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

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Date:   Fri, 23 Feb 2018 08:56:06 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>, linux-nvdimm@...ts.01.org,
        Michal Hocko <mhocko@...e.com>, Jan Kara <jack@...e.cz>,
        KVM list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>, Linux MM <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 <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 Fri, Feb 23, 2018 at 12:55 AM, Haozhong Zhang
<haozhong.zhang@...el.com> wrote:
> 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>
>

Thank you!