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 F3E65401A1D for ; Wed, 13 May 2026 12:10:33 +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=1778674234; cv=none; b=KoRk9BU8QZF4HKjmrvnAD0iwWJOBUHKEN8x9NxFKq7D2cCq3qJnCM8OZpBUBgbus09RqzSL7ZCKdG0mjvy5+s6/SUM4HFNhhex5qJ8EOS3Koi+KB6JnwO+UfCvfo+ykJGyqb4xOr0+rAtnPfV+V0WlAZHbnckRYQ0atBxBL/C6k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778674234; c=relaxed/simple; bh=6rfhGFFoVU1ZOiQR3m9G/ek9hBpseQZfJiZqEPfqQDg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=i5TCPQCm7zSseg9O467tAGrvjoEkekFUz3ejo2+OYeUPHeJMNj85JmJ63uqlwXHXUdINhOspujsHLDXoo0xXb2Z2zW9GW/+UlOclOxXwBQDUFSg25tIF4I6d7WbZztYAE4nGloze3+ib0/2ER7hqvMZUA+9SYmd6AEsJiYhaaiU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=rlL1pN2t; 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="rlL1pN2t" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 47439C2BCF5; Wed, 13 May 2026 12:10:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778674233; bh=6rfhGFFoVU1ZOiQR3m9G/ek9hBpseQZfJiZqEPfqQDg=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=rlL1pN2te9N+SZDlvSLoyrjpJUj/4WIyYFoZaS1GHIsyY3fYqgvL0Wtfm09/dTw2Z w9agvTGfatnpseOfVSnk3Ta3yMn3me34w5z9dD8YqV/Nwyz7fMKXetIJkLMiLj+Yoa M2WY8yKkgmqUDO/HPt4KSvkyK7P6g6FZAorikP0HDV84xWHQ0dzb+k+xc/rv6sZJt4 uuOQ897ZV50aaDMa3I9QyyugM5EG+40IBoHadmGuE7PvLu5gNPLRBTHluep4wSHslD A3ZNIgn7wRgyvRJ3MvWrqsz3XCE83zBWg80d1aYAXSQgUzPHc6iyeVz2hYDt4J0Gp+ 5S0SBCCRxqXsA== Message-ID: <0432f2d7-3151-49d1-911c-0e227223df10@kernel.org> Date: Wed, 13 May 2026 14:10:29 +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 v4 4/5] ksm: Optimize rmap_walk_ksm by passing a suitable address range To: xu.xin16@zte.com.cn, akpm@linux-foundation.org, ljs@kernel.org Cc: hughd@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, michel@lespinasse.org References: <20260503205013850sym7UO0jvKJzyRiTYmO8V@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 5/3/26 14:50, 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 > ======== > We cannot rely solely on anon_vma to locate all PTEs mapping this page but > also need to have the original page's pgoff. In fact, I believe only the > original vma->vm_pgoff is just enough. The implementation of > anon_vma_interval_tree_foreach — it essentially iterates to find a suitable > VMA such that the provided pgoff falls within the candidate's vm_pgoff range. > > vm_pgoff <= pgoff_parameter <= (vm_pgoff + vma_pages(v) - 1) > > Fortunately, we have already vm_pgoff in ksm_rmap_item in the previos patch > of series, so that we use it to get the pgoff to accelerate the searching. > > 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 shown 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 > > We also provide a rmap testbench: tools/testing/rmap/rmap_benchmark.c > The testing result in QEMU is shown as follows: > > 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) > > Co-developed-by: Wang Yaxin > Signed-off-by: Wang Yaxin > Signed-off-by: xu xin > --- > mm/ksm.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 0299a53ba7c9..a13184d00759 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -3200,6 +3200,7 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc) > hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) { > /* Ignore the stable/unstable/sqnr flags */ > const unsigned long addr = rmap_item->address & PAGE_MASK; > + const unsigned long vm_pgoff = rmap_item->vm_pgoff; > struct anon_vma *anon_vma = rmap_item->anon_vma; > struct anon_vma_chain *vmac; > struct vm_area_struct *vma; > @@ -3213,8 +3214,12 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc) > anon_vma_lock_read(anon_vma); > } > > + /* > + * Currently KSM folios are order-0 normal pages, so pgoff_end > + * should be the same as pgoff_start. > + */ > anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root, > - 0, ULONG_MAX) { > + vm_pgoff, vm_pgoff) { But vm_pgoff would just correspond to the start of the VMA, not where the page is actually mapped? I'd assume you really want the linear page index of the original page? -- Cheers, David