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b=oMu/GVUU6uQTbTnA3N5QZm1YoJdWSwp2y2Ppr3BjSDBcgiarwHhEPTKnPq4pD2aDL wAU2NvoPEaawuI0ElZdzfBm6lxouNgaks13uuiAa8uvMPmHi5lypzmDf7tpROoh/w5 WQeiSvTp1jwNgsqOJCVp3+ITBKxQgjamnHTOL8CJHeEZNJTCbXePq82/jeZHMu90rF qalNBfPvrytS1Wv+AMPRdGxRxiFOpTXMs+xCOPghWFk8vRevnikM2YIaiM4/iT1g2g iYaNuoNUVqKZcOhu6LZhIQF4nYyxnnFpFW/m1F2wYyDUT1j53NcyBSrjgS3zJCURGt GNQ03kjrHUfTw== Message-ID: <2a6c4e62-1663-4a98-9adc-406a6a1ebfd3@kernel.org> Date: Mon, 23 Feb 2026 10:29:56 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails To: Lance Yang , akpm@linux-foundation.org, peterz@infradead.org Cc: dave.hansen@intel.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260223033604.10198-1-lance.yang@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: <20260223033604.10198-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: 5ej11tcipxwnaiwshrzaa6cwr9axugy5 X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: B0B55180002 X-HE-Tag: 1771839001-641998 X-HE-Meta: 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 qcNwYt2A pbuWo X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/23/26 04:36, Lance Yang wrote: > From: Lance Yang > > When freeing page tables, we try to batch them. If batch allocation fails > (GFP_NOWAIT), __tlb_remove_table_one() immediately frees the one without > batching. > > On !CONFIG_PT_RECLAIM, the fallback sends an IPI to all CPUs via > tlb_remove_table_sync_one(). It disrupts all CPUs even when only a single > process is unmapping memory. IPI broadcast was reported to hurt RT > workloads[1]. > > tlb_remove_table_sync_one() synchronizes with lockless page-table walkers > (e.g. GUP-fast) that rely on IRQ disabling. These walkers use > local_irq_disable(), which is also an RCU read-side critical section. > synchronize_rcu() waits for all such sections to complete, providing the > same guarantee as IPI but without disrupting all CPUs. > > Since batch allocation already failed, we are in a way slow path, so > replacing the IPI with synchronize_rcu() is fine. > > We are in process context (unmap_region, exit_mmap) with only mmap_lock > held, a sleeping lock. synchronize_rcu() will catch any invalid context > via might_sleep(). > > [1] https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@kernel.org/ > > Link: https://lore.kernel.org/linux-mm/20260202150957.GD1282955@noisy.programming.kicks-ass.net/ > Link: https://lore.kernel.org/linux-mm/dfdfeac9-5cd5-46fc-a5c1-9ccf9bd3502a@intel.com/ > Link: https://lore.kernel.org/linux-mm/bc489455-bb18-44dc-8518-ae75abda6bec@kernel.org/ > Suggested-by: Peter Zijlstra > Suggested-by: Dave Hansen > Suggested-by: David Hildenbrand (Arm) I think it was primarily Peter and Dave suggesting that :) > Signed-off-by: Lance Yang > --- > mm/mmu_gather.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c > index fe5b6a031717..df670c219260 100644 > --- a/mm/mmu_gather.c > +++ b/mm/mmu_gather.c > @@ -339,7 +339,8 @@ static inline void __tlb_remove_table_one(void *table) > #else > static inline void __tlb_remove_table_one(void *table) > { > - tlb_remove_table_sync_one(); > + if (IS_ENABLED(CONFIG_MMU_GATHER_RCU_TABLE_FREE)) > + synchronize_rcu(); That should work. Reading all the comments for tlb_remove_table_smp_sync(), I wonder whether we should wrap that in a tlb_remove_table_sync_rcu() function, with a proper kerneldoc for the CONFIG_MMU_GATHER_RCU_TABLE_FREE variant where we discuss how this relates to tlb_remove_table_sync_one (and tlb_remove_table_smp_sync() . -- Cheers, David