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

From: Tomasz Figa <tfiga@chromium.org>
To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "DRI Development" <dri-devel@lists.freedesktop.org>,
	LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"list@263.net:IOMMU DRIVERS <iommu@lists.linux-foundation.org>,
	Joerg Roedel <joro@8bytes.org>,"
	<linux-arm-kernel@lists.infradead.org>,
	linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	"Linux Media Mailing List" <linux-media@vger.kernel.org>,
	"Daniel Vetter" <daniel.vetter@intel.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Pawel Osciak" <pawel@osciak.com>,
	"Marek Szyprowski" <m.szyprowski@samsung.com>,
	"Kyungmin Park" <kyungmin.park@samsung.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Jérôme Glisse" <jglisse@redhat.com>, "Jan Kara" <jack@suse.cz>,
	"Dan Williams" <dan.j.williams@intel.com>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM
Date: Fri, 30 Oct 2020 15:11:14 +0100	[thread overview]
Message-ID: <CAAFQd5ANOAzVf+tC1iYKXeY0ALahtYrG7xtKHXHmvv1xh7si3g@mail.gmail.com> (raw)
In-Reply-To: <20201030100815.2269-6-daniel.vetter@ffwll.ch>

On Fri, Oct 30, 2020 at 11:08 AM Daniel Vetter <daniel.vetter@ffwll.ch> 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@intel.com>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> 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: Mauro Carvalho Chehab <mchehab@kernel.org>
> 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
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> --
> 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@chromium.org>

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?

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
>