From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C08C03FA5CC for ; Mon, 6 Jul 2026 06:53:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783320826; cv=none; b=ok67EvOL7bFCx9YRrrPhPELQA3IZxBrNDcq2iiCXBshSg1eox/bx/rzDubCphgVfRmSq6YqC3yssHVADw3+AFJEORvlzBFcfqjVIDW4Ky+Cz6m3PHAluMvFp/XY02CcUOZoxjGrmpBDAsmzsEIY3F0NliXlqMBlF6wzmyjuq/Qc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783320826; c=relaxed/simple; bh=EQ11ZR0XDH+a7F9pXvRUQtHCcdtA6WjFKF3Aaw9dxfM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ZEZNmd1aKYaIo4dY0CyNyFay2AuBN+WvUbYNaPaRohbkw9nEKvASsNJzmbH1uK/gPitwHR26XSgp/iu4RiADVEi1oLURZdDehYXTXD+IucVSaKyIKhzdrtDodFYrKo1JqwRMwwbFN1BEZ1fKj0Ml0kJjzeIWbIFum7TiRVW3Ltc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=atJNIUBD; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="atJNIUBD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 06E7F1F000E9; Mon, 6 Jul 2026 06:53:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783320819; bh=r599ktiKbulgE6viGKOYXX1IIDFeRM72i/7JTqGw39Q=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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; 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charset=UTF-8 Content-Transfer-Encoding: 8bit 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