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SubjectRe: [PATCH 3/3] vfio: disable filesystem-dax page pinning
On Mon, Feb 5, 2018 at 11:53 PM, Haozhong Zhang
<haozhong.zhang@intel.com> wrote:
> 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.

Thanks for that thorough debug, I'll take a look today.

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