Commit 94db151d authored Feb 04, 2018 by Dan Williams

vfio: disable filesystem-dax page pinning



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.

Acked-by: Alex Williamson <alex.williamson@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: kvm@vger.kernel.org
Cc: <stable@vger.kernel.org>
Reported-by: Haozhong Zhang <haozhong.zhang@intel.com>
Tested-by: Haozhong Zhang <haozhong.zhang@intel.com>
Fixes: d475c634 ("dax,ext2: replace XIP read and write with DAX I/O")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

parent 230f5a89

drivers/vfio/vfio_iommu_type1.c

+15 −3

Original line number	Original line	Diff line number	Diff line
	@@ -338,11 +338,12 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
	{		{
	struct page *page[1];		struct page *page[1];
	struct vm_area_struct *vma;		struct vm_area_struct *vma;
			struct vm_area_struct *vmas[1];
	int ret;		int ret;

	if (mm == current->mm) {		if (mm == current->mm) {
	ret = get_user_pages_fast(vaddr, 1, !!(prot & IOMMU_WRITE),		ret = get_user_pages_longterm(vaddr, 1, !!(prot & IOMMU_WRITE),
	page);		page, vmas);
	} else {		} else {
	unsigned int flags = 0;		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);		down_read(&mm->mmap_sem);
	ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,		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);		up_read(&mm->mmap_sem);
	}		}