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b=cZ1Nv4E5M8ZqvrssciqIPmbcB5tnUu3LISQlyC31sZV4aLmai4oBlDr0ZCMJJtt3O Zwzp39lzTFzX94viFolQ+w6JHMMxiVd3yt/YwuJl15kd1pa3Y2XK36Uioz915XaIXy ADNAu+j61uQXJJogMbTMssodfeJEbzWjPHIaBa0SP5qkL8Okggq7YiVnjEXf6ZItyq KkS71wjnsZDMh+iXkUo4auLw3X08RPkjSfXECNvtxinNP60v6cSskmcff8CQujWKl3 WneAAh86Egy/phHyKYiGLo7lDuLPDfJ/qM2sax0hm/rPW3almwiNYYRM1OPwo03Lhf fD+pONFOrnWCQ== Message-ID: Date: Thu, 17 Sep 2026 10:43:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: batch PTE-mapped large folios in gup_fast_pte_range() From: "David Hildenbrand (Arm)" To: Yuan-Hao Hsu , Andrew Morton Cc: Jason Gunthorpe , John Hubbard , Peter Xu , Lorenzo Stoakes , liam@infradead.org, Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Rik van Riel , Aristeu Rozanski , Ryan Roberts , Dev Jain , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260917083747.786-1-aa9736195201@gmail.com> 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: D5C601C0007 X-Stat-Signature: p1tthsdreriiiejxdqdkq318jmg4yjot X-Rspamd-Server: rspam01 X-HE-Tag: 1789634610-119976 X-HE-Meta: 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 61VxpkaD BhFcN5NpDVckKRq/nK2lPgsznCqd6JHPaDdoGr9e/i1gjBXiO5FFveayOg/7dvXD+yNadAzJWhZt6Hvdw9b+ObaB23UjSjhdzza+6FccmzN+YikyH0Wun+73gW7LkzRenpCsS+TTjs5pvLvasJ35vmviSAnZPVi3L+sFRBQ7EsVtrU2fcosyIG71NBnMnLOmbdTnaEKC94BWhvGTgVeC5FQGLlfAW2WuXt41An92QhkSB8HqqUQxgYOQRAy9scLBafFbD7T6RW3CmihOvRhBLgcNb2eLD2LiQUxJPPFocjYMRfZFzHxIFmdgqvA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/17/26 10:42, David Hildenbrand (Arm) wrote: > On 9/17/26 10:37, Yuan-Hao Hsu wrote: >> GUP-fast grabs a PTE-mapped large folio one page at a time. Every PTE >> costs a try_grab_folio_fast() (a refcount cmpxchg, plus the pincount >> atomic, a full barrier and a node stat update for FOLL_PIN), a >> gup_fast_folio_allowed() and a folio_set_referenced(), so a 64 kB mTHP >> pays sixteen of each and a PTE-mapped 2 MB THP pays 512. The PMD and >> PUD leaf paths already take the whole range with one >> try_grab_folio_fast() call, and the slow path is getting the same >> treatment for PTEs in Rik's follow_page_mask() series. >> >> The users are the hot ones: iov_iter_extract_user_pages() pins every >> O_DIRECT buffer through pin_user_pages_fast(), io_uring registers its >> buffers through it, and so does RDMA memory registration. With mTHP >> those buffers are PTE-mapped large folios. >> >> After the head page has been grabbed and verified as before, extend the >> run to the following PTEs that map the next pages of the same folio >> with the same protection bits, take the remaining references for the >> run with try_grab_folio(), and fill pages[] from the run. The PTEs of >> the run are read while the reference on the folio is held, so the folio >> can neither be freed nor split underneath the scan; a THP collapse can >> still detach the page table, so the pmd is checked again after the run >> is grabbed and the run is dropped if it changed, leaving the next >> iteration to hit the existing pmd check and bail out. A read-only run >> of an anonymous folio stops at the first page that gup_must_unshare() >> rejects, so the per-page exclusivity guarantee for FOLL_PIN is kept. >> >> folio_pte_batch_flags() does the scan. It reads the entries with >> ptep_get(), which is not safe without the page table lock on >> CONFIG_GUP_GET_PXX_LOW_HIGH (x86 PAE, mips32, sh) or with arm64 contpte, >> and GUP-fast reads every PTE with ptep_get_lockless() for that reason. >> Add FPB_LOCKLESS to make the helper do the same; it is inlined per call >> site, so the other callers do not change. >> >> The order-0 path keeps its instruction sequence. The folio flags are >> tested before folio_set_referenced() on purpose: a load of the flags >> right after that locked instruction cost about 25% on order-0 pages in >> the gup_test benchmark below, and reading them first brings that path >> back to the baseline. >> >> gup_test ioctl over a 256 MB anonymous region, FOLL_WRITE, 65536 pages >> per call, median of 15 runs, two baseline boots / one patched boot, on >> an x86-64 VM: >> >> get_user_pages_fast() pin_user_pages_fast() >> before after before after >> 4 kB pages 849/789 us 783 us 1174/1138 us 1094 us >> 64 kB mTHP 812/811 us 126 us 1215/1113 us 188 us >> 1 MB mTHP 799/774 us 59 us 1185/1077 us 64 us >> 2 MB THP, PTE-mapped 803/786 us 57 us 1159/1120 us 58 us >> 2 MB THP, PMD-mapped 42/40 us 40 us 46/41 us 42 us >> >> fio O_DIRECT randread of a null_blk device (CPU bound) with the I/O >> buffer in 64 kB mTHP, median of 15 five-second runs: >> >> bs=1M psync 126.2/125.6 GB/s -> 220.6 GB/s >> bs=1M io_uring, iodepth 16 71.2/71.6 GB/s -> 112.1 GB/s >> bs=64k io_uring, iodepth 16 50.2/50.7 GB/s -> 60.1 GB/s >> >> IORING_REGISTER_BUFFERS of 1 GB, median of 15: >> >> 64 kB mTHP 6179/5998 us -> 1599 us >> 1 MB mTHP 5790/5801 us -> 893 us >> 4 kB pages 7173/6741 us -> 6763 us >> > > I think Rik was already working on this and sent some patches. Anyhow, there is > quite some GUP review backlog I have t go through, so this will have to wait. > Ah, no, Rik looked at GUP not GUP-fast. So I'll have a look but it will take a while. -- Cheers, David