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SubjectRe: [PATCH 3/3] vfio: disable filesystem-dax page pinning
Hi Dan,

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@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(-)
>
> 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);
> } else {
> 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);
> 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);
> }
>
>

Besides this patch series, are there other patches needed to make
vma_is_fsdax() to work with device-dax?

I applied this patch series on the libvdimm-for-next branch of nvdimm
tree (ee95f4059a83), and found this patch series also failed
device-dax mapping with vfio. It can be reproduced by following steps:

1. Attach PCI device at BDF 0000:03:10.2 to vfio-pci.
# modprobe vfio-pci
# lspci -n -s 0000:03:10.2
03:10.2 0200: 8086:1515 (rev 01)
# echo 0000:03:10.2 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
# echo 8086:1515 > /sys/bus/pci/drivers/vfio-pci/new_id

2. Use RAM to emulate NVDIMM and create a device-dax device /dev/dax0.0
# cat /proc/iomem
...
100000000-2ffffffff : Persistent Memory (legacy)
100000000-2ffffffff : namespace0.0
...

# ndctl create-namespace -f -e namespace0.0 -m dax
{
"dev":"namespace0.0",
"mode":"dax",
"size":8453619712,
"uuid":"e1db00bc-f830-4f1b-ac18-091ae7df4f93",
"daxdevs":[
{
"chardev":"dax0.0",
"size":8453619712
}
]
}

3. Create a VM with assigned PCI device in step 1 and the device-dax
device in step 2.
# qemu-system-x86_64 -machine pc,accel=kvm,nvdimm=on -smp host \
-m 4G,slots=32,maxmem=128G \
-drive file=VM_DISK_IMG.img,format=raw,if=virtio \
-object memory-backend-file,id=nv_be1,share=on,mem-path=/dev/dax0.0,size=4G,align=2M \
-device nvdimm,id=nv1,memdev=nv_be1 \
-device ioh3420,id=root.0,slot=4 \
-device vfio-pci,sysfsdev=/sys/bus/pci/devices/0000:03:10.2,id=nic1,bus=pci.0,addr=0x6

It then fails with the following QEMU error messages:
qemu-system-x86_64: -device vfio-pci,sysfsdev=/sys/bus/pci/devices/0000:03:10.2,id=nic1,bus=pci.0,addr=0x6: VFIO_MAP_DMA: -95
qemu-system-x86_64: -device vfio-pci,sysfsdev=/sys/bus/pci/devices/0000:03:10.2,id=nic1,bus=pci.0,addr=0x6: vfio_dma_map(0x5643804a92c0, 0x140000000, 0xffe00000, 0x7f2ed5200000) = -95 (Operation not supported)
qemu-system-x86_64: -device vfio-pci,sysfsdev=/sys/bus/pci/devices/0000:03:10.2,id=nic1,bus=pci.0,addr=0x6: vfio error: 0000:03:10.2: failed to setup container for group 52: memory listener initialization failed for container: Operation not supported

I added the following debug messages after the
get_user_pages_longterm() call in this patch,
if (vmas[0] && vma_is_dax(vmas[0]))
printk(KERN_DEBUG "%s: longterm failed for pfn 0x%lx, ret %d\n",
__func__, page_to_pfn(page[0]), ret);
and shows get_user_pages_longterm() returns -EOPNOTSUPP on the
first device-dax page mapping.



Haozhong

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