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[2003:d8:2f0c:c200:fa4a:c4ff:1b32:21ce]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-427f009a6c5sm14619574f8f.28.2025.10.20.02.42.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Oct 2025 02:42:29 -0700 (PDT) Message-ID: <320b8572-bdaf-4aa9-83d7-701d18a9628f@redhat.com> Date: Mon, 20 Oct 2025 11:42:28 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] ksm: use range-walk function to jump over holes in scan_get_next_rmap_item To: Andrew Morton , Pedro Demarchi Gomes Cc: Xu Xin , craftfever , Chengming Zhou , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20251016012236.4189-1-pedrodemargomes@gmail.com> <20251016140735.d7d4c282f0fbf22954a4b4ee@linux-foundation.org> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+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: <20251016140735.d7d4c282f0fbf22954a4b4ee@linux-foundation.org> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 1kaaky8QdLoj0PmV1TCxEeI9b1Pb_jDduXjaso_MzPw_1760953351 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 3D176100003 X-Rspamd-Server: rspam03 X-Stat-Signature: 4db5z3dffkgsjz5gwqdo8sgayin6f514 X-HE-Tag: 1760953354-94605 X-HE-Meta: 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 y4WVgUPk 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 16.10.25 23:07, Andrew Morton wrote: > On Wed, 15 Oct 2025 22:22:36 -0300 Pedro Demarchi Gomes wrote: > >> Currently, scan_get_next_rmap_item() walks every page address in a VMA >> to locate mergeable pages. This becomes highly inefficient when scanning >> large virtual memory areas that contain mostly unmapped regions. >> >> This patch replaces the per-address lookup with a range walk using >> walk_page_range(). The range walker allows KSM to skip over entire >> unmapped holes in a VMA, avoiding unnecessary lookups. >> This problem was previously discussed in [1]. >> >> ... >> >> Reported-by: craftfever >> Closes: https://lkml.kernel.org/r/020cf8de6e773bb78ba7614ef250129f11a63781@murena.io >> Suggested-by: David Hildenbrand >> Signed-off-by: Pedro Demarchi Gomes > > Is Fixes: b1d3e9bbccb4 ("mm/ksm: convert scan_get_next_rmap_item() from > follow_page() to folio_walk") appropriate? No. That commit is not the problem. The problem probably goes back when scan_get_next_rmap_item() was first introduced (likely when KSM was added): it simply never was optimized to deal with large sparse memory areas. > > The problem which is being addressed seems pretty serious. What do > people think about proposing a -stable backport of this fix? We'll likely have to backport it to each and every stable tree. We could think about limiting backports only to kernels that actually allow for enabling KSM for a complete process. So that would make sense to me. -- Cheers David / dhildenb