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=1794234442; b=RgDAtg5RQxpg4dwZZDpUZ3GL7wBmo+futaYb0cob0FSe5Ym17oTPmgQdzQZOi1mR6+9I 3ZFnsLc1VSzvbPT+fOaDiD0BvLg0EoMIcBk7kCZHnOREO1wlz2vSh00D3bHn4vzTFtZ4O GsSnrt3P61lxbWceSbBauxVzxaOVfxJ1IPMvvS9Ebmmj8EsmiNRBvNh+3lykhUXRtf4HN 5bZYbkMpOwKVTKqNrj6CLJv52sBxq3yvYwGfIx/bepB0EZFHSeaVn8V8qJWODMcHct0ln 99qMun3UswMuOPMKo/5iS94oexfTgqms1UY7NNqgGXQPqcieQAeIE3wY5G23jg4hAgg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794234442; 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=DQBR/dBCMcYTXkdOeOOog1rJR98U8AmPJCb23jPzIvA=; b=AlhI2kHAc6NsWIVR1SBbOjwINFc7aAOgIExbhqyW5yXqyOPgpz/jPWZgx0YLPpLHoMEx 0nzJycpMcavClExChuOar+PWUjvaWh9fF5qhgC7koH2KBKH9Cu92m0GZQ7jDL8+wNetrW RWV3QuZLagM3VRb0/BO2jx41Y609gyDHdGdvhBk9+90VXQwTzDijHatjZQFu1Li4cuagZ rcnzn1xTABDlQ3zfcPeDz+zBja6aioPDzO1RQRJKxtgtkbqsD1zTQvfU0VzhkkKOR02AA OcZiBQ0cuSNmA8I83auyKoQii4/m5sDNG9GuY3xOJb3U2yMYnI8ax+kQMMiZLJMdGIw== 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=C3H8wQYE; 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 D4763606A326; Mon, 9 Nov 2026 14:27:21 +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=C3H8wQYE Received: by smtp.kernel.org (Postfix) with ESMTPSA id E83B31F0089F; Mon, 9 Nov 2026 14:27:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794234441; bh=DQBR/dBCMcYTXkdOeOOog1rJR98U8AmPJCb23jPzIvA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=C3H8wQYEaereW5Y+ykz1l50Et0cRmZvNz5OyubO8k6Mzm3JRcJ8ZwAzbIpn1O0VnW WoQZamewYE3OxZzQIlKv3AdsQpU4iqUvDUDOUFFBxGhaas9ujo4Rviw5CnK1wasFNZ BUbvNzWWpx8nshdFgRq/q4q6yeNnrkSKKxHXxuWA1lNtb1Qdi219igtdjcLi+Ld4bM PZPeinVRzFXxJ1Ls9Oa8cCwUWPLcmMFskJLS5DmV9kGsMA4Qcpf6ev0HbUWLZBFjBw eLU5zsQLYsYo7mXGzMGvrSGOBEjSMBn1GmAXyNmOInW70mml0WujDbfWK6rByi2MN5 ZGKuwK5yM2DAw== Message-ID: <07b298ca-d258-4cda-b5ae-0231ec39e033@kernel.org> Date: Mon, 9 Nov 2026 15:27:15 +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 v2 2/2] mm: don't allocate a page table under RCU in do_set_pmd() To: Usama Arif Cc: Andrew Morton , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Kiryl Shutsemau , Qi Zheng , Hugh Dickins , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261106155604.3999641-1-usama.arif@linux.dev> 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: <20261106155604.3999641-1-usama.arif@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/6/26 16:56, Usama Arif wrote: > On Fri, 06 Nov 2026 13:36:13 +0100 "David Hildenbrand (Arm)" wrote: > >> do_fault_around() skips PTE table preallocation when the PMD already points >> to a PTE table. A concurrent collapse can then retract that table and >> clear the PMD before filemap_map_pages() locks the new PMD-sized folio. >> >> filemap_map_pmd() can then call do_set_pmd() with an empty PMD and no >> preallocated table. On PowerPC hash MMU, do_set_pmd() then tries >> allocating a PTE table to deposit. But filemap_map_pmd() calls >> do_set_pmd() under RCU, so allocating from that context is not good. >> >> This race became possible when retract_page_tables() stopped taking the >> mmap and VMA locks. Previously, those locks excluded retraction while >> the fault was in progress. >> >> Add a may_alloc argument to do_set_pmd() and pass false from >> filemap_map_pmd(), so we can fail with VM_FAULT_FALLBACK instead of >> trying to allocate. >> >> This issue should be fairly hard to hit in practice. >> > > Yes, I imagine it was very difficult to reproduce, doing a sleeping > pte_alloc_one() inside filemap_map_pmd()'s RCU read-side section > when a concurrent retract_page_tables() empties the PMD. Only > possible on PowerPC. Yes. Did I express how much I hate us having to support this ppc handling? I'll buy you a double digit number of beers if you manage to rip that out ;) -- Cheers, David