Re: [PATCH v2 0/5] vfio, dax: prevent long term filesystem-dax pins and other fixes

[Haozhong Zhang]

On 02/22/18 23:17 -0800, Dan Williams wrote:
> Changes since v1 [1]:
> 
> * Fix the detection of device-dax file instances in vma_is_fsdax().
>   (Haozhong, Gerd)
> 
> * Fix compile breakage in the FS_DAX=n and DEV_DAX=y case. (0day robot)
> 
> [1]: https://lists.01.org/pipermail/linux-nvdimm/2018-February/014046.html
> 
> ---
> 
> The vfio interface, like RDMA, wants to setup long term (indefinite)
> pins of the pages backing an address range so that a guest or userspace
> driver can perform DMA to the with physical address. Given that this
> pinning may lead to filesystem operations deadlocking in the
> filesystem-dax case, the pinning request needs to be rejected.
> 
> The longer term fix for vfio, RDMA, and any other long term pin user, is
> to provide a 'pin with lease' mechanism. Similar to the leases that are
> hold for pNFS RDMA layouts, this userspace lease gives the kernel a way
> to notify userspace that the block layout of the file is changing and
> the kernel is revoking access to pinned pages.
> 
> ---
> 
> Dan Williams (5):
>       dax: fix vma_is_fsdax() helper
>       dax: fix dax_mapping() definition in the FS_DAX=n + DEV_DAX=y case
>       dax: fix S_DAX definition
>       dax: short circuit vma_is_fsdax() in the CONFIG_FS_DAX=n case
>       vfio: disable filesystem-dax page pinning
> 
> 
>  drivers/vfio/vfio_iommu_type1.c |   18 +++++++++++++++---
>  include/linux/dax.h             |    9 ++++++---
>  include/linux/fs.h              |    6 ++++--
>  3 files changed, 25 insertions(+), 8 deletions(-)

Tested on QEMU with fs-dax and device-dax as vNVDIMM backends
respectively with vfio passthrough. The fs-dax case fails QEMU as
expected, and the device-dax case works normally now.

Tested-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>