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b=atJNIUBDok2nCDX2dEaLw3k7nTp6IPtCLm2Xswl5lz4p6HjaWtmIFddfoYcbIFy9c 13csMEzZOATX0JU4evaBvmIG04nWSL7bWMm7rp3ZS+MsD3NEpoe0vOCRtwBkR8Sf2B oZhQlPTpeR2mpkUKarnydz45Q8+LY5WY4pumBBr2dvZ8YHfx8VvFKwHZw81Dw8XytN EbzPyI/hjxlEm3lDq8C7Oj498c/xUC1EIuees31yzm7BuOerMvVC38bvDRXsSfbvz5 Y6TUfB8fESlL63LkwaEM5FKaTep2B8rVRsybRGlsx3eNftn6SBIPE/Z6Q5gudGIn3/ vq0T2LkTYzs1A== Message-ID: <35b46b88-6f8a-4eb6-bef3-58389784f750@kernel.org> Date: Mon, 6 Jul 2026 08:53:36 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 2/3] ksm: Optimize rmap_walk_ksm by passing a suitable page index To: xu.xin16@zte.com.cn, akpm@linux-foundation.org Cc: chengming.zhou@linux.dev, hughd@google.com, wang.yaxin@zte.com.cn, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ljs@kernel.org References: <20260703162510242nxmjbcLy5ccp1dbZSK3EU@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: <20260703162510242nxmjbcLy5ccp1dbZSK3EU@zte.com.cn> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: htfrgapyq4s78j81ypwpetbkxec6dcno X-Rspamd-Queue-Id: A4034140002 X-Rspamd-Server: rspam01 X-Rspam-User: X-HE-Tag: 1783320820-62353 X-HE-Meta: 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 3wnBanmZ FBUUwpb4Oy6vXOGR1GQCc/ZXKW18vePbTUcAP5qWWqWNC1EuzjLrjWhyPy1m7+OU9O8rkYkZBFXVL6f35DUVlXPQYNZ0yUeORIfDhCvOP8cPS/6rCPs3LMCUHreIUammP2D+rJraZcJZNi8M4H9Mhevd//jX9u1lfIYXgrKZg3iTiZb0XZPKCmXirpTRJxHrRSwmWOYaPwt4lKWuzVzTriDBX6HeFSgGpTG/UBzRrDMSe9kMd/BmnG6xQruNibiC1kdM1IPbNnil8lu6xOSyRJla9U+d89dd/L55dZIy0VvA9dKxi7gWJ7n8ICiAmTzLyECNDYVKJxN2SxwMUYp52vFrHKUoOHDVtcOhF6JdETyk6NzRuSs7uWYNMrl70UqLSTMgY+9REtYZG7UQhHlzjoeDmdhaI7sPcri7m Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/3/26 10:25, xu.xin16@zte.com.cn wrote: > From: xu xin > > User impact / Why this matters to Linux users > ============================================= > When a system runs with KSM enabled and memory becomes tight, KSM pages > may be swapped out or migrated. The kernel then performs a reverse map > walk by rmap_walk_ksm to locate all page table entries that reference > these pages. If A large number of unrelated VMAs can attach to a single > anon_vma related with this KSM page, then rmap_walk might be severe > performance bottleneck. In our embedded test environment, we observed > ~20,000 VMAs sharing one anon_vma without any fork – purely from VMA > splits， which cause 200~700ms duration of rmap_walk_ksm. > > When one of those VMAs mapped a KSM page, then this KSM page's rmapping > will become bottleneck with hold its anon_vma lock for a long time. The > anon_vma lock is not only used by KSM; it is a core lock protecting the > VMA interval tree and is acquired by many critical memory operations: > > • Page faults: do_anonymous_page(), do_wp_page() (during COW) > • Memory reclaim: try_to_unmap() > • Page migration & compaction: migrate_pages(), compact_zone() > • mlock / munlock: mlock_fixup() > • Process exit: exit_mmap() (tearing down VMAs) > • Cgroup memory accounting: mem_cgroup_move_charge() > > If one thread holds the anon_vma lock for hundreds of milliseconds > because of an inefficient KSM rmap walk, any other thread that > tries to acquire the same lock (e.g., an application taking a page > fault, kswapd reclaiming pages, or a migration thread) will block. > This leads to stalled application threads, increased latency > spikes, and in extreme cases container timeouts or watchdog > triggers. > > This patch reduces the worst-case anon_vma lock hold time during > ksm_rmap_walk from >500 ms to <1 ms, thereby almost eliminating > this source of lock contention and improving system responsiveness > under memory pressure. > > Real-world examples: > ==================== > - JVM / Go runtime: These use mmap for heap regions and later call > mprotect(PROT_NONE) for garbage collection barriers or guard pages, > splitting the original VMA into thousands of small pieces over time. > > - Database engines (MySQL, PostgreSQL): Large shared memory buffers > or anonymous mappings are managed with madvise(MADV_DONTNEED) to > release specific pages, which also splits VMAs. > > Root Cause > ========== > Through local debugging trace analysis, we found that most of the > latency of rmap_walk_ksm 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. > > 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 start page index and the end page index 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 > ======== > We cannot rely solely on anon_vma to locate all PTEs mapping this page > but also need to have the original page's linear_page_index. Since the > implementation of anon_vma_interval_tree_foreach — it essentially > iterates to find a suitable VMA such that the provided page index > falls within the candidate's vm_pgoff range. > > vm_pgoff <= original linear page offset <= (vm_pgoff + vma_pages(v) - 1) > > Fortunately, an earlier commit introduced the linear_page_index to struct > ksm_rmap_item, allowing for optimizing the RMAP walk. > > Test results > ============ > A rmap testbench can be obtained with two Out-Of-Tree patches at [1][2]. > After applying the OOT patches and building rmap_benchmark from: > tools/testing/rmap/rmap_benchmark.c, we can start the performance test. > > The testing result in QEMU is shown as follows: > > KSM rmapping Maximum duration Average duration > > Before: 705.12 ms (705119858 ns) 532.04 ms (532041586 ns) > After: 1.67 ms (1665917 ns) 1.44 ms (1443784 ns) > > The benchmark numbers are realistic, since we observed ~20,000 VMAs > sharing one anon_vma on a production system running a Java application > with KSM enabled. The lock hold time before the patch was measured at > 228 ms (max) during rmap walks triggered by memory compaction and page > migration. The benchmark reproduces that VMA count and lock‑hold > behavior in a controlled environment. > > [1] https://lore.kernel.org/all/202605301703094695zmVgcSC27BNR0rH0N8_x@zte.com.cn > [2] https://lore.kernel.org/all/20260530170404509QpJmBtpSjn3uQHeVKA2iA@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