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);@@ -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]);}