Re: [PATCH 3/3] vfio: disable filesystem-dax page pinning

[Alex Williamson <alex.williamson@redhat.com>]

On Sun, 04 Feb 2018 15:05:30 -0800
Dan Williams <dan.j.williams@intel.com> 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.williamson@redhat.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: kvm@vger.kernel.org
> Cc: <stable@vger.kernel.org>
> Reported-by: Haozhong Zhang <haozhong.zhang@intel.com>
> Fixes: d475c6346a38 ("dax,ext2: replace XIP read and write with DAX I/O")
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c |   18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)

This isn't without some expense, a vfio mapping and un-mapping unit
test incurs ~1.5% increase in system time losing access to gup_fast().
Also, I think tce_iommu_use_page() is going to have the same problem, it
provides the same sort of functionality for a different vfio IOMMU
backend.  Please take this through your tree and I'll add a todo list
item to see how we might improve this.

Acked-by: Alex Williamson <alex.williamson@redhat.com>

Thanks,
Alex

> 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);
>  	} 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);
>  	}
>  
>