Patch "vfio: disable filesystem-dax page pinning" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

vfio: disable filesystem-dax page pinning

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
vfio-disable-filesystem-dax-page-pinning.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Dan Williams <[email protected]>
Date: Sun, 4 Feb 2018 10:34:02 -0800
Subject: vfio: disable filesystem-dax page pinning

From: Dan Williams <[email protected]>

commit 94db151dc89262bfa82922c44e8320cea2334667 upstream.

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.

---
 drivers/vfio/vfio_iommu_type1.c |   18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

@@ -338,11 +338,12 @@ static int vaddr_get_pfn(struct mm_struc
 {
 	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_struc
 
 		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);
 	}
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/dax-fix-vma_is_fsdax-helper.patch queue-4.14/vfio-disable-filesystem-dax-page-pinning.patch