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

[Tomasz Figa <tfiga@chromium.org>]

On Fri, Nov 20, 2020 at 5:28 PM 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 all for dropping that code.

Best regards,
Tomasz

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