Re: [PATCH v4 05/15] mm/frame-vector: Use FOLL_LONGTERM

Tomasz Figa
2020-10-26T22:15:26Z

Hi Daniel,

On Mon, Oct 26, 2020 at 11:58:08AM +0100, Daniel Vetter 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)
> ---
> mm/frame_vector.c | 50 ++++++++++++++---------------------------------
> 1 file changed, 15 insertions(+), 35 deletions(-)
>

Thank you for the patch. Please see my comments inline.

> diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> index 10f82d5643b6..d44779e56313 100644
> --- a/mm/frame_vector.c
> +++ b/mm/frame_vector.c
> @@ -38,7 +38,6 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> struct vm_area_struct *vma;
> int ret = 0;
> int err;
> - int locked;
>
> if (nr_frames == 0)
> return 0;
> @@ -48,40 +47,25 @@ 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);

Should we drop the gup_flags argument, since it's ignored now?

> - goto out;
> + goto out_unlocked;
> }
>

Should we initialize ret with 0 here, since pin_user_pages_fast() can
return a negative error code, but below we use it as a counter for the
looked up frames?

Best regards,
Tomasz

> + mmap_read_lock(mm);
> vec->got_ref = false;
> vec->is_pfns = true;
> 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 +76,13 @@ 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
>