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 66C0B321457 for ; Mon, 6 Apr 2026 09:23:45 +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=1775467425; cv=none; b=N4Y/ytZmJ8OYYtTzYRagZSYnnlRcQ6lhjbu5v0tSDYw+CC0bSW/Yj7HztDFHw8//AOieWLqKTrIu0jYuO9UWLrB8Fk8Q69Vynvt7DY9FocxGRaNPuBWweSHVuLennSvapsUM7Jmmk9cZZn0dOk3zD/MytchXumM7IKea+FmuHiE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775467425; c=relaxed/simple; bh=SQ0GTh8RyULPElvjMm8BF/sb5+Fdu/zQIOIbg8gaNDc=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=uDXjcV8o+kGwD9GuADJ5aIk+snunKuATgIn1ngRrqCm0puGxCGhthY0R4nN2iFeLCo3TZ2QkLcNiRAdNhGiJPYOqRCdUCz59h8lUR3RcD1zxU6xJTBX0D+rVGfEOVPqQuao1Upnqenbg9QbTzX5Kh5L/HqOibD+do8caN7lguKI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Zdo4RW2c; 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="Zdo4RW2c" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ACB71C19425; Mon, 6 Apr 2026 09:23:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775467425; bh=SQ0GTh8RyULPElvjMm8BF/sb5+Fdu/zQIOIbg8gaNDc=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=Zdo4RW2cpgdNvxaWum7O5zJkvuGECCZIWKHhZdwFI9HzfU+RtqKSScx6UAyHEEbMx WVNxy/Q/VaNxgs9EWiOietMidqQHMHfn+dPeBZGfTc2q0o25cH6FCf0l+CbOUkXX7/ jLDVRsxbouq2NIm15jUknUQzqxHITEiQkjOFe3A3wfpyx1ALgIQDw6IgVqx4U9PJfO OyLyu7Mnh4knUZEPn94IG7NfWinRZjHWXoE748KbIWDQWA0BAnijl3DbKUQqfldY14 KfxXYi/6URjSTJ/+U/KnB4z9jt6CDGU0xjhAhhlWnNZhKVfcN2OqyaJVoue3400W5d zre0IVePpDuOQ== Message-ID: Date: Mon, 6 Apr 2026 11:23:40 +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 From: "David Hildenbrand (arm)" 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> <8481d0dd-2471-4acc-a429-cbc02451a812@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 4/6/26 11:21, David Hildenbrand (arm) wrote: > 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 :) > [realizing threading is somehow messed up and Xu Xin commented that already] -- Cheers, David