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From: John Hubbard <jhubbard@nvidia.com>
To: Jason Gunthorpe <jgg@ziepe.ca>, Christoph Hellwig <hch@infradead.org>
Cc: linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	Jan Kara <jack@suse.cz>, Pawel Osciak <pawel@osciak.com>,
	KVM list <kvm@vger.kernel.org>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	LKML <linux-kernel@vger.kernel.org>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Tomasz Figa <tfiga@chromium.org>, Linux MM <linux-mm@kvack.org>,
	J??r??me Glisse <jglisse@redhat.com>,
	"open list:DMA BUFFER SHARING FRAMEWORK"
	<linux-media@vger.kernel.org>,
	Daniel Vetter <daniel.vetter@intel.com>,
	Kyungmin Park <kyungmin.park@samsung.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Marek Szyprowski <m.szyprowski@samsung.com>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM
Date: Wed, 4 Nov 2020 10:44:56 -0800	[thread overview]
Message-ID: <d3497583-2338-596e-c764-8c571b7d22cf@nvidia.com> (raw)
In-Reply-To: <20201104181708.GU36674@ziepe.ca>

On 11/4/20 10:17 AM, Jason Gunthorpe wrote:
> On Wed, Nov 04, 2020 at 04:41:19PM +0000, Christoph Hellwig wrote:
>> On Wed, Nov 04, 2020 at 04:37:58PM +0000, Christoph Hellwig wrote:
>>> On Wed, Nov 04, 2020 at 05:26:58PM +0100, Daniel Vetter wrote:
>>>> What we're discussing is whether gup_fast and pup_fast also obey this,
>>>> or fall over and can give you the struct page that's backing the
>>>> dma_mmap_* memory. Since the _fast variant doesn't check for
>>>> vma->vm_flags, and afaict that's the only thing which closes this gap.
>>>> And like you restate, that would be a bit a problem. So where's that
>>>> check which Jason&me aren't spotting?
>>>
>>> remap_pte_range uses pte_mkspecial to set up the PTEs, and gup_pte_range
>>> errors out on pte_special.  Of course this only works for the
>>> CONFIG_ARCH_HAS_PTE_SPECIAL case, for other architectures we do have
>>> a real problem.
>>
>> Except that we don't really support pte-level gup-fast without
>> CONFIG_ARCH_HAS_PTE_SPECIAL, and in fact all architectures selecting
>> HAVE_FAST_GUP also select ARCH_HAS_PTE_SPECIAL, so we should be fine.
> 
> Mm, I thought it was probably the special flag..
> 
> Knowing that CONFIG_HAVE_FAST_GUP can't be set without
> CONFIG_ARCH_HAS_PTE_SPECIAL is pretty insightful, can we put that in
> the Kconfig?
> 
> config HAVE_FAST_GUP
>          depends on MMU
>          depends on ARCH_HAS_PTE_SPECIAL
>          bool
> 
Well, the !CONFIG_ARCH_HAS_PTE_SPECIAL case points out in a comment that
gup-fast is not *completely* unavailable there, so I don't think you want
to shut it off like that:

/*
  * If we can't determine whether or not a pte is special, then fail immediately
  * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
  * to be special.
  *
  * For a futex to be placed on a THP tail page, get_futex_key requires a
  * get_user_pages_fast_only implementation that can pin pages. Thus it's still
  * useful to have gup_huge_pmd even if we can't operate on ptes.
  */


thanks,
-- 
John Hubbard
NVIDIA
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  reply	other threads:[~2020-11-04 18:45 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-30 10:08 [PATCH v5 00/15] follow_pfn and other iomap races Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 01/15] drm/exynos: Stop using frame_vector helpers Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 02/15] drm/exynos: Use FOLL_LONGTERM for g2d cmdlists Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 03/15] misc/habana: Stop using frame_vector helpers Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 04/15] misc/habana: Use FOLL_LONGTERM for userptr Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM Daniel Vetter
2020-10-30 14:11   ` Tomasz Figa
2020-10-30 14:37     ` Daniel Vetter
2020-11-02 18:19       ` Tomasz Figa
2020-10-31  2:55   ` John Hubbard
2020-10-31 14:45     ` Daniel Vetter
2020-11-01  5:22       ` John Hubbard
2020-11-01 10:30         ` Daniel Vetter
2020-11-01 21:13           ` John Hubbard
2020-11-01 22:50             ` Daniel Vetter
2020-11-04 14:00               ` Jason Gunthorpe
2020-11-04 15:54                 ` Daniel Vetter
     [not found]                   ` <20201104162125.GA13007@infradead.org>
2020-11-04 16:26                     ` Daniel Vetter
     [not found]                       ` <20201104163758.GA17425@infradead.org>
     [not found]                         ` <20201104164119.GA18218@infradead.org>
2020-11-04 18:17                           ` Jason Gunthorpe
2020-11-04 18:44                             ` John Hubbard [this message]
2020-11-04 19:02                               ` Jason Gunthorpe
2020-11-05  9:25                               ` Daniel Vetter
2020-11-05 12:49                                 ` Jason Gunthorpe
2020-11-06  4:08                                   ` John Hubbard
2020-11-06 10:01                                     ` Daniel Vetter
2020-11-06 10:27                                       ` Daniel Vetter
2020-11-06 12:55                                         ` Jason Gunthorpe
2020-11-09  8:44                                           ` Thomas Hellström
2020-11-09 20:19                                             ` Jason Gunthorpe
2020-11-06 12:58                                       ` Jason Gunthorpe
2020-10-30 10:08 ` [PATCH v5 06/15] media: videobuf2: Move frame_vector into media subsystem Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 07/15] mm: Close race in generic_access_phys Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 08/15] mm: Add unsafe_follow_pfn Daniel Vetter
     [not found]   ` <20201102072931.GA16419@infradead.org>
2020-11-02 12:56     ` Daniel Vetter
2020-11-02 13:01       ` Jason Gunthorpe
2020-11-02 13:23         ` Daniel Vetter
2020-11-02 15:52           ` Jason Gunthorpe
2020-11-02 16:42             ` Daniel Vetter
2020-11-02 17:16               ` Jason Gunthorpe
2020-10-30 10:08 ` [PATCH v5 09/15] media/videbuf1|2: Mark follow_pfn usage as unsafe Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 10/15] vfio/type1: Mark follow_pfn " Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 11/15] PCI: Obey iomem restrictions for procfs mmap Daniel Vetter
2020-11-03 21:28   ` Bjorn Helgaas
2020-11-03 22:09     ` Dan Williams
2020-11-04  8:44       ` Daniel Vetter
2020-11-04 16:50         ` Bjorn Helgaas
2020-11-04 20:12           ` Dan Williams
2020-11-05  9:34             ` Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 12/15] /dev/mem: Only set filp->f_mapping Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 13/15] resource: Move devmem revoke code to resource framework Daniel Vetter
2020-10-31  6:36   ` John Hubbard
2020-10-31 14:40     ` Daniel Vetter
2020-11-03  6:06   ` [resource] 22b17dc667: Kernel panic - not syncing: Fatal exception lkp
2020-11-03  6:15     ` John Hubbard
2020-11-03 10:10       ` Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 14/15] sysfs: Support zapping of binary attr mmaps Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 15/15] PCI: Revoke mappings like devmem Daniel Vetter
2020-10-30 19:22   ` Dan Williams
2020-11-03 21:30   ` Bjorn Helgaas

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