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

Jason Gunthorpe
2019-11-12T20:43:44Z

On Mon, Nov 11, 2019 at 04:06:45PM -0800, John Hubbard 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@ziepe.ca>
> Cc: Jerome Glisse <jglisse@redhat.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
> drivers/vfio/vfio_iommu_type1.c | 30 +++++++++++++-----------------
> mm/gup.c | 13 ++++++++-----
> 2 files changed, 21 insertions(+), 22 deletions(-)

This matches what I thought, but I think DanW should check it too, and
the vfio users should test..

> 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);
> + /*
> + * 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]);
> }

AFAIK this chunk is redundant now as it is some hack to emulate
FOLL_LONGTERM? So vmas can be deleted too.

Also unclear why this function has this:

up_read(&mm->mmap_sem);

if (ret == 1) {
*pfn = page_to_pfn(page[0]);
return 0;
}

down_read(&mm->mmap_sem);

Jason