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=1794687430; b=aVT7OOJXNnyzO2yH9z6uCqvITejzzY9RHMtjbN8jV1pHjHev7ZX+lwqkCulWw6BKvNJo vUfPd5Vzk14mDZLRP+cnIMhazxM09w+31v+rJ2fjTGRQeZOaTe91cv3rTWB6FNw8lBGz6 2f5qNFTwOlEefKY1e6p2i0plLsi9d+wMWiahlFaEDL3C1Yff9UssA7CaNfwY2KBCfTD+f P4UEi9C+rrEwtHSol26VyXeADRF2dQg2SJ9bBFA3g8jkozcFHb8a1L7QLItPH5n/tirg3 yjvbJ8RC+4OS4+axuG2W55j9DCvROcXKC8L+TfjiJ2/mO6TEsznJ0x1BMA+1Vd7xlHw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794687430; 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=OZxdnEWnSrxJbR1e8/SaG5gFDq6H5EE2W5CAZ6cJ4ww=; b=QxVpFi7PGv5HCPMjG4O1v8IbKGj97P9emmkSUQnja1gX2FGNKUJNnDxlInrmWkov1HnZ XXotFGWpJ0QsawSRhbNhqPpdveX3P7LTIr88hqQ1rERwdDgKobsqfNEpHzakl2egUdKVE h2ROMehM1ltWr2/DPaBxjfIT8kGmYdEEZsacQvD4ms3Z6UKksR0S+JHXzylrQR2iYD5qz UQ8lFh8Zmig10/Rtxf42nqqY/0/Egtf3D0yDsNPu6trfamb2eOVu5h8++O2/Fb7euMZQd nS5xRaSgsbFbOZ0gEpnZV+QQETdW16LS34uDb99U8kHmOB+pFAck45z4iq5yCf4DaJw== 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=cWWbsaIx; 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 B2FA3606A327; Sat, 14 Nov 2026 20:17:10 +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=cWWbsaIx Received: by smtp.kernel.org (Postfix) with ESMTPSA id 378E91F000C4; Sat, 14 Nov 2026 20:17:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794687430; bh=OZxdnEWnSrxJbR1e8/SaG5gFDq6H5EE2W5CAZ6cJ4ww=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=cWWbsaIxnBdWa/4AtdpygHFk0lD1RcnYrPAcN4DBz5Q5NkxBnHyFRqSAUZbMVVxCs cqqYmpw2kN7ZeQMC17g2X/lCT5KrKit12s2zkXOFzqTKueVoYFLqtTD9gnPuG10TZm 4shc6Swp2kDscdFX4FDnBWHsRrahP/DBs+OKcmwany3vPUTj1Y7IoH6FPJznXYu9by D6CVZZc3q9Dq7+I0VU48mgU46PVXp45lJE+g572jZ+lxZy6zFY8CekPh1nWD/Vff5H xSV/SNu534+cyGIjo7P+OS8s7wbGDR/iKOGNBK7BmS1jlFvbkcSYxM7UUjuWIr2JAP 4yMKFNQu4whAA== Message-ID: <70740a32-92f5-4425-8676-68863f3e44c3@kernel.org> Date: Sat, 14 Nov 2026 21:16:59 +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 2/2] mm/mempolicy: limit shared policy ranges per inode To: "Gregory Price (Meta)" Cc: Hugh Dickins , Baolin Wang , Zi Yan , Joshua Hahn , Rakie Kim , Ying Huang , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Shuah Khan , Randy Dunlap , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org References: <20261113-mempolicy_limit-v1-0-16688b2644dc@kernel.org> <20261113-mempolicy_limit-v1-2-16688b2644dc@kernel.org> <179467808112.482102.13365594235443107281.b4-reply@b4> 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: <179467808112.482102.13365594235443107281.b4-reply@b4> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/14/26 18:41, Gregory Price (Meta) wrote: > On 2026-11-13 22:36 +0100, David Hildenbrand (Arm) wrote: >> Shared NUMA policy ranges can accumulate on an inode after its mappings >> are removed. Currently, these nodes are effectively unbounded (even though >> the allcoations are charged). >> >> An unbounded tree can cause long rwlock hold times when replacing or >> freeing policies. Instead of trying to fix that, it sounds more like >> having many nodes is actually only a scenario for processes trying to >> cause trouble. Most use cases will probably only use a handful of them, >> not millions. >> >> Add vm.max_shared_policy_count (inspired by vm.max_map_count), defaulting >> to 1024 ranges per inode. Reject updates with -ENOMEM if they would >> increase the count beyond the limit, taking merging, splitting etc. >> into account. >> >> Signed-off-by: David Hildenbrand (Arm) > > > This arguably breaks userspace. Which is fine if there is no valid userspace that does something stupid. > > A single mapper that tries to set >1024 ranges would start getting > -ENOMEMs unexpectedly with this change. Sure, that's expected. The admin can raise the default for these odd cases. I did some digging and was not able to come up with any valid use case. The only is that you have many VMAs for the same file mapping (mprotect etc) and then do a mind(). But that's precisely what patch #1 does. > > Multi-task mappers were previously unlimited, but that seems like more > of a nasty unintended corner case - not a real feature. > > A single process is already limited on the number of vmas/policies based > on `max_map_count`, so it seems natural to just use max_map_count > directly, but I took a brief look at some distros and most seem to > ship a raised `max_map_count` (>1 million) by default). > > I tested that and even 1 million can cause a soft lockup. > > Not sure there's an easy solution here, we might have to rework the shm > policy locks. Do you know a single valid use case that's not solved by patch #1 and would trigger that many policies? Ai was not able to come up with anything for me. -- Cheers, David