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From: Hans Verkuil <hverkuil@xs4all.nl>
To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Jan Kara" <jack@suse.cz>, "KVM list" <kvm@vger.kernel.org>,
	"DRI Development" <dri-devel@lists.freedesktop.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"Daniel Vetter" <daniel.vetter@intel.com>,
	"Michel Lespinasse" <walken@google.com>,
	"Marek Szyprowski" <m.szyprowski@samsung.com>,
	linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	"Daniel Jordan" <daniel.m.jordan@oracle.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Linux ARM" <linux-arm-kernel@lists.infradead.org>,
	"open list:DMA BUFFER SHARING FRAMEWORK"
	<linux-media@vger.kernel.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Pawel Osciak" <pawel@osciak.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Laurent Dufour" <ldufour@linux.ibm.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	LKML <linux-kernel@vger.kernel.org>,
	"Tomasz Figa" <tfiga@chromium.org>,
	"Kyungmin Park" <kyungmin.park@samsung.com>,
	"Andrew Morton" <akpm@linux-foundation.org>
Subject: Re: [PATCH v6 09/17] media/videbuf1|2: Mark follow_pfn usage as unsafe
Date: Fri, 20 Nov 2020 11:38:55 +0100
Message-ID: <9035555a-af6b-e2dd-dbad-41ca70235e21@xs4all.nl> (raw)
In-Reply-To: <CAKMK7uEzFAtr9yxjaxi-kiuZhb+hWT3q6E41OegJr+J2-zkT8w@mail.gmail.com>

On 20/11/2020 10:18, Daniel Vetter wrote:
> On Fri, Nov 20, 2020 at 9:28 AM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>>
>> On 20/11/2020 09:06, Hans Verkuil wrote:
>>> On 19/11/2020 15:41, Daniel Vetter wrote:
>>>> The media model assumes that buffers are all preallocated, so that
>>>> when a media pipeline is running we never miss a deadline because the
>>>> buffers aren't allocated or available.
>>>>
>>>> This means we cannot fix the v4l follow_pfn usage through
>>>> mmu_notifier, without breaking how this all works. The only real fix
>>>> is to deprecate userptr support for VM_IO | VM_PFNMAP mappings and
>>>> tell everyone to cut over to dma-buf memory sharing for zerocopy.
>>>>
>>>> userptr for normal memory will keep working as-is, this only affects
>>>> the zerocopy userptr usage enabled in 50ac952d2263 ("[media]
>>>> videobuf2-dma-sg: Support io userptr operations on io memory").
>>>>
>>>> Acked-by: Tomasz Figa <tfiga@chromium.org>
>>>
>>> Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
>>
>> Actually, cancel this Acked-by.
>>
>> So let me see if I understand this right: VM_IO | VM_PFNMAP mappings can
>> move around. There is a mmu_notifier that can be used to be notified when
>> that happens, but that can't be used with media buffers since those buffers
>> must always be available and in the same place.
>>
>> So follow_pfn is replaced by unsafe_follow_pfn to signal that what is attempted
>> is unsafe and unreliable.
>>
>> If CONFIG_STRICT_FOLLOW_PFN is set, then unsafe_follow_pfn will fail, if it
>> is unset, then it writes a warning to the kernel log but just continues while
>> still unsafe.
>>
>> I am very much inclined to just drop VM_IO | VM_PFNMAP support in the media
>> subsystem. For vb2 there is a working alternative in the form of dmabuf, and
>> frankly for vb1 I don't care. If someone really needs this for a vb1 driver,
>> then they can do the work to convert that driver to vb2.
>>
>> I've added Mauro to the CC list and I'll ping a few more people to see what
>> they think, but in my opinion support for USERPTR + VM_IO | VM_PFNMAP
>> should just be killed off.
>>
>> If others would like to keep it, then frame_vector.c needs a comment before
>> the 'while' explaining why the unsafe_follow_pfn is there and that using
>> dmabuf is the proper alternative to use. That will make it easier for
>> developers to figure out why they see a kernel warning and what to do to
>> fix it, rather than having to dig through the git history for the reason.
> 
> I'm happy to add a comment, but otherwise if you all want to ditch
> this, can we do this as a follow up on top? There's quite a bit of
> code that can be deleted and I'd like to not hold up this patch set
> here on that - it's already a fairly sprawling pain touching about 7
> different subsystems (ok only 6-ish now since the s390 patch landed).
> For the comment, is the explanation next to unsafe_follow_pfn not good
> enough?

