Re: [PATCH v6 09/17] media/videbuf1|2: Mark follow_pfn usage as unsafe

[Hans Verkuil]

On 20/11/2020 10:18, Daniel Vetter wrote:
> On Fri, Nov 20, 2020 at 9:28 AM Hans Verkuil <hverkuil@xxxxxxxxx> 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@xxxxxxxxxxxx>
>>>
>>> Acked-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx>
>>
>> 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@xxxxxxxxx>
>>>> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
>>>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>>>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>>> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
>>>> Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
>>>> Cc: Jan Kara <jack@xxxxxxx>
>>>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>>>> Cc: linux-mm@xxxxxxxxx
>>>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>>> Cc: linux-samsung-soc@xxxxxxxxxxxxxxx
>>>> Cc: linux-media@xxxxxxxxxxxxxxx
>>>> Cc: Pawel Osciak <pawel@xxxxxxxxxx>
>>>> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
>>>> Cc: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
>>>> Cc: Tomasz Figa <tfiga@xxxxxxxxxxxx>
>>>> Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
>>>> Cc: Vlastimil Babka <vbabka@xxxxxxx>
>>>> Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
>>>> Cc: Michel Lespinasse <walken@xxxxxxxxxx>
>>>> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
>>>> --
>>>> 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;
>>>>
>>>>
>>>
>>
> 
>