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=1793901126; b=D1WjE9jt1a4yYOWmoDUdBNC6npjlO6hQCNkGciZcrJigUzMEzxB07Bc8vulf4t73S9Ye 7vGSsUc1FRYDAjvfDmO2+IoSfzgia2NYrDl4F5IO2Fz6Jh/9Y57stIDKFVC5Y9In/ZPac /R6227T2XFAGo0H9px33NTW6p14mNwZHmoFks3EqSH/Nn03x8aY1XbT21xhW7XAu6h8Md GPl83on54JknFkcH+JJiaAe+FkkSz98bTHxQy+m8IUvR2rK3+m3UxRuhxhFwHnRQXbFtH oLC0xc0m7WoclWIg0Ex6e2cIn8Ubs7QEtnnMfU29lprkCnRyUBYaa9f65EqXu+kjR7w== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793901126; 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=TU2co5OHwPqZGsjHAK77lVb6RAQX9yPBM909tv534g8=; b=5hYwksnhsiYYDM0Qj9BjbJNLwlAcJyzAx6Hmqtx7QPUetrvZTNW0ewJqiysN3bP+w4lJ zrxzVcAveJ9b+JxuKqf3ufZgWFZ//UXvzxck4u0SPKBlnst1w5DRvXIFnIpIO+uKNLAUC ZlibuopIHdOvN9K3QLBabdFGpboDGjKcCKjK4j4FtScYTRGFa6PpK9kQW5j1OgR0UHGXq H0gsJk9+bKQZ/hSP53hyPqjUnetLebHCwcMgw+PuhuBxguUq+SyOewWGQAFWnn1+WeLWA zHpxstu8wlimHUGRbvLnJ+r79nyXSdYxR1oeMfQKX8qe1RuKyH9Y24kQK9uYGBbiisw== 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="BrB1bRD/"; 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 4DA86606A326; Thu, 5 Nov 2026 17:52:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=BrB1bRD/ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 10D681F000C1; Thu, 5 Nov 2026 17:51:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793901125; bh=TU2co5OHwPqZGsjHAK77lVb6RAQX9yPBM909tv534g8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=BrB1bRD/Soh7jF52sv//X1vQCquFJ8zymfkK/FXO3wwGJCCa1tEzIwzqm+fYvdCY2 DhOXVYoY20C6gB4jzI06/aa3FJKL16cXAVTE6GEma7rI1e7g4iXKOlePRLiF288XT4 VranzWw07bgVjhZQsOa83Q/zQ3MuhhyHnyb2Y6HAkPVvrS/Wn+pivLXwQqw5DGkCmY Cz1VF9gZFLMgCfghzav+g+H8FtHrcNJ/t8tgMMFQF3ADgfQtXjJ3DcjRu6FOLIlaUE W42fuX0MRzbloN/Ga6DFTl1WfqZzBIulN3KaPzlLmT4L3jFyDf4xTB+qfOgbKxHlU6 KSfFjlSeP944Q== Message-ID: Date: Thu, 5 Nov 2026 18:51:53 +0100 X-Mailing-List: linux-arch@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 0/5] mm: limit pagecache folio refcount To: Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Peter Xu , Johannes Weiner , Song Liu , Rik van Riel , Hugh Dickins , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Peter Zijlstra , Jason Gunthorpe , Kevin Tian , "Joerg Roedel (AMD)" , Robin Murphy , Steve Sistare , John Hubbard Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, iommu@lists.linux.dev, Chris Mason , stable@vger.kernel.org References: <20261105-refcount_overflow_stable-v1-0-70b9c655da3e@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: <20261105-refcount_overflow_stable-v1-0-70b9c655da3e@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/5/26 14:10, David Hildenbrand (Arm) wrote: > Users can create a lot of folio references to pagecache folios, making > it possible to overflow the refcount of large folios. > > While we knew that one could in theory overflow a folio's refcount, we > mostly assumed that bad things would happen and it wouldn't go > unnoticed. > > Not so sure if that is really always the case, and leaving that problem > unsolved is irresponsible, so let's limit pagecache folio refcounts. > > This is really only relevant for large folios / thps in the page cache. > With anon folios (including THPs), we should not be > able to trigger folio refcount overflows (until we get PUD-sized ones ;) ). Just a note that I don't think anon THP are as really relevant today, at least for 4k kernels, as one would usually need that so many processes (consuming so much memory for everything else) that this is mostly irrelevant in practice as an attack vector. At least with 512 pages per THP. The exception might be arm64 with 16k or 64k page size with 2k. (putting aside that 512M THPs are barely usable in practice). #processes to overflow the folio refcount 4k kernel: 512 pages per THP: 4m processes (irrelevant in practice) 16k kernel: 2048 pages per THP: 1m processes (mostly irrelevant in practice) 64k kernel: 8192 pages per THP: 256k processes The 64k case might be the only relevant one, but I think even there all processes combined would already consume way more than 100 GiB of kernel memory. If we really care about the latter case we could throw in some cheap checks during fork() checks. Handling migration entry restore / swapin would be a bit more nasty, but not sure if that's really something to worry about in the big scheme of things. -- Cheers, David