFzRDpg0gJSs6o6wRH8kSCRmLCvQZZb08MET4LV+1mVyUBFc0Pm/xxFBU0xelO600MqmbXzjF7vuK5rP5e/K+vtkztoux5JKHBtn7QgOgrBmymEeCEDAQZBdeyg5AP7bYcpc32pOKfAA3/7cMa57r351nYVpDWPzv1j9a0hXXricvR/5fnmdM5K6oiSXzSNZazZJULwh5ZoRTveXT9+yeXrrnBvKaWhfgvoAbuqxMiVRoMYTlRbM/lOFvr23eXPvuW7Dsc/VDcQttUeCWV9IEhfA7YU0QamW3dKuJiSZvpuhIlUdDgfIVK59ELqY8sydDvqpRaTCj5shY7TSbLzcE+psYJk14NZoY4zpnJWY7suGme1exiKLq4rdQpagknRCNhWxigwBL0ZyofeEQhkmTVGBuo4xF0cGpXQOJL1c7A== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/11/26 12:45, Jan Kara wrote: > On Tue 10-11-26 21:33:12, David Hildenbrand (Arm) wrote: >> mapping_seek_hole_data() walks the page cache under RCU and only calls >> xas_set() after entries larger than a page. Between base-page entries, >> it continues the walk with xas_find(), which skips sibling entries >> to return each multi-index entry only once. If a large folio replaces >> small folio entries between iterations, the slot holding the new folio >> can lie behind the cursor. Skipping its sibling entries then skips data >> that has not yet been examined. >> >> On tmpfs, SEEK_HOLE can then return an offset inside existing data. >> SEEK_DATA can skip data after a fallocated page that is not uptodate. >> >> For example, assume a 2 MiB tmpfs file backed by 4 KiB pages on x86-64, >> with page index 63 fallocated but not uptodate and all other pages >> uptodate. Index 63 occupies the last slot in the old xarray leaf. A >> SEEK_DATA starting there can skip that page, then race with collapse of >> pages 0-511 into one 2 MiB folio. The next xas_find() moves past the old >> leaf and skips the new folio's sibling entries in the parent, returning >> -ENXIO instead of finding data at page index 64, or 256 KiB. >> >> The inode lock held by lseek() does not exclude collapse or swapin. >> >> Restarting the xarray walk after every entry fixes this, but increases >> SEEK_HOLE time about 24%, when scanning a fully written 1 GiB memfd backed >> by base-page folios in an x86-64 microbenchmark. >> >> Instead, use xas_next() for adjacent indices. Restart the lookup from the >> root at the requested index if it returns an empty slot or an internal >> entry, including a sibling entry. Also restart for nonadjacent indices, >> invalid iterator state, or failed folio validation. This avoids skipping >> unexamined data without restarting each lookup during base-page scans. >> >> Fixes: 6b24ca4a1a8d ("mm: Use multi-index entries in the page cache") >> Reported-by: Chris Mason >> Signed-off-by: David Hildenbrand (Arm) >> --- >> I'll be more than happy if someone of the pagecache folks wants to take >> over ;) > > This looks good to me. Feel free to add: > > Reviewed-by: Jan Kara > > I just find it unfortunate that we have to play complicated games like "unfortunate" is a good word. I'd have used "annoying" :) > these for such a simple operation as sequentially iterating over all folios > in a mapping. And it is not a first bug of this nature trigered by > compaction. So longer term I believe we need to provide saner apis > shielding pagecache users from these nasty details. Yeah, I would have preferred the simpler fix, but ~24% slowdown gave me a pause :( thanks! -- Cheers, David