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b=mxXNFSZEgof4oWvDh69w8cl9UASs5pzSlvh6ePsgKJIA+Z4aZCoL8o/NzkhyaACAq jVb4bMPUBDBV4zV6B26G80K1qFX4PaqMs1eJmaEuv8v4fp3OJNczMtMN5h7OHpf9K6 Ial2n412Zt46Wl2/lTWfypPx6ZgyKD5BpLTPyghJ4foYVp6adWPm5ojdyV528uVulg wpoDa8jWI3nScevOBgmucmnWsFCqqLq7z8Dt4SrhuQgL37L45+sgWCvaUXLQ7K/j1s vS5ZTkyFevwbM74GQeKMZPo5VcFiwzGgbDhSCm9NaUzvI7jtZLuCc+m0xgYygbCVkh XQV9aGj7Kb4pQ== Message-ID: Date: Tue, 24 Feb 2026 12:04:00 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 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: <20260224030700.35857-1-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: <20260224030700.35857-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspam-User: X-Rspamd-Queue-Id: 93CB61A000D X-Stat-Signature: soeejc4krd6thenqk9xbnh5jn9fwc3ae X-HE-Tag: 1771931049-337954 X-HE-Meta: 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 mxQ5bzd2 4zossxVbQFY1lJ3WVsA1PP9y5dxns54s63kZuYsBwEVB0BpBIKoW7gMkv7whvNvcNO8Wu+Fz1s2Yuch3fCgzBAfGX/+xxNMTlf/FJICxBmpda6kBPH8OK6uyQ4sUhzdmibCQU1OmZvc8Hl6Tk+le8oeGtSG4yhKG58RgowHqa+fg6c2vGEmZMEUKzTQlCJqey6Hxo3g+ePZ8mEf9HZXMoNHwGTUaMG5136/0jK1fO3FnC1Qz7bs4fraZzYfZWCt+OPq2m9XUgC1/bbHPQM2KmiczvbFvCkyjmSEh5/fqNLMmkbQ2IWCSZjqSmeHr/LrNv69m7LEC45DXhqHcIPOnd/EDqSxJjjLIxsXNYAASLbvxWHFUq1iCcgeKr/WBvLreJGUsVIwHAYfQ1AOiGd7b9CNyfG4RJljY1oUxOIZo6QwH2H+idSoh0rpInQfaD9V4MlP99xcci6o1uansoHVCcbwCkFQ== 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 2/24/26 04:07, 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. > > This patch introduces tlb_remove_table_sync_rcu() which uses RCU grace > period (synchronize_rcu()) instead of IPI broadcast. This provides the > same guarantee as IPI but without disrupting all CPUs. Since batch > allocation already failed, we are in a way slow path where sleeping is > acceptable - we are in process context (unmap_region, exit_mmap) with only > mmap_lock held. might_sleep() will catch any invalid context. > > [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) > Signed-off-by: Lance Yang > --- > v1 -> v2: > - Wrap synchronize_rcu() in tlb_remove_table_sync_rcu() with proper > kerneldoc (per David) > - Add might_sleep() to make sleeping constraint explicit (per Dave) > - Clarify this is for synchronization, not memory freeing (per Dave) > - https://lore.kernel.org/linux-mm/20260223033604.10198-1-lance.yang@linux.dev/ > > include/asm-generic/tlb.h | 4 ++++ > mm/mmu_gather.c | 22 +++++++++++++++++++++- > 2 files changed, 25 insertions(+), 1 deletion(-) > > diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h > index 4aeac0c3d3f0..bdcc2778ac64 100644 > --- a/include/asm-generic/tlb.h > +++ b/include/asm-generic/tlb.h > @@ -251,6 +251,8 @@ static inline void tlb_remove_table(struct mmu_gather *tlb, void *table) > > void tlb_remove_table_sync_one(void); > > +void tlb_remove_table_sync_rcu(void); > + > #else > > #ifdef tlb_needs_table_invalidate > @@ -259,6 +261,8 @@ void tlb_remove_table_sync_one(void); > > static inline void tlb_remove_table_sync_one(void) { } > > +static inline void tlb_remove_table_sync_rcu(void) { } > + > #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */ > > > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c > index fe5b6a031717..2c6fa8db55df 100644 > --- a/mm/mmu_gather.c > +++ b/mm/mmu_gather.c > @@ -296,6 +296,26 @@ static void tlb_remove_table_free(struct mmu_table_batch *batch) > call_rcu(&batch->rcu, tlb_remove_table_rcu); > } > > +/** > + * tlb_remove_table_sync_rcu() - synchronize with software page-table walkers Nit: no need for the "()" Thanks! Acked-by: David Hildenbrand (Arm) -- Cheers, David