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

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Message-ID: <20180223085547.3kkbo5lbt3orkqqn@hz-desktop>
Date:   Fri, 23 Feb 2018 16:55:47 +0800
From:   Haozhong Zhang <haozhong.zhang@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm@...ts.01.org, Michal Hocko <mhocko@...e.com>,
        Jan Kara <jack@...e.cz>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux-mm@...ck.org, Alex Williamson <alex.williamson@...hat.com>,
        Gerd Rausch <gerd.rausch@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org,
        kbuild test robot <fengguang.wu@...el.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 0/5] vfio, dax: prevent long term filesystem-dax pins
 and other fixes

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@...el.com>