2:54 p.m.
On Sun, Feb 4, 2018 at 7:46 PM, Haozhong Zhang <haozhong.zhang(a)intel.com> wrote:
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.williamson(a)redhat.com>
> Cc: Michal Hocko <mhocko(a)suse.com>
> Cc: Christoph Hellwig <hch(a)lst.de>
> Cc: kvm(a)vger.kernel.org
> Cc: <stable(a)vger.kernel.org>
> Reported-by: Haozhong Zhang <haozhong.zhang(a)intel.com>
> Fixes: d475c6346a38 ("dax,ext2: replace XIP read and write with DAX I/O")
> Signed-off-by: Dan Williams <dan.j.williams(a)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?
I'd rather go the other way and refactor this a bit further to skip
the find_vma_intersection() below since get_user_pages() already does
that work.