From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 46CE2EF4EC9 for ; Mon, 6 Apr 2026 09:21:50 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id AC0C06B00AB; Mon, 6 Apr 2026 05:21:49 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A988E6B00AD; Mon, 6 Apr 2026 05:21:49 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9AE456B00AE; Mon, 6 Apr 2026 05:21:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id 89B4B6B00AB for ; Mon, 6 Apr 2026 05:21:49 -0400 (EDT) Received: from smtpin30.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 43BD0C18BC for ; Mon, 6 Apr 2026 09:21:49 +0000 (UTC) X-FDA: 84627588738.30.A9A33F7 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf20.hostedemail.com (Postfix) with ESMTP id 640991C0003 for ; Mon, 6 Apr 2026 09:21:47 +0000 (UTC) Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=OPDC+7uX; spf=pass (imf20.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1775467307; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=7brLVBbt/2vJX86SniNxOC4s3K+St3pyFUU7bpORVDA=; b=YIj3Zq5JsfzTVMHTQnx2tq57+mF7gKtgwJgsvumsmZrGisfD0H9xR84EkpnRnm26gzejj0 wtFJQC8o+5L3BzOZTuwXo69lUgKHek7759THk4RFYC6nucgM6lNJOKK9wExe4VqdTc3I0Q 2A7UGwy849kmVotTfK32K+4fv5JeEDM= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1775467307; a=rsa-sha256; cv=none; b=GyD2p0694I1WfvoVjdhd78D7RJm1tB4aUPlgE/xCpY4KnphGWNXclVmSdrqAcl3gDR5SO9 EVIQuPSi8h9yRrTH3DbpftT377+FhppWC4YVE+yEVWG73KGMlOSeBWYVpH7zjxD3VMCs6A 2mcm+nABY/h2ZeKDjVcOj1lnY+kJuIY= ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=OPDC+7uX; spf=pass (imf20.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 69CCF402DE; Mon, 6 Apr 2026 09:21:46 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id E0E64C19425; Mon, 6 Apr 2026 09:21:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775467306; bh=fhg6BtIGhbmFMSd9SUmDv0Uh+zl0kmiOHGSsPw4uSYk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=OPDC+7uXvv8RPigLxb7P6WGuldZMzaT4gpMCKFxsI8rE3B7f83451+wSMcqtJNQiS peQxUdtPT2HvktkNW6MuHynWTTS8+T7YXITEIJUJxpHEUcYAqQCRSUOQLcX24CvbJ0 nppaXd7F2oe1Su8tWkycHZh0lM6XtaAa8VXuhvl/tDlYbQk7Hqu8vb+S0rkSQaZ3R5 kji8V/YeKtwjCQu7HSlBn228dzmLdZbw8UWdJLcBovl1AjhzEcnY6LdPuhGajpiN1F UCAByJSqNKIZbSDX/eGyHm8LT3UXqbu8ZcSJE0sBsptpvV1g8a6z5uTTCDYI5Vr6d1 sNRzYHvp34W6A== Message-ID: <8481d0dd-2471-4acc-a429-cbc02451a812@kernel.org> Date: Mon, 6 Apr 2026 11:21:41 +0200 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: Hugh Dickins , xu.xin16@zte.com.cn Cc: akpm@linux-foundation.org, chengming.zhou@linux.dev, wang.yaxin@zte.com.cn, yang.yang29@zte.com.cn, Michel Lespinasse , Lorenzo Stoakes , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260212193045556CbzCX8p9gDu73tQ2nvHEI@zte.com.cn> <02e1b8df-d568-8cbb-b8f6-46d5476d9d75@google.com> 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAa2VybmVsLm9yZz7CwY0EEwEIADcWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCaKYhwAIbAwUJJlgIpAILCQQVCgkIAhYCAh4FAheAAAoJEE3eEPcA/4Naa5EP/3a1 9sgS9m7oiR0uenlj+C6kkIKlpWKRfGH/WvtFaHr/y06TKnWn6cMOZzJQ+8S39GOteyCCGADh 6ceBx1KPf6/AvMktnGETDTqZ0N9roR4/aEPSMt8kHu/GKR3gtPwzfosX2NgqXNmA7ErU4puf zica1DAmTvx44LOYjvBV24JQG99bZ5Bm2gTDjGXV15/X159CpS6Tc2e3KvYfnfRvezD+alhF XIym8OvvGMeo97BCHpX88pHVIfBg2g2JogR6f0PAJtHGYz6M/9YMxyUShJfo0Df1SOMAbU1Q Op0Ij4PlFCC64rovjH38ly0xfRZH37DZs6kP0jOj4QdExdaXcTILKJFIB3wWXWsqLbtJVgjR