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

From: Tomasz Figa
Date: Fri Oct 30 2020 - 10:11:35 EST

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?

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
>