No, because that doesn't mention that you should use dma-buf as a replacement.
That's really the critical piece of information I'd like to see. That doesn't
belong in unsafe_follow_pfn, it needs to be in frame_vector.c since it's
vb2 specific.

> 
> So ... can I get you to un-cancel your ack?

Hmm, I really would like to see support for this to be dropped completely.

How about this: just replace follow_pfn() by -EINVAL instead of unsafe_follow_pfn().

Add a TODO comment that this code now can be cleaned up a lot. Such a clean up patch
can be added on top later, and actually that is something that I can do once this
series has landed.

Regardless, frame_vector.c should mention dma-buf as a replacement in a comment
since I don't want users who hit this issue to have to dig through git logs
to find that dma-buf is the right approach.

BTW, nitpick: the subject line of this patch says 'videbuf' instead of 'videobuf'.

Regards,

	Hans

> 
> Thanks, Daniel
> 
>>
>> Regards,
>>
>>         Hans
>>
>>>
>>> Thanks!
>>>
>>>       Hans
>>>
>>>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>>>> Cc: Jason Gunthorpe <jgg@ziepe.ca>
>>>> Cc: Kees Cook <keescook@chromium.org>
>>>> Cc: Dan Williams <dan.j.williams@intel.com>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: John Hubbard <jhubbard@nvidia.com>
>>>> Cc: Jérôme Glisse <jglisse@redhat.com>
>>>> Cc: Jan Kara <jack@suse.cz>
>>>> Cc: Dan Williams <dan.j.williams@intel.com>
>>>> Cc: linux-mm@kvack.org
>>>> Cc: linux-arm-kernel@lists.infradead.org
>>>> Cc: linux-samsung-soc@vger.kernel.org
>>>> Cc: linux-media@vger.kernel.org
>>>> Cc: Pawel Osciak <pawel@osciak.com>
>>>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
>>>> Cc: Kyungmin Park <kyungmin.park@samsung.com>
>>>> Cc: Tomasz Figa <tfiga@chromium.org>
>>>> Cc: Laurent Dufour <ldufour@linux.ibm.com>
>>>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>>> Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
>>>> Cc: Michel Lespinasse <walken@google.com>
>>>> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>>>> --
>>>> v3:
>>>> - Reference the commit that enabled the zerocopy userptr use case to
>>>>   make it abundandtly clear that this patch only affects that, and not
>>>>   normal memory userptr. The old commit message already explained that
>>>>   normal memory userptr is unaffected, but I guess that was not clear
>>>>   enough.
>>>> ---
>>>>  drivers/media/common/videobuf2/frame_vector.c | 2 +-
>>>>  drivers/media/v4l2-core/videobuf-dma-contig.c | 2 +-
>>>>  2 files changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/media/common/videobuf2/frame_vector.c b/drivers/media/common/videobuf2/frame_vector.c
>>>> index a0e65481a201..1a82ec13ea00 100644
>>>> --- a/drivers/media/common/videobuf2/frame_vector.c
>>>> +++ b/drivers/media/common/videobuf2/frame_vector.c
>>>> @@ -70,7 +70,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>>>>                      break;
>>>>
>>>>              while (ret < nr_frames && start + PAGE_SIZE <= vma->vm_end) {
>>>> -                    err = follow_pfn(vma, start, &nums[ret]);
>>>> +                    err = unsafe_follow_pfn(vma, start, &nums[ret]);
>>>>                      if (err) {
>>>>                              if (ret == 0)
>>>>                                      ret = err;
>>>> diff --git a/drivers/media/v4l2-core/videobuf-dma-contig.c b/drivers/media/v4l2-core/videobuf-dma-contig.c
>>>> index 52312ce2ba05..821c4a76ab96 100644
>>>> --- a/drivers/media/v4l2-core/videobuf-dma-contig.c
>>>> +++ b/drivers/media/v4l2-core/videobuf-dma-contig.c
>>>> @@ -183,7 +183,7 @@ static int videobuf_dma_contig_user_get(struct videobuf_dma_contig_memory *mem,
>>>>      user_address = untagged_baddr;
>>>>
>>>>      while (pages_done < (mem->size >> PAGE_SHIFT)) {
>>>> -            ret = follow_pfn(vma, user_address, &this_pfn);
>>>> +            ret = unsafe_follow_pfn(vma, user_address, &this_pfn);
>>>>              if (ret)
>>>>                      break;
>>>>
>>>>
>>>
>>
> 
> 