YhOrPokd6mDA3gAque7481KkpKM4JraOEELg8pF6eRb3KcAwPRekvf/nYVIbOVyT9lXD5mJn IZUY0LwZsFN0YhGhQJ8xronZy0A59faGBMuVnVb3oy2S0fO1y/r53IeUDTF1wCYF+fM5zo14 5L8mE1GsDJ7FNLj5eSDu/qdZIKqzfY0/l0SAUAAt5yYYejKuii4kfTyLDF/j4LyYZD1QzxLC MjQl36IEcmDTMznLf0/JvCHlxTYZsF0OjWWj1ATRMk41/Q+PX07XQlRCRcE13a8neEz3F6we 08oWh2DnC4AXKbP+kuD9ZP6+5+x1H1zEzsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCgh Cj/CA/lc/LMthqQ773gauB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseB fDXHA6m4B3mUTWo13nid0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts 6TZ+IrPOwT1hfB4WNC+X2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiu Qmt3yqrmN63V9wzaPhC+xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKB Tccu2AXJXWAE1Xjh6GOC8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvF FFyAS0Nk1q/7EChPcbRbhJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh 2YmnmLRTro6eZ/qYwWkCu8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRk F3TwgucpyPtcpmQtTkWSgDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0L LH63+BrrHasfJzxKXzqgrW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4v q7oFCPsOgwARAQABwsF8BBgBCAAmAhsMFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAmic2qsF CSZYCKEACgkQTd4Q9wD/g1oq0xAAsAnw/OmsERdtdwRfAMpC74/++2wh9RvVQ0x8xXvoGJwZ rk0Jmck1ABIM//5sWDo7eDHk1uEcc95pbP9XGU6ZgeiQeh06+0vRYILwDk8Q/y06TrTb1n4n 7FRwyskKU1UWnNW86lvWUJuGPABXjrkfL41RJttSJHF3M1C0u2BnM5VnDuPFQKzhRRktBMK4 GkWBvXlsHFhn8Ev0xvPE/G99RAg9ufNAxyq2lSzbUIwrY918KHlziBKwNyLoPn9kgHD3hRBa Yakz87WKUZd17ZnPMZiXriCWZxwPx7zs6cSAqcfcVucmdPiIlyG1K/HIk2LX63T6oO2Libzz 7/0i4+oIpvpK2X6zZ2cu0k2uNcEYm2xAb+xGmqwnPnHX/ac8lJEyzH3lh+pt2slI4VcPNnz+ vzYeBAS1S+VJc1pcJr3l7PRSQ4bv5sObZvezRdqEFB4tUIfSbDdEBCCvvEMBgoisDB8ceYxO cFAM8nBWrEmNU2vvIGJzjJ/NVYYIY0TgOc5bS9wh6jKHL2+chrfDW5neLJjY2x3snF8q7U9G EIbBfNHDlOV8SyhEjtX0DyKxQKioTYPOHcW9gdV5fhSz5tEv+ipqt4kIgWqBgzK8ePtDTqRM qZq457g1/SXSoSQi4jN+gsneqvlTJdzaEu1bJP0iv6ViVf15+qHuY5iojCz8fa0= In-Reply-To: <02e1b8df-d568-8cbb-b8f6-46d5476d9d75@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Stat-Signature: 53qo9xaahiyn59cus1ctuskxh8rk7t9i X-Rspamd-Queue-Id: 640991C0003 X-Rspam-User: X-HE-Tag: 1775467307-726738 X-HE-Meta: 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 47k4V9Z5 Lg2J4HGa6RtNOHPrvhORxktOYNpyWQGPmlKOYFEkh/QXaZU1QR0birnjwt3+MtfJjBCooR2PP5kaZZTmceq4nyzqMZCBsgkTgS4rjqzerpii5Jz5tu4SfGUXJsmyKs3leuL4+eJZE9KYn/W+9oHdkzMVR8sO9nuSthzx5b75I0zCT7klz+piHDLGuvSl1+0bH3Hm2CK1CqOOmf07CN9dqN1X8qjps3fGS71wv2VQfNsuD+IxTFoIKDy1l+ZE0jxheB/TjQUZjzCYd0pKeagRa68DfGqvltf3R2bTMxDE9ovRKY9Of+joPOwVOPvQb44tX+ydIn4I+dDlWsa7qbZtJmPBOgCQVmLKsbKNefCsXtmmW72/yjr/QbTVRx90hcXGayCUvOq1D+NtiePA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/5/26 06:44, Hugh Dickins wrote: > On Thu, 12 Feb 2026, 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 > > This is a very attractive speedup, but I believe it's flawed: in the > special case when a range has been mremap-moved, when its anon folio > indexes and anon_vma pgoff correspond to the original user address, > not to the current user address. [as discussed in earlier versions of this patch set] mremap() breaks KSM in the range to be moved. See prep_move_vma()->ksm_madvise(MADV_UNMERGEABLE) So I am not sure what you say can trigger. But I'm just scrolling by, as I'm still busy celebrating Easter :) -- Cheers, David