From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794401547; b=bKfPi1pg0W43Zck2SnL8XshKQmirQrZOgeAeV3fBELS+XK/Hcy4bg1tUrbgqdqavDRuv mUsxWlzwZyPSzUbG5c23bIWbNMf6+1TRUs5BcQOc6GDF6j7340uUzDtjilO6QBsVI8X1e gmKtEOUwTxwPAuI9YnWfdCUdpwCccYyZl3amtIwJj4a/KyMqv1Vt2RUihiBgkI6PKYWJF dzC5cWUNIEhohxw2Ojz9ivnCTmvTjiX7Tv7icvgDeidDDs9Y99M/UfYPbLDLgujj1gNKl z21VxCqSnCLXjm60x1dz8NSUFja85uMXK9BQnhOClr+lzzKJWODd9b2erwUUHTCcqkA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794401547; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=8EaOYw3Ns7m59pAEKopA3/8l+QWoCfeJZjHCMiNoUTw=; b=2cpx9tbPTg4ybBizG11VFUUzWMmRHK48ziKhNvySO2qZjKfjLJDirVsv4ZmDkiVTo0TG 7c8QE/qdwx3vso61DrUMRs+fc/8yz16d0rivcmGdChoVPUlkquC3zzWRwYze+FQNnaE4+ dm770UYt+TUJFmaPhBYWUgtYpk6c7eS6BM+dFoGBpWSJ7a8DaRlsSjdzqhVkhIrT/RxBB xQYQJgybW/zcMlRr8NgSOi7RCI3m1nZocPEmynGVUXshdEUBES9y8+1WnoKpnq2yFUxIo NeOuzKpwT99oajnEjZu0QoeSzGPTXWqDGAmg1f1UkGTmhypBdbhD7fJZiuBu8nCYJKg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="lV9/dT8F"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B8516606A331; Wed, 11 Nov 2026 12:52:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=lV9/dT8F Received: by smtp.kernel.org (Postfix) with ESMTPSA id D6A8D1F00A05; Wed, 11 Nov 2026 12:52:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794401547; bh=8EaOYw3Ns7m59pAEKopA3/8l+QWoCfeJZjHCMiNoUTw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list 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 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