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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 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; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260503205013850sym7UO0jvKJzyRiTYmO8V@zte.com.cn> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 3FAA5180014 X-Stat-Signature: 571ns5yfkkub9pifycyao1uhgazia3w6 X-Rspam-User: X-HE-Tag: 1778674234-927698 X-HE-Meta: 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 f08y5UCa CrXhuHBCylylv0V6kgPy4wB2DywA1M6M98gNtQP6GMqcuD4P4Rc281KgB7n3/nmBz8V2mOn967uvt7clZZj5otVBEAHJoSD5wlclYY4bfFhVS2l2vl1R+K1CnUaSUjHmyskgAOfKgGVu8w86R98OQ3D62q2cZrgCWIVXazJEsVquJa4G6KFjZ6l//cpX3LZ9akZGHpI/z5Kwmzan/lB5t+l8/VGo0+4stuS/n6erkcPL2PaJ+zCz7fhSf9qu0b8qEVvStd/oEvFG8zIpcsgGkCN7gJKcdF77UYCB6sTU/77d29B1W8gdsRs51RVQdJnpalbN1AAekK1RU3NIuJF9xjjEtTZL/950O53k0XVUQ1OWJlTl3trMjFXX+4CdXUpaFVZ8mGL30epan4bE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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