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

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Fri, Nov 20, 2020 at 11:39 AM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>
> 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.

Ah makes sense, I'll add that.

> >
> > 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'.

Will fix to, and next round will have the additional -EINVAL on top.
Iirc Mauro was pretty clear that we can't just delete this, so I kinda
don't want to get stuck in this discussion with my patches :-)
-Daniel

>
> 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;
> >>>>
> >>>>
> >>>
> >>
> >
> >
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch