Commit 94db151d authored by Dan Williams's avatar 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: default avatarAlex Williamson <alex.williamson@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: kvm@vger.kernel.org
Cc: <stable@vger.kernel.org>
Reported-by: default avatarHaozhong Zhang <haozhong.zhang@intel.com>
Tested-by: default avatarHaozhong Zhang <haozhong.zhang@intel.com>
Fixes: d475c634 ("dax,ext2: replace XIP read and write with DAX I/O")
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
parent 230f5a89
...@@ -338,11 +338,12 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr, ...@@ -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, ...@@ -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);
} }
......
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