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

[Daniel Vetter]

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