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

[Jason Gunthorpe]

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@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(-)

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