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b=VlUcElYub9DBYJ7OhfBzT83mIYtFZk+b7GV8Yc2OxMb55U279PAMlUoIYKbc+lP9k IcL9R3S0tfkvClz9l31/nYbK7bZIuLUQeXG0tqu7GlemTGalRrhbUGz9B8Ghepg597 WHxO0dyzH2GABw8ZivJkcL/syeN8nIodknFT75lBFfJ20KomrrusjUMuxq5KNYk6+K 7vEpPVFEQ6+A14Jnnx3JW2tmOVFSpJ6u03nU46hTKoocNWGkAOCtSR7MV+QiaGYid/ PO2iY+AhpUY4s7odgDEREOI0vyJFSqeQRLjJUmuCdFq2bWfHRVSo6Lh6twSeohqTyT ipl4sc6yO2Zpg== Message-ID: Date: Thu, 9 Apr 2026 17:11:30 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: fix GUP-fast fallback for NULL-mapping order-0 folios To: Zi Yan Cc: John Hubbard , Andrew Morton , Jason Gunthorpe , Peter Xu , Mike Rapoport , LKML , linux-mm@kvack.org, Sourab Gupta , "Matthew Wilcox (Oracle)" References: <20260409014647.397515-1-jhubbard@nvidia.com> <4d6c1af0-dee5-4f1b-b74c-2ebc23de7baf@kernel.org> 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Stat-Signature: a9paddae7mpo47ochshu4fxanscdkj56 X-Rspamd-Queue-Id: D92A3160006 X-Rspam-User: X-HE-Tag: 1775747495-36692 X-HE-Meta: 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 NzvYeP+7 wOY6E4Xbq5uxU5wsDvIESw4XDTOvfadTGz0CedYFHKXQ/hL1as5FJkzQp491nJH+Yoj4oRTdJTlgF9YdY36tO3eO/Zut2i+Q3eRXYkMOQSa2nYLD+jI1RUbDD2d4/Lp3rH5jNcLF5pDfv+PGHmI+cFL+z0GS0sorJ+jdSmsl2qkcoEW+vqYWc+g5dUAsoTBrPf7WUagiHFsvxwWOTnE65S0mOnKiVCW+EjCPZry+A1OEcgHwR4FERyQ8KfYOtmo2ZuQNzjeOhvqHBZTFZqblnkToZ+w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/9/26 17:05, Zi Yan wrote: > On 9 Apr 2026, at 3:52, David Hildenbrand (Arm) wrote: > >> On 4/9/26 03:46, John Hubbard wrote: >>> Since commit f002882ca369 ("mm: merge folio_is_secretmem() and >>> folio_fast_pin_allowed() into gup_fast_folio_allowed()"), >>> gup_fast_folio_allowed() falls back to the slow path for any order-0 >>> folio with a NULL mapping when CONFIG_SECRETMEM=y. This causes a >>> performance regression for drivers that allocate pages with alloc_page() >>> and insert them into VMAs via vm_insert_page(). These pages legitimately >>> have a NULL folio->mapping, but they cannot be secretmem pages. >>> >>> Secretmem pages are always added to the secretmem inode's page cache via >>> filemap_add_folio(), which sets folio->mapping to the inode's i_mapping. >>> A folio with a NULL mapping can never be a secretmem folio. The >>> NULL-mapping check was intended to handle truncated file-backed pages (a >>> reject_file_backed concern), not secretmem detection. >>> >>> When only check_secretmem is true (and reject_file_backed is false), a >>> NULL mapping is sufficient to prove the folio is not secretmem, so the >>> fast path can proceed. >> >> Hm, what if secretmem folio just got truncated? I hate to rely on some >> handling in the caller to detect truncation differently during GUP-fast, >> but this function returning "true". >> >> Zi is working on a way to distinguish folios from non-folio things: that >> we can identify whatever was added through vm_insert_page(). > > The idea is to give every allocated page a NotRmappable page_type and > 1) let page_rmappable_folio() remove the page_type by setting mapcount > to 0, since page_type overlaps with mapcount; > 2) allow callers of alloc_page*() to change page_type from NotRmappable > to other types. Could we instead just set the special type in vm_insert_page(), after verifying that it does not have any other type? > > After these, rmappable folios with any order is a folio without any > page_type. HugeTLB is an exception, but rmap code has special handling > for them, so it should be fine. > > I have a draft[1], but have not had time to refresh it yet. I am planning > to work on this after my two other jobs (make READ_ONLY_THP_FOR_FS enabled > for all FS with large folio support and ensure ->private is 0 for tail pages). > > [1] https://github.com/x-y-z/linux-dev/tree/remove_mapcount_for_non_folio-2026-02-16-23-27 > >> >> Because that's really the key problem here: vm_insert_page() pages are >> not actually folios, they just look like a folio today, but looking at >> fields like ->mapping does not make any sense. > > Based on my understanding of some callers of alloc_page()/get_free_pages(), > vm_insert_page(), they use GFP_COMP to get a compound page and vm_insert_page() > individual pages of the compound page. These pages might be pinned. > During past THP cabal meeting, we discussed about it if > 1. we should disallow such uses and only allow order-0 allocations, or Right. > 2. we need a new type like new_type_folio for them and the type does not > have folio fields except ->pincount. Right. -- Cheers, David