On 02/04/18 15:05 -0800, Dan Williams wrote:
> Filesystem-DAX is incompatible with 'longterm' page pinning. Without
> page cache indirection a DAX mapping maps filesystem blocks directly.
> This means that the filesystem must not modify a file's block map while
> any page in a mapping is pinned. In order to prevent the situation of
> userspace holding of filesystem operations indefinitely, disallow
> 'longterm' Filesystem-DAX mappings.
> 
> RDMA has the same conflict and the plan there is to add a 'with lease'
> mechanism to allow the kernel to notify userspace that the mapping is
> being torn down for block-map maintenance. Perhaps something similar can
> be put in place for vfio.
> 
> Note that xfs and ext4 still report:
> 
>    "DAX enabled. Warning: EXPERIMENTAL, use at your own risk"
> 
> ...at mount time, and resolving the dax-dma-vs-truncate problem is one
> of the last hurdles to remove that designation.
> 
> Cc: Alex Williamson <alex.william...@redhat.com>
> Cc: Michal Hocko <mho...@suse.com>
> Cc: Christoph Hellwig <h...@lst.de>
> Cc: k...@vger.kernel.org
> Cc: <sta...@vger.kernel.org>
> Reported-by: Haozhong Zhang <haozhong.zh...@intel.com>
> Fixes: d475c6346a38 ("dax,ext2: replace XIP read and write with DAX I/O")
> Signed-off-by: Dan Williams <dan.j.willi...@intel.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c |   18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index e30e29ae4819..45657e2b1ff7 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -338,11 +338,12 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned 
> long vaddr,
>  {
>       struct page *page[1];
>       struct vm_area_struct *vma;
> +     struct vm_area_struct *vmas[1];
>       int ret;
>  
>       if (mm == current->mm) {
> -             ret = get_user_pages_fast(vaddr, 1, !!(prot & IOMMU_WRITE),
> -                                       page);
> +             ret = get_user_pages_longterm(vaddr, 1, !!(prot & IOMMU_WRITE),
> +                                           page, vmas);

vmas is not used subsequently if this branch is taken, so can we use
NULL here?

Thanks,
Haozhong

>       } else {
>               unsigned int flags = 0;
>  
> @@ -351,7 +352,18 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned 
> long vaddr,
>  
>               down_read(&mm->mmap_sem);
>               ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
> -                                         NULL, NULL);
> +                                         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]);
> +             }
>               up_read(&mm->mmap_sem);
>       }
>  
> 
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