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Mon, 6 Apr 2026 09:23:47 +0000 (UTC) X-FDA: 84627593694.11.4C6439E Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf26.hostedemail.com (Postfix) with ESMTP id 1EA97140003 for ; Mon, 6 Apr 2026 09:23:45 +0000 (UTC) Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=Zdo4RW2c; spf=pass (imf26.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-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1775467426; a=rsa-sha256; cv=none; b=kCT2bk89pl7yIsRxk/Nh2i9VYteZiFEKHmXzrdYEoWR2UCUxYxa6JyJ8AcjcLzpn7LwJsP Z8bZhufBgt6m7ZUhyN/EvCAa2h9132to7o2wgaoMH70MpVPi3ijNRBdFqSJZUmGsQDjsoF t4jGYFzjZ0E+ACXwY3ZyFcB1MYh8XoM= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1775467426; 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; 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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 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 X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 1EA97140003 X-Stat-Signature: 7aioja3bew4seubnqi98ef4my89symn4 X-HE-Tag: 1775467425-230267 X-HE-Meta: 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 ShHthxAX 8ptXH/4tR9rRBmZZ1K/65wx9V1u5VmjzQPxyBZGK9UvvsBS74EBCM1mFefRrsF7+5N2UPejkRBuiV789uIw2ogHDtGVYlu9jnE7r7XkEMQqwFs7xjqTxjH9inTHhWXfJRRpKkgNC835Mr4DeXN89nsjFhcvO8uuL4aql1n8IL7UWCCTy1fWaZsEIyQMXuvb/FPRFSxJHWSvPk25x3Li9bD0nCtTSf1cBQSHpoueRhq6lQt3nOjErN4/vQ5FoWt7Elkcz6ypNi3EpkBaQxWsPks3a/nKFM2g5Fz2D937IJSb2h6OfTdEIta+w5pOuSN8LylSTBdgX0Ywo7A5g7rsscRuxe+6CPeoi5LdeN9HBwWVqmYWKiRgkJ+jmPcXrfllYmkJes8K/4XR3cC+k= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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