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[2003:d8:2f25:700:d846:15f3:6ca0:8029]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4b48f7a531asm102949671cf.49.2025.09.08.05.04.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Sep 2025 05:04:08 -0700 (PDT) Message-ID: Date: Mon, 8 Sep 2025 14:04:05 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/1] mm: skip mlocked THPs that are underused early in deferred_split_scan() To: Kiryl Shutsemau Cc: Lance Yang , akpm@linux-foundation.org, Liam.Howlett@oracle.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, npache@redhat.com, ryan.roberts@arm.com, usamaarif642@gmail.com, ziy@nvidia.com References: <20250908090741.61519-1-lance.yang@linux.dev> <9a0c07b9-5bf0-4251-8609-fbaf0ca75bf9@redhat.com> <5j6i2o6umqwxabdfncbrdytmvdma4yrraxe6hu4csckcniduya@sm3mlablwbad> 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: <5j6i2o6umqwxabdfncbrdytmvdma4yrraxe6hu4csckcniduya@sm3mlablwbad> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: CRGvUpNhKtm5jdmi_60MANKc0Bj-ifWIk5n2Nc6Xj2k_1757333050 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: ggg35in8hh7ue9bi11x5e9f4e8zj691a X-Rspam-User: X-Rspamd-Queue-Id: 7908E1C0004 X-Rspamd-Server: rspam01 X-HE-Tag: 1757333052-709677 X-HE-Meta: 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 ioizRbyD 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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 08.09.25 13:44, Kiryl Shutsemau wrote: > On Mon, Sep 08, 2025 at 01:32:05PM +0200, David Hildenbrand wrote: >> On 08.09.25 12:38, Kiryl Shutsemau wrote: >>> On Mon, Sep 08, 2025 at 05:07:41PM +0800, Lance Yang wrote: >>>> From: Lance Yang >>>> >>>> When we stumble over a fully-mapped mlocked THP in the deferred shrinker, >>>> it does not make sense to try to detect whether it is underused, because >>>> try_to_map_unused_to_zeropage(), called while splitting the folio, will not >>>> actually replace any zeroed pages by the shared zeropage. >>> >>> It makes me think, does KSM follows the same logic as >>> try_to_map_unused_to_zeropage()? >>> >>> I cannot immediately find what prevents KSM from replacing zeroed mlocked >>> folio with ZERO_PAGE(). >>> >>> Hm? >> >> I assume if you're using mlock and at the same time enable KSM for a >> process/VMA, you're doing something wrong. >> >> In contrast, THP is supposed to be transparent (yeah, I know ...). > > Yeah, I guess it is user error. > > Maybe we should make ksm_compatible() return false for VM_LOCKED? > KSM breaks mlock() contract. I was thinking the same and falsely remembered that we would already be checking for that. > > But it can be risky if someone already relies on this broken behaviour. Could be. Staring at QEMU, we have the following parameters: mem-merge=on|off Enables or disables memory merge support. This feature, when supported by the host, de-duplicates identical memory pages among VMs instances (enabled by default). And -overcommit mem-lock=on|off|on-fault "Run qemu with hints about host resource overcommit. The default is to assume that host overcommits all resources." Now, I would assume that anybody who sets "-overcommit mem-lock=on" either (a) Has KSM disabled on that machine. (b) Sets mem-merge=off as well. But QEMU would allow for configuring it. Interestingly, mm_populate()->populate_vma_page_range() wants to break COW. [*] But if the app later calls fork(), we still allow for cow-sharing pages with the child. (another case of "don't do it", like KSM I guess) [*] it doesn't do it for mappings that start out R/O. I think we might end up with sharedzero pages in that case, but not sure if worth fixing. -- Cheers David / dhildenb