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

Daniel Vetter
2020-10-27T08:07:21Z

On Mon, Oct 26, 2020 at 11:15 PM Tomasz Figa <tfiga@chromium.org> wrote:
>
> 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?

Hm right I think an earlier version even had that, but then I moved to
inlining the functionality in all the places it's used.

I'll drop the gup flag.

> > - 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?

Indeed, that's a bug. Will fix for v5.
-Daniel

> 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
> >

--
Daniel Vetter
Software Engineer, Intel Corporation