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b=YBd+dY2L8dcZD/RG1CKqIDwjI5F+dsiagft+Jt+z0ZrBbYQ4zTDNA9xAfQzlyJzAg 6hzGPTV/w7tTQ8ifJHIJzJtFgJ9ORcxu2oLyo9CFHrQLFbJ77++db/6Ar2evW9wU12 SBAZzqKmEupSOmbs1iXS9046heDaaQVlkgpev1Iv4hClWY2JWDZk2I1u6Iublf9/BR HC0MZmt57vaNw+orAJ2O9fUcEJYzTRS1N8ITnKR3zaq8oCmUIeicQQGO0MIBEck8D8 fUcxZok+XwpnFDwj69a6fLEIBdtTXsnM/rLna1naUCHMX70DVyJ1ib3wwT0FQGj9dz 7psYVoCzpUJnw== Message-ID: <62404b4a-d8a5-480b-8e22-a406b3e31471@kernel.org> Date: Thu, 12 Feb 2026 13:21:11 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range To: xu.xin16@zte.com.cn, akpm@linux-foundation.org Cc: chengming.zhou@linux.dev, hughd@google.com, wang.yaxin@zte.com.cn, yang.yang29@zte.com.cn, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260212193045556CbzCX8p9gDu73tQ2nvHEI@zte.com.cn> 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: <20260212193045556CbzCX8p9gDu73tQ2nvHEI@zte.com.cn> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 84C90140008 X-Stat-Signature: 5minfo1whjerw3p18yghrz8wubeztr4k X-Rspam-User: X-HE-Tag: 1770898878-882124 X-HE-Meta: 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 URfcPh5h hu41lPG7ejvI0CNh887Ye32Fks3Qfn6cJ8SRfoYsF3L8wbfgrTgrUUWAMyjzIVs2bC5xhbbx1CIJ/1N91iC3GVZhZfIrOX4KmFiS7dMskPNHXhv+whVgvCPQuKYM+1VtUwnMPw0oP5j3+KkXP8DYEZYS+VEOj1H3Y6YWYmShwkCQAbW0sKsk+AV1kCXGMU6MSWgh0bKwqL0RLeRy0bP/NEbY3tcbvxIiM+va9LxAKA1vXTjGFl69hUfY2aWGnA5fC6OBx8Xe27CXPO6wJ7B6U6MpQlbE6qxq8izkLj1HUPyPCyVr6d/BAfQEdaabfQvsZek+R8LnnRtGNx10QZn0EVpn1GPyCPneuArD2+i6Ws2qPk2x9TveTX3zxJ9cKF5LKFAz/ 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 2/12/26 12:30, xu.xin16@zte.com.cn wrote: > From: xu xin > > Problem > ======= > When available memory is extremely tight, causing KSM pages to be swapped > out, or when there is significant memory fragmentation and THP triggers > memory compaction, the system will invoke the rmap_walk_ksm function to > perform reverse mapping. However, we observed that this function becomes > particularly time-consuming when a large number of VMAs (e.g., 20,000) > share the same anon_vma. Through debug trace analysis, we found that most > of the latency occurs within anon_vma_interval_tree_foreach, leading to an > excessively long hold time on the anon_vma lock (even reaching 500ms or > more), which in turn causes upper-layer applications (waiting for the > anon_vma lock) to be blocked for extended periods. > > Root Cause > ========== > Further investigation revealed that 99.9% of iterations inside the > anon_vma_interval_tree_foreach loop are skipped due to the first check > "if (addr < vma->vm_start || addr >= vma->vm_end)), indicating that a large > number of loop iterations are ineffective. This inefficiency arises because > the pgoff_start and pgoff_end parameters passed to > anon_vma_interval_tree_foreach span the entire address space from 0 to > ULONG_MAX, resulting in very poor loop efficiency. > > Solution > ======== > In fact, we can significantly improve performance by passing a more precise > range based on the given addr. Since the original pages merged by KSM > correspond to anonymous VMAs, the page offset can be calculated as > pgoff = address >> PAGE_SHIFT. Therefore, we can optimize the call by > defining: > > pgoff = rmap_item->address >> PAGE_SHIFT; > > Performance > =========== > In our real embedded Linux environment, the measured metrcis were as > follows: > > 1) Time_ms: Max time for holding anon_vma lock in a single rmap_walk_ksm. > 2) Nr_iteration_total: The max times of iterations in a loop of anon_vma_interval_tree_foreach > 3) Skip_addr_out_of_range: The max times of skipping due to the first check (vma->vm_start > and vma->vm_end) in a loop of anon_vma_interval_tree_foreach. > 4) Skip_mm_mismatch: The max times of skipping due to the second check (rmap_item->mm == vma->vm_mm) > in a loop of anon_vma_interval_tree_foreach. > > The result is as follows: > > Time_ms Nr_iteration_total Skip_addr_out_of_range Skip_mm_mismatch > Before: 228.65 22169 22168 0 > After : 0.396 3 0 2 > > The referenced reproducer of rmap_walk_ksm can be found at: > https://lore.kernel.org/all/20260206151424734QIyWL_pA-1QeJPbJlUxsO@zte.com.cn/ > > Co-developed-by: Wang Yaxin > Signed-off-by: Wang Yaxin > Signed-off-by: xu xin > --- Acked-by: David Hildenbrand (Arm) -- Cheers, David