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Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-19 14:41 [PATCH v6 00/17] follow_pfn and other iomap races Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 01/17] drm/exynos: Stop using frame_vector helpers Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 02/17] drm/exynos: Use FOLL_LONGTERM for g2d cmdlists Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 03/17] misc/habana: Stop using frame_vector helpers Daniel Vetter
2020-11-21 12:47   ` Oded Gabbay
2020-11-19 14:41 ` [PATCH v6 04/17] misc/habana: Use FOLL_LONGTERM for userptr Daniel Vetter
2020-11-21 10:15   ` Oded Gabbay
2020-11-19 14:41 ` [PATCH v6 05/17] mm/frame-vector: Use FOLL_LONGTERM Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 06/17] media: videobuf2: Move frame_vector into media subsystem Daniel Vetter
2020-11-20  8:07   ` Hans Verkuil
2020-11-19 14:41 ` [PATCH v6 07/17] mm: Close race in generic_access_phys Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 08/17] mm: Add unsafe_follow_pfn Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 09/17] media/videbuf1|2: Mark follow_pfn usage as unsafe Daniel Vetter
2020-11-20  8:06   ` Hans Verkuil
2020-11-20  8:28     ` Hans Verkuil
2020-11-20  8:32       ` Tomasz Figa
2020-11-20  9:18       ` Daniel Vetter
2020-11-20 10:38         ` Hans Verkuil [this message]
2020-11-20 10:51           ` Daniel Vetter
2020-11-20 12:08             ` Hans Verkuil
2020-11-20 12:23               ` Tomasz Figa
2020-11-24 14:16                 ` Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 10/17] vfio/type1: Mark follow_pfn " Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 11/17] PCI: Obey iomem restrictions for procfs mmap Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 12/17] /dev/mem: Only set filp->f_mapping Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 13/17] resource: Move devmem revoke code to resource framework Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 14/17] sysfs: Support zapping of binary attr mmaps Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 15/17] PCI: Revoke mappings like devmem Daniel Vetter
2020-11-19 14:41 ` [PATCH v6 16/17] RFC: kvm: pass kvm argument to follow_pfn callsites Daniel Vetter
2020-11-20 15:33   ` Paolo Bonzini
2020-11-20 15:44     ` Daniel Vetter
2020-11-20 15:55       ` Paolo Bonzini
2020-11-19 14:41 ` [PATCH v6 17/17] RFC: mm: add mmu_notifier argument to follow_pfn Daniel Vetter
2020-11-20 18:30   ` Jason Gunthorpe
2020-11-24 14:28     ` Daniel Vetter
2020-11-24 15:55       ` Jason Gunthorpe
2020-11-25  9:00         ` Daniel Vetter
2020-11-27 13:12 ` [PATCH v6 00/17] follow_pfn and other iomap races Jason Gunthorpe
2020-11-27 15:36   ` Daniel Vetter

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