Re: [PATCH 3/3] vfio: disable filesystem-dax page pinning

[Dan Williams]

On Mon, Feb 5, 2018 at 1:44 PM, Alex Williamson
<alex.williamson@xxxxxxxxxx> wrote:
> On Sun, 04 Feb 2018 15:05:30 -0800
> Dan