Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

[Dan Williams]

On Mon, Nov 11, 2019 at 4:07 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>
> As it says in the updated comment in gup.c: current FOLL_LONGTERM
> behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the
> FS DAX check requirement on vmas.
>
> However, the corresponding restriction in get_user_pages_remote() was
> slightly stricter than is actually required: it forbade all
> FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers
> that do not set the "locked" arg.
>
> Update the code and comments accordingly, and update the VFIO caller
> to take advantage of this, fixing a bug as a result: the VFIO caller
> is logically a FOLL_LONGTERM user.
>
> Thanks to Jason Gunthorpe for pointing out a clean way to fix this.
>
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Jerome Glisse <jglisse@xxxxxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 30 +++++++++++++-----------------
>  mm/gup.c                        | 13 ++++++++-----
>  2 files changed, 21 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index d864277ea16f..017689b7c32b 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -348,24 +348,20 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>                 flags |= FOLL_WRITE;
>
>         down_read(&mm->mmap_sem);
> -       if (mm == current->mm) {
> -               ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page,
> -                                    vmas);
> -       } else {
> -               ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
> -                                           vmas, NULL);
> -               /*
> -                * The lifetime of a vaddr_get_pfn() page pin is
> -                * userspace-controlled. In the fs-dax case this could
> -                * lead to indefinite stalls in filesystem operations.
> -                * Disallow attempts to pin fs-dax pages via this
> -                * interface.
> -                */
> -               if (ret > 0 && vma_is_fsdax(vmas[0])) {
> -                       ret = -EOPNOTSUPP;
> -                       put_page(page[0]);
> -               }
> +       ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
> +                                   page, vmas, NULL);

Hmm, what's the point of passing FOLL_LONGTERM to
get_user_pages_remote() if get_user_pages_remote() is not going to
check the vma? I think we got to this code state because the
get_user_pages() vs get_user_pages_remote() split predated the
introduction of FOLL_LONGTERM.

I think check_vma_flags() should do the ((FOLL_LONGTERM | FOLL_GET) &&
vma_is_fsdax()) check and that would also remove the need for
__gup_longterm_locked.