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=1793959568; b=VdbPUrJYVCuJnAm9rojp/yXowh0X7fJ3ZQzGA3lX5tLMvVJ+AYGmuRfFA6PGRi/ENoUF ShJuzuF5Zu8HWCpzHQeSe2GPILfsTbDH6s1SmGK6JcRUxOlk0Jts7AN4LuINwf/G2Da0r 2skQzM5I1Q1kRfhrtgL2cZexNGwtsjHDHw67HHk5gyhGWt3zS5f+MXjYGDLSc30//F3oM ISBe8aEteVnXXkDypBXHH/1B/Foqjh/TPNtZ+MOgUU14/gotNVVSKmLTVa5lCBHFOVdxZ v++MRjocqnjVKdvwXCw3eaEwWtw8/sk4VNIL5iE6Aifv92dA0RXnH3Gm1Puiwh3xDYg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793959568; 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=Px8YGrUbpB2T2jEhenGoA16y8NMxi2MB1FLmbQ0eAMw=; b=YyPrVSv5fqXKEC8XGXQrohVbthKmopD3ikgWAqHTqT64h+TqAKVOBs66XWN4AKubbFxM p6pQgqmh23g+Rc8uLcPfQZVcH3xd+txt7Pei8OuN4NGeEEjnxD/saAf8kBKtXYU7ugrpZ HXpk5yVS8ONeoGrhgp2eUW06MAO4EhtfzLSNqheO/lqD6oNHhORW+G56raUNyLGyKTSUU GAa3SGvmW4QEsYCooB5RtsThKHmXlgUdo2Duc2HtxDdrt9TB1umO9XMAFLbwznao20P0Z Sbj6QLfASBgP3h4JwH7fLWt+IoIG7z8d316VlLZRMYhYPMtA3kGGxITSKSZQdjyI6Sg== 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=LnIYthTQ; 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 56F61606A32D; Fri, 6 Nov 2026 10:06:07 +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=LnIYthTQ Received: by smtp.kernel.org (Postfix) with ESMTPSA id E68161F008A2; Fri, 6 Nov 2026 10:05:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793959567; bh=Px8YGrUbpB2T2jEhenGoA16y8NMxi2MB1FLmbQ0eAMw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=LnIYthTQJEXOjvvLs54ZGX/uUfr0klYw3b86wsbcnRnjMkpBZxFYk4+mow/6/uap8 Ivtt9RN/Kg0Vc9xqaRycTlEc2bPPJKnPZ6QatK+5uisGzt8lOls3vzsRNfvYPei+Gb XU7CLMDzEh5hkUwM5oR8dSDjbO5AXhJfvwxMegtiuS3dtstOuW+UNGQgD3B/Z7aKDc B/jYYug6sRFk+ZM0OhppHwhlPQCm4lPM5vlZE7T20ObUuqvApxXnI/Ra+UjDJ1Tram cJlEkqKW0G6jBDB1Y5PPWZc6twszsfy5hzKboS8hBCJmh0OmA0vOHCV4OrE7HdLivD BIwEstq14SG0A== Message-ID: <086c3d5f-84cb-412b-b61a-8cb3ca3e2e28@kernel.org> Date: Fri, 6 Nov 2026 11:05:54 +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 1/5] mm: limit pagecache folio refcount when mapping pages To: "Jose A. Perez de Azpillaga" , 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> <20261105-refcount_overflow_stable-v1-1-70b9c655da3e@kernel.org> <528e3a0e-04fb-427f-88c7-63a7f4bbc505@gmail.com> 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: <528e3a0e-04fb-427f-88c7-63a7f4bbc505@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/6/26 10:42, Jose A. Perez de Azpillaga wrote: > On 11/5/26 2:10 PM, David Hildenbrand (Arm) wrote: >> With large file folios, overflowing the folio mapcount and refcount >> became possible. For anon folios it is currently not really an issue: >> each large anon folio can only be mapped once into a process. >> >> That's not really big news, but it's about time that we do something >> about it because some modern technology might find ways to abuse it :) >> >> One of the enablers for that was commit ddc58f27f9ee ("mm: drop tail page >> refcounting"), however, file thp support was added later. >> >> File folios can be mapped multiple times into the same process, allowing >> for processes to hold many references to the same folio. >> >> As a short-term solution, let's introduce two folio refcount limits: >> one for ordinary consumers and one for admins. Leave some headroom >> before the refcount goes negative, which we really don't want (having >> the mapcount go negative is also not expected). >> >> Start reporting SIGBUS if we run into limits. When restoring mappings >> from migration entries, instead zap the pte/pmd entry. We allow for >> slightly exceeding the limit to keep things simpler. >> >> This is far from ideal, but better than all easy alternatives we can >> backport comfortably. For now it fixes the big problem: letting >> the refcount (+mapcount) go negative and cause trouble. >> >> This is only a temporary fix, we'll follow up with something better. >> >> While the refcount checks are not required for order 0 folios, let's >> just add them for consistency: each folio_add_file_rmap*() and >> folio_dup_file_rmap*() needs a prior check. > in mm-new copy_huge_pmd() now does a folio_dup_file_rmap_pmd() at fork, > since Konstantin's "mm/huge_memory: copy file THPs at fork like > copy_present_ptes() does". doesn't that one need the check too? Yes, that one will need care as well, thanks. I think I missed it when rebasing on top of the other fixes. Thanks! -- Cheers, David