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b=RyDAibe96Eq/7kf+TLq5bawmi37igOsg/UMj3h8nppbsBm4/0BO4DS9C2Kmn7qeq3 crVliEz67Dwxbp9KPAmewYN2KaM+5HN7ehBeEKXZI74Xw84Djzhpd/ITtoOSeurzKB imskknXHsIhAMXD/TE5ljilnge8SW0Wh247bO3Ay4in9N2UkzDsYFGgHb8+ZWppYzh filExIkMvEToz8TregUkBzcU001evOU1KzpRqu5vrtMAbVHO+9I6FerFDmaUHxsOi/ pcdYAXjNqgTu2kZsGVcs0lNsaffA54L13s20tpZzce4SVc/Eq0pYWzxFH5QuHIR+iK 1sF4YNgtGwp/w== Message-ID: <69c33490-e5a3-4cf2-a5ed-d2bc8ed336d2@kernel.org> Date: Wed, 7 Oct 2026 13:01:14 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] riscv/mm: fix soft-dirty migration PMDs being treated as present To: Lance Yang Cc: pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, akpm@linux-foundation.org, zhangchunyan@iscas.ac.cn, rppt@kernel.org, kas@kernel.org, andrew+kernel@donnellan.id.au, rmclure@linux.ibm.com, debug@rivosinc.com, baolin.wang@linux.alibaba.com, usama.arif@linux.dev, wangruikang@iscas.ac.cn, namcao@linutronix.de, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, alexghiti@rivosinc.com, viro@zeniv.linux.org.uk, ajones@ventanamicro.com, arnd@arndb.de, axelrasmussen@google.com, brauner@kernel.org, conor.dooley@microchip.com, conor@kernel.org, jack@suse.cz, liam@infradead.org, ljs@kernel.org, mhocko@suse.com, paul.walmsley@sifive.com, peterx@redhat.com, robh@kernel.org, surenb@google.com, vbabka@kernel.org, yuanchu@google.com, stable@vger.kernel.org, pasha.tatashin@soleen.com, linux-mm@kvack.org, me@ziyao.cc References: <20261005134641.5801-1-lance.yang@linux.dev> 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: <20261005134641.5801-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 4367E100003 X-Rspam-User: X-Stat-Signature: aixgfzyd38td6jnduysmjmgaaq9iumsk X-HE-Tag: 1791370886-718527 X-HE-Meta: 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 +CdLRD0O hT4uR687bTCsXLXllcrtgI+2sMGS7DaFOLTXVK6YL1tTqRs5tzSE3uzgz7Y7jfctHCrpUKcmuoaC+TX6hkxwrI31sIjurjcpHBZnlkT/YSUpEsz7RWzCp//p2KXUlV9J6afXVLAEBiRQwNqGil9rUEM2MegARcLB9Zp1A/TA66RdJlYaNzZ0s8JrwxXVWuyT+dml0VwGPagnkiWr6nDwelA7Ii5LXrCGRMchrqVGaGOM6ukUd6iu6N/d9CdWzEqhEZngUQYl6RCO371P4N9dB4MLASYQ21FOsnt38sZCIylB6UM+sYWsxsZWSMUW52KmzNudlrOeRJ+x5BaIlEivnfxcZ+EO0oGFg3/JNmoiNs6xgTQUlZY/La1q9PQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/5/26 15:46, Lance Yang wrote: > > On Mon, Oct 05, 2026 at 12:37:20PM +0200, David Hildenbrand (Arm) wrote: >> On 10/4/26 05:03, Lance Yang wrote: >>> RISC-V uses _PAGE_EXEC for swap soft-dirty tracking when >>> CONFIG_MEM_SOFT_DIRTY is enabled and Svrsw60t59b is available. That's >>> a problem for PMD migration entries, since pmd_present() also checks >>> _PAGE_LEAF (R/W/X) to recognize THPs with _PAGE_PRESENT temporarily >>> cleared during splitting. >> >> I'm curious: why do we have to set leaf indications for non-present things? The >> HW sure will ignore it, right? > > Yeah, that surprised me too :) Still wrapping my head around the details > ... > >> Is this a sw problem? Who needs that? > > IIUC, it's for software during a PMD split. > > __split_huge_pmd_locked() invalidates the huge PMD and flushes the TLB > before installing the PTE table. Software still needs pmd_present() and > pmd_trans_huge() to recognize the THP in between. > > RISC-V clears V but keeps the R/W/X bits for that, so the entry is invalid > to hardware but still identifiable as a THP by software. > > Hopefully I didn't miss something. I thought some change we performed to GUP-fast wouldn't require that anymore. But my memory is a bit vague ... :) > >>> >>> When a soft-dirty THP is migrated, set_pmd_migration_entry() preserves >>> soft-dirty with pmd_swp_mksoft_dirty(), setting the X bit in the >>> migration PMD. Even with _PAGE_PRESENT clear, we end up treating a >>> migration PMD as a present THP! The fault handler skips >>> pmd_migration_entry_wait(), and a write fault can end up in >>> do_huge_pmd_wp_page(), where pmd_page() decodes the migration entry >>> as a mapped PFN. >> >> That sounds bad. > [...] > YES, looks a bit off ... >> That reduces the effective swap size (and PFN we can store). Could that be a >> problem? > > We don't need all 52 bits of the swap offset. > > RV64 PFNs only need 44 bits for migration entries, and actual swap is > already limited to about 16 TiB per area with 4 KiB pages by > last_page (__u32) and swap_info_struct.max (unsigned int). > > So there's room to reserve a bit without reducing the supported swap > size or PFN range. > > CONFIG_MEM_SOFT_DIRTY is only available on RV64, so RV32 keeps its 20-bit > offset. Good, can you spell that out in the patch description? -- Cheers, David