Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

From: Tomasz Figa
Date: Mon Nov 02 2020 - 13:19:32 EST

On Fri, Oct 30, 2020 at 3:38 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
>
> On Fri, Oct 30, 2020 at 3:11 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
> >
> > On Fri, Oct 30, 2020 at 11:08 AM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> > >
> > > This is used by media/videbuf2 for persistent dma mappings, not just
> > > for a single dma operation and then freed again, so needs
> > > FOLL_LONGTERM.
> > >
> > > Unfortunately current pup_locked doesn't support FOLL_LONGTERM due to
> > > locking issues. Rework the code to pull the pup path out from the
> > > mmap_sem critical section as suggested by Jason.
> > >
> > > By relying entirely on the vma checks in pin_user_pages and follow_pfn
> > > (for vm_flags and vma_is_fsdax) we can also streamline the code a lot.
> > >
> > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > > Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> > > Cc: Pawel Osciak <pawel@xxxxxxxxxx>
> > > Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> > > Cc: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
> > > Cc: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > > Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
> > > 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
> > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > > --
> > > v2: Streamline the code and further simplify the loop checks (Jason)
> > >
> > > v5: Review from Tomasz:
> > > - fix page counting for the follow_pfn case by resetting ret
> > > - drop gup_flags paramater, now unused
> > > ---
> > > .../media/common/videobuf2/videobuf2-memops.c | 3 +-
> > > include/linux/mm.h | 2 +-
> > > mm/frame_vector.c | 53 ++++++-------------
> > > 3 files changed, 19 insertions(+), 39 deletions(-)
> > >
> >
> > Thanks, looks good to me now.
> >
> > Acked-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> >
> > From reading the code, this is quite unlikely to introduce any
> > behavior changes, but just to be safe, did you have a chance to test
> > this with some V4L2 driver?
>
> Nah, unfortunately not.

I believe we don't have any setup that could exercise the IO/PFNMAP
user pointers, but it should be possible to exercise the basic userptr
path by enabling the virtual (fake) video driver, vivid or
CONFIG_VIDEO_VIVID, in your kernel and then using yavta [1] with
--userptr and --capture=<number of frames> (and possibly some more
options) to grab a couple of frames from the test pattern generator.

Does it sound like something that you could give a try? Feel free to
ping me on IRC (tfiga on #v4l or #dri-devel) if you need any help.

[1] https://git.ideasonboard.org/yavta.git

Best regards,
Tomasz

> -Daniel
>
> >
> > Best regards,
> > Tomasz
> >
> > > diff --git a/drivers/media/common/videobuf2/videobuf2-memops.c b/drivers/media/common/videobuf2/videobuf2-memops.c
> > > index 6e9e05153f4e..9dd6c27162f4 100644
> > > --- a/drivers/media/common/videobuf2/videobuf2-memops.c
> > > +++ b/drivers/media/common/videobuf2/videobuf2-memops.c
> > > @@ -40,7 +40,6 @@ struct frame_vector *vb2_create_framevec(unsigned long start,
> > > unsigned long first, last;
> > > unsigned long nr;
> > > struct frame_vector *vec;
> > > - unsigned int flags = FOLL_FORCE | FOLL_WRITE;
> > >
> > > first = start >> PAGE_SHIFT;
> > > last = (start + length - 1) >> PAGE_SHIFT;
> > > @@ -48,7 +47,7 @@ struct frame_vector *vb2_create_framevec(unsigned long start,
> > > vec = frame_vector_create(nr);
> > > if (!vec)
> > > return ERR_PTR(-ENOMEM);
> > > - ret = get_vaddr_frames(start & PAGE_MASK, nr, flags, vec);
> > > + ret = get_vaddr_frames(start & PAGE_MASK, nr, vec);
> > > if (ret < 0)
> > > goto out_destroy;
> > > /* We accept only complete set of PFNs */
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index ef360fe70aaf..d6b8e30dce2e 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -1765,7 +1765,7 @@ struct frame_vector {
> > > struct frame_vector *frame_vector_create(unsigned int nr_frames);
> > > void frame_vector_destroy(struct frame_vector *vec);
> > > int get_vaddr_frames(unsigned long start, unsigned int nr_pfns,
> > > - unsigned int gup_flags, struct frame_vector *vec);
> > > + struct frame_vector *vec);
> > > void put_vaddr_frames(struct frame_vector *vec);
> > > int frame_vector_to_pages(struct frame_vector *vec);
> > > void frame_vector_to_pfns(struct frame_vector *vec);
> > > diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> > > index 10f82d5643b6..f8c34b895c76 100644
> > > --- a/mm/frame_vector.c
> > > +++ b/mm/frame_vector.c
> > > @@ -32,13 +32,12 @@
> > > * This function takes care of grabbing mmap_lock as necessary.
> > > */
> > > int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> > > - unsigned int gup_flags, struct frame_vector *vec)
> > > + struct frame_vector *vec)
> > > {
> > > struct mm_struct *mm = current->mm;
> > > struct vm_area_struct *vma;
> > > int ret = 0;
> > > int err;
> > > - int locked;
> > >
> > > if (nr_frames == 0)
> > > return 0;
> > > @@ -48,40 +47,26 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> > >
> > > start = untagged_addr(start);
> > >
> > > - mmap_read_lock(mm);
> > > - locked = 1;
> > > - vma = find_vma_intersection(mm, start, start + 1);
> > > - if (!vma) {
> > > - ret = -EFAULT;
> > > - goto out;
> > > - }
> > > -
> > > - /*
> > > - * While get_vaddr_frames() could be used for transient (kernel
> > > - * controlled lifetime) pinning of memory pages all current
> > > - * users establish long term (userspace controlled lifetime)
> > > - * page pinning. Treat get_vaddr_frames() like
> > > - * get_user_pages_longterm() and disallow it for filesystem-dax
> > > - * mappings.
> > > - */
> > > - if (vma_is_fsdax(vma)) {
> > > - ret = -EOPNOTSUPP;
> > > - goto out;
> > > - }
> > > -
> > > - if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
> > > + ret = pin_user_pages_fast(start, nr_frames,
> > > + FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,
> > > + (struct page **)(vec->ptrs));
> > > + if (ret > 0) {
> > > vec->got_ref = true;
> > > vec->is_pfns = false;
> > > - ret = pin_user_pages_locked(start, nr_frames,
> > > - gup_flags, (struct page **)(vec->ptrs), &locked);
> > > - goto out;
> > > + goto out_unlocked;
> > > }
> > >
> > > + mmap_read_lock(mm);
> > > vec->got_ref = false;
> > > vec->is_pfns = true;
> > > + ret = 0;
> > > do {
> > > unsigned long *nums = frame_vector_pfns(vec);
> > >
> > > + vma = find_vma_intersection(mm, start, start + 1);
> > > + if (!vma)
> > > + break;
> > > +
> > > while (ret < nr_frames && start + PAGE_SIZE <= vma->vm_end) {
> > > err = follow_pfn(vma, start, &nums[ret]);
> > > if (err) {
> > > @@ -92,17 +77,13 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> > > start += PAGE_SIZE;
> > > ret++;
> > > }
> > > - /*
> > > - * We stop if we have enough pages or if VMA doesn't completely
> > > - * cover the tail page.
> > > - */
> > > - if (ret >= nr_frames || start < vma->vm_end)
> > > + /* Bail out if VMA doesn't completely cover the tail page. */
> > > + if (start < vma->vm_end)
> > > break;
> > > - vma = find_vma_intersection(mm, start, start + 1);
> > > - } while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));
> > > + } while (ret < nr_frames);
> > > out:
> > > - if (locked)
> > > - mmap_read_unlock(mm);
> > > + mmap_read_unlock(mm);
> > > +out_unlocked:
> > > if (!ret)
> > > ret = -EFAULT;
> > > if (ret > 0)
> > > --
> > > 2.28.0
> > >
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch