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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Usama Arif References: <20250905141137.3529867-1-david@redhat.com> <7B0B1E09-5083-449F-851D-FD63D32D2B3D@nvidia.com> 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: <7B0B1E09-5083-449F-851D-FD63D32D2B3D@nvidia.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: GIzmhxvfHFWOgiwKFgPGcmA5N7VlxdC4h-XNdQsgib4_1757083193 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 99569140010 X-Stat-Signature: qut1b3so36t89etyurp6mrs7ekfo19bp X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1757083195-872973 X-HE-Meta: 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 T9QvK5d4 x0FqlPrfmj7fUOplEbNA+KaAQIS7psOqjeiYf0uf9Ylf2IqduuA9aFQU1hOOaRvr3c5lBoIoTbUbLY1Q6EbOFZlcWLxqpwp+KtdpsxNpkZLgRIDenrNmKbl2wOZ82XjtmKvjFU9xxsB4vcNvUD0wsjevXIeTBefb8113VRYrjyotvNerb7JFyMV7X3kdcloLJ/dAqR/lqdNT1fdas+urO7HtGErR+jJOnZd7mSUKGNA3sxWE1a8EYmnmx7P8vzvr8wrUNukKQxWY3pX6YvjrJhmGe7hpbk5Mrr32caPTw96bi0hVPoEeC5STea6r89h7AvY5RY4Cn2SepIRgcnwEUwMqK2ruM2wazWnDF0B0aTM0qOATijRzGpyUUcadYmqRY7hRLEIucs3y/bpUPNY7h2Zeo0IQsb3NL0iyoRU8AkOiGRZXNEgmAOpFKaYALeymvRoPt104qYg51jdSRv+UObuRannyz2stEfnOMJzsZojFNjGFt7+s0iBz3BesElpE6AW7N/oxkeo37QuXigO80DlsWIzueHiiq9ZhQc2x/5H/3ynD1HIC3P6V3T1T7vICjSiPWgtk78ICahaX5yYXrEofd4FrSh2U0WvKceu3bPu4eWpCwM1uW67c7qX02a5QhwxuC0E5ZfUWuM8uDNr/BfnvZ+sPWYNdc5Mz3+ip+/14yFIp6CrQcttVHyyPpadSCHPeVJ8FgnVpkL8zIC00MCvz6tA== 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 05.09.25 16:37, Zi Yan wrote: > On 5 Sep 2025, at 10:11, David Hildenbrand wrote: > >> We added an early exit in thp_underused(), probably to avoid scanning >> pages when there is no chance for success. >> >> However, assume we have max_ptes_none = 511 (default). >> >> Nothing should stop us from freeing all pages part of a THP that >> is completely zero (512) and khugepaged will for sure not try to >> instantiate a THP in that case (512 shared zeropages). >> >> This can just trivially happen if someone writes a single 0 byte into a >> PMD area, or of course, when data ends up being zero later. >> >> So let's remove that early exit. >> >> Do we want to CC stable? Hm, not sure. Probably not urgent. >> >> Note that, as default, the THP shrinker is active >> (/sys/kernel/mm/transparent_hugepage/shrink_underused = 1), and all >> THPs are added to the deferred split lists. However, with the >> max_ptes_none default we would never scan them. We would not do that. If >> that's not desirable, we should just disable the shrinker as default, >> also not adding all THPs to the deferred split lists. >> >> Easy to reproduce: >> >> 1) Allocate some THPs filled with 0s >> >> >> #include >> #include >> #include >> #include >> #include >> >> const size_t size = 1024*1024*1024; >> >> int main(void) >> { >> size_t offs; >> char *area; >> >> area = mmap(0, size, PROT_READ | PROT_WRITE, >> MAP_ANON | MAP_PRIVATE, -1, 0); >> if (area == MAP_FAILED) { >> printf("mmap failed\n"); >> exit(-1); >> } >> madvise(area, size, MADV_HUGEPAGE); >> >> for (offs = 0; offs < size; offs += getpagesize()) >> area[offs] = 0; >> pause(); >> } >> <\prog.c> >> >> 2) Trigger the shrinker >> >> E.g., memory pressure through memhog >> >> 3) Observe that THPs are not getting reclaimed >> >> $ cat /proc/`pgrep prog`/smaps_rollup >> >> Would list ~1GiB of AnonHugePages. With this fix, they would get >> reclaimed as expected. >> >> Fixes: dafff3f4c850 ("mm: split underused THPs") >> Cc: Andrew Morton >> Cc: Lorenzo Stoakes >> Cc: Zi Yan >> Cc: Baolin Wang >> Cc: "Liam R. Howlett" >> Cc: Nico Pache >> Cc: Ryan Roberts >> Cc: Dev Jain >> Cc: Barry Song >> Cc: Usama Arif >> Signed-off-by: David Hildenbrand >> --- >> mm/huge_memory.c | 3 --- >> 1 file changed, 3 deletions(-) >> > LGTM. Acked-by: Zi Yan > > I also notice that thp_underused() checks num_zero_pages directly > against khugepaged_max_ptes_none. This means mTHPs will never be regarded > as underused. A similar issue you are discussing in Nico’s khugepaged > mTHP support. Maybe checks against these khugepaged_max* variables > should be calculated based on nr_pages of a large folio, like > making these variables a ratio in other discussion. Yeah, factoring that out and cleaning it up is my next step. But note that mTHPs are not a candidate for the shrinker right now. (see my explanation in reply to Nicos patch) -- Cheers David / dhildenb