From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6C38D3806DA; Thu, 6 Aug 2026 14:26:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786026386; cv=none; b=bblk+0Vkfztd2vIQTxLcB1zP4OJJWjFL/aqL/IoauyIwjfi7R9lOPqvqREwaMDFKivInprEUpNj+psD1iabGOLvBco7REqOnAXxDcDwJLJu69OdoJ2tLSy7Zf2zAjO22kA6gFm8fhlojYpS3wJTp8liHvsBClQKffONSFfP64A8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786026386; c=relaxed/simple; bh=5JPG/IE7MpyRCHKDB0v2afsVTjAV0GARn0zF5Z5f7z8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VoMS9IYKgXgf03+DwOmfAUyB2ux4gNUg9F/qdRWMccwEdt5fLzmV1+OO4CK4/aU1OGI93y44DzfprSnxAq5SlmKPO4BU5hAbQkZK5mQSkyQfCAtiVdN4uzCkeajiET9Ndoa2pfrLN61pzm8poR+hV4ymzeaVb2n+nbZofuiciNw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SnJsh9bV; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SnJsh9bV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4F73A1F00A3A; Thu, 6 Aug 2026 14:26:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786026385; bh=pm44+rjMaMmNzpedN1v7slJ59DpJ7j6EBTW26pHYJik=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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; 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charset=UTF-8 Content-Transfer-Encoding: 8bit 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