From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 A935631353B for ; Mon, 6 Apr 2026 09:21:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775467306; cv=none; b=neNe79qv7XthO2ZGhKM/Z0Ft3E0+oiBC3TDe5hZ+0QdoU2JP4S3xa+3EEpfLOyaZwjwHJgo6wi95E30tOjJmgh3Vtr+BnqPhfX+DAnX/busNePeitgfQaVPpzJO8iwQ4fi+8CU6lgu7fAilsuVUoqGF+IpY6iknT2eThAOV8gA0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775467306; c=relaxed/simple; bh=fhg6BtIGhbmFMSd9SUmDv0Uh+zl0kmiOHGSsPw4uSYk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=AvBE9Ck733lMwZUD7g3La08qChbsbHFBhT6t4EStayhveF2HcIQ75PCFQdATBi0eD4cMWolWKR0JJomIojGvfdNyp5ixBx+pb0xLel8SHdv0VM8SqhMsbK1tGUkwhiaWdMoHPFmZCznwjgj4gDvZ8WqCPONyMI2s74hFFGOEftA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=OPDC+7uX; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="OPDC+7uX" 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 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 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; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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