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b=hVL1qCoESBzyncNyjZOjHebgaRnwQq7OTLas7qK5ytnPnGT/qVnxaJ8epup1Q3Gnv 3fZX01jlfiPJ8xBm8egrhGjOfsoBy6lwtuUjXreRoArmc78LQGYhgHobBdJ4/YJBrl 88oaQeAmO1iosbVzMtlohN2iPpfueXA873ipSpOYerzdG7DTRR15+OHWjaDCDBz7lE UHO5/LG7wcW8TivdO2WWQzflga8qsaBF/kT3ZIZ6NqVWme8+uvxZtG22o+Xxxtf4fk 4rIHo7Kw7SC1kQhcu+DXKFWKfR+7s5udZgXs0KgtmTycvUOn5Sztcy/nehHBUGvPXB 5/wzFQCynHLjA== Message-ID: <47a6817e-e98e-4a00-8e2c-6a66d15ddb37@kernel.org> Date: Mon, 7 Sep 2026 17:29:42 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hugetlb: fix overbroad MMU notifiers for unshared PMDs To: Andrew Morton , Li Zhe Cc: muchun.song@linux.dev, osalvador@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, aiqi.i7@bytedance.com References: <20260831091023.66581-1-lizhe.67@bytedance.com> <20260831173220.72ab28190a44a305dacf4d04@linux-foundation.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: <20260831173220.72ab28190a44a305dacf4d04@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: BB8C61C0002 X-Stat-Signature: py4spjwni93g76dfjbqz11bf8wycpaae X-HE-Tag: 1788794987-638921 X-HE-Meta: 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 3Eru14d6 HfQAbBB8iVQi0ITGebv8kMHJw1hdAMwPE3VYANyPKp1b1Pstrs7B1vCNXloyZ/oyQeCX5dOcfqsWKAVdWLzpJSBKDt2e4wYzxnlMbknK60tYs5+3U9oz7SkClmLLfccDQc9mO+J7GLME10QkXh1P89V+j2g4TxuJoOreshLdDDz2ORWVzoG+SO/wCnDj5H0douF27yrJn5ZlcnfPIbcHkM0izZORcknVy4WAAG8Jz8XwAe40UldCtbrA3S2TjZo3LFz/BHBzDZpQ144RmSpLTfeddoA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/1/26 02:32, Andrew Morton wrote: > On Mon, 31 Aug 2026 17:10:23 +0800 "Li Zhe" wrote: > >> Hugetlb currently expands MMU notifier ranges to PUD boundaries whenever >> PMD sharing is possible. That is only needed when huge_pmd_unshare() >> actually detaches a shared PMD page table, because clearing the PUD >> invalidates the whole PUD-sized virtual address range. >> >> For hugetlbfs hole punch and MADV_DONTNEED, a shared mapping can pass >> the "PMD sharing is possible" range test in function >> adjust_range_if_pmd_sharing_possible() even when the PMD table covering >> the target 2M page is not shared. KVM then receives a 1G invalidation for >> a 2M operation and zaps unrelated secondary mappings, so the guest has to >> fault them back in. >> >> Fix this by using the existing cheap "sharing possible" test only as a >> gate, then inspect the candidate PMD tables under the locks held by the >> hugetlb unmap paths. The notifier is expanded only for PUDs whose PMD >> table is actually shared, while the other callers keep the existing >> conservative expansion. > > Thanks. > >> On a Redis-in-VM workload that punches cold 2M hugetlb pages, this >> patch improves P99 QPS stability while punching pages, reducing the QPS >> degradation ratio from 7.09% to 1.45%. > > So a modest performance improvement? Is that worth the complexity, though? -- Cheers, David