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b=SnJsh9bVghFGUQLo4q7FXusg8tdyw+GFmZER/SFNErq+VKGkpCkBXtO4LmjNxfrwp KGaYM8zUJrkf/xjHudJpOhdLWh3Hqlun6SpUenj2lTuHNcuK96NMP0sFE9iBP3VVZf s/HR7Ojx4kpqnuHcYoLx7kjOfy5H4HUAG1/qCBlXwcmVjBTYo4GdDRIDYS4PrsXca0 FPtqnEuiLdwnnJy6GQ42kn8wTd7nJYSqYGHsydTcuBTwFqB5M1wB5veqG3bS1tWGN+ 2HKMlDAowRAggRJLYKE/Sdj/Ux7BBjXYc4n8rRZXkNOaHKxueZ/qUD2r5pT0aAVzdV dyGnrv1hn3a/Q== Message-ID: <0e52d0b4-064d-4602-8e7b-5744b05f24ea@kernel.org> Date: Thu, 6 Aug 2026 16:26:20 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/huge_memory: let special huge VMAs bypass the THP policy check To: =?UTF-8?Q?C=C3=A9dric_Le_Goater?= , Andrew Morton , linux-mm@kvack.org Cc: Peter Xu , Lorenzo Stoakes , Alex Williamson , Jason Gunthorpe , Zi Yan , stable@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260805055544.1568534-1-clg@redhat.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: <20260805055544.1568534-1-clg@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 443DD40008 X-Rspam-User: X-Stat-Signature: pmcdf3y4jciottdp9xzj37344tidy1gt X-HE-Tag: 1786026386-854243 X-HE-Meta: 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 tW2gb3zy kvlKIvo8+UVhmY0NyBkK1fXfaXDADO7YWhtodixY1txAHZ/m4uIEKd0t3bz3I0VWzM99Wh/sZl8LcCarFxJQ2B10KztaOsZwb4SlBKJ7kdsb4w6yEkWgnimMaaJ0r4uNto/wW133W21cRjIop3T4XT+eHXZltO237MFSXkc2Px6plMtd1qA/zD4T+5vPiAGSfxlqCGTJvJaHk9ggBmfSJkNolSVGmgb0oaVUNJ6gp6jbWA/88lxfT/Bf7iL9rVswFye1wWtjUsX/h5PEHV3kApTNCJY7z4EMb9i94uRxOa+tiI5aAmPk2GTid1GRf+KniumzS7qRIhPacigtwveqgUKdfiVf0zJulvhn/pfDyNDZL79GGwsysfRUYxoaEWAfPkMtd+++6P94hTV9pC3bNJVl+g9GWHcxhskwDwIZbpwxMgZ2MOoSQiHFA0e+GoFsFgqXDnNgrXWiK/ws= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/5/26 07:55, Cédric Le Goater wrote: > From: Cedric Le Goater > > The global THP sysfs policy (transparent_hugepage=never/madvise/always) > gates the huge fault dispatch path in __thp_vma_allowable_orders() for > all non-anonymous VMAs, including PFN-mapped device BARs (VM_PFNMAP). > > DAX VMAs already bypass this check via an early return: > > if (vma_is_dax(vma)) > return in_pf ? orders : 0; > > But "special huge" VMAs -- identified by vma_is_special_huge() -- do not > get this early return, even though they share the same fundamental > property: they map physical addresses directly into page tables and > involve no memory allocation, no compaction, no splitting, and no > reclaim. The THP policy has no meaningful effect on them. > > This matters for VFIO PCI passthrough of large-BAR devices such as > NVIDIA H200 NVL GPUs (256 GB BAR each). The VFIO driver registers a > .huge_fault handler (vfio_pci_mmap_huge_fault) that dispatches to > vmf_insert_pfn_pmd/pud, and QEMU's vfio_region_mmap() aligns the BAR > mappings for huge page table entries. Both prerequisites are met, but > with THP=never or THP=madvise, __thp_vma_allowable_orders() returns 0 > before reaching the "trust huge_fault handlers" code. > > The result: each 256 GB BAR is mapped at 4 KiB granularity -- 67 million > page faults per GPU instead of a few thousand PMD/PUD faults. On hosts > with 8 GPUs (2 TB of BAR space), this causes VM boot times to degrade > severely, with 99.98% of CPU time spent in the VFIO BAR mapping path. > > Configurations that trigger this: > - transparent_hugepage=never on the kernel command line > - The tuned cpu-partitioning profile (inherits network-latency, which > sets transparent_hugepages=never via sysfs) > - transparent_hugepage=madvise (the RHEL default), since VFIO VMAs > lack VM_HUGEPAGE and QEMU does not call madvise(MADV_HUGEPAGE) on > BAR mmap regions > > Extend the existing DAX early return to also cover vma_is_special_huge() > VMAs. This is consistent with how vma_is_special_huge() is already > treated for supported_orders (grouped with DAX). The mm/Kconfig TODO > comment "Allow to be enabled without THP" also acknowledges this > coupling is wrong. > > Cc: Peter Xu > Cc: Andrew Morton > Cc: Lorenzo Stoakes > Cc: David Hildenbrand > Cc: Alex Williamson > Cc: Jason Gunthorpe > Cc: Zi Yan > Fixes: 5dd40721f147 ("mm: allow THP orders for PFNMAPs") > Cc: stable@vger.kernel.org > Assisted-by: Claude:claude-opus-4 > Signed-off-by: Cedric Le Goater > --- > mm/huge_memory.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 58cabe6af33d031e48250e21db51506bc46c97b2..6dfef5500a054f09f9ece6df8bf7a0194624350f 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -139,8 +139,12 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma, > if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags, forced_collapse)) > return 0; > > - /* khugepaged doesn't collapse DAX vma, but page fault is fine. */ > - if (vma_is_dax(vma)) > + /* > + * khugepaged doesn't collapse DAX or special huge VMAs, but page > + * fault is fine. These map physical addresses directly — the THP > + * policy is irrelevant for them. emdash in a code comment? Then I spot Assisted-by: Claude:claude-opus-4 and really have to shake my head. -- Cheers, David