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b=RvVJ8UzmOEqrs0G14kFM++8rr6iSHglz2PZ93CS4iIyeprIkvH8VqMiq/edzOnkCw Si4666iPcgT3pRU8sE0Wf+cubbsOxmSkc58sEXEE0TCWStDdwgdVmxRQmfBm8wrY71 mzI3VspNd6nW1JYnfViIiONMRI2Kej7tVv4FKomuFTLENDR/OcJ18kSdXGarSyzoR5 r118jxfBv6pJ7S8C5KL9DbZVwIWGBoDobp64ckPeC4QjdTdxu0/gPXSdcQNREJvO9n jJLUS6GwJ0Vjf3l1frsawwNuCCzoXfWttFhEnBiG3G2B7XWsVMhPxhMs+7qvRANkx0 J1LsyFry/mCQw== Message-ID: <4d6c1af0-dee5-4f1b-b74c-2ebc23de7baf@kernel.org> Date: Thu, 9 Apr 2026 09:52:09 +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: John Hubbard , Andrew Morton Cc: Jason Gunthorpe , Peter Xu , Mike Rapoport , LKML , linux-mm@kvack.org, Sourab Gupta , Zi Yan References: <20260409014647.397515-1-jhubbard@nvidia.com> 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: <20260409014647.397515-1-jhubbard@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: 7htiauzsmr585impu4oe18ux9qj8bqch X-Rspamd-Queue-Id: CA4DE1C0005 X-Rspamd-Server: rspam09 X-HE-Tag: 1775721134-244941 X-HE-Meta: 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 mL8iEmwt fAnsD11p6AKuMvxLLp0tPjYMWfZASvpw74tWRwbTlo3UBQt2O6Et4mhNaZ0uclsgKJxXruYvU4x50eo2njbIYlzrBSIbbDq2TQ+O3FIgOcZYKdhrSn2Hsl3DgPZgnUtHnOUq2XhyJrcl372CsEVFLLnljm8R4pvrIGrRlkBht5+Bnnc2ZtZB7nl8WgKmYQKEgzj2wg5ULqGKp5ywTPY2f3QT/ArpyZcbdRrvidaEY3rMIoCKMLIAoNXoMhSeYxvtbVjsAEzvuoP/DoPEp+nj4twGqFw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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(). 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. > > Tested-by: Sourab Gupta > Fixes: f002882ca369 ("mm: merge folio_is_secretmem() and folio_fast_pin_allowed() into gup_fast_folio_allowed()") > Signed-off-by: John Hubbard > --- > mm/gup.c | 13 +++++++++---- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 8e7dc2c6ee73..3ea661e67eea 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2784,12 +2784,17 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags) > mapping = READ_ONCE(folio->mapping); > > /* > - * The mapping may have been truncated, in any case we cannot determine > - * if this mapping is safe - fall back to slow path to determine how to > - * proceed. > + * If the mapping is NULL (truncated, or never set), we cannot > + * determine whether the folio is file-backed, so a long-term writable > + * pin must fall back to the slow path. > + * > + * Otherwise, a NULL mapping proves this is not a secretmem folio > + * (secretmem folios always have a valid mapping to the secretmem > + * inode's address_space), so in that case, we can continue with the > + * fast path. > */ > if (!mapping) > - return false; > + return !reject_file_backed; > > /* Anonymous folios pose no problem. */ > mapping_flags = (unsigned long)mapping & FOLIO_MAPPING_FLAGS; > > base-commit: 7f87a5ea75f011d2c9bc8ac0167e5e2d1adb1594 -- Cheers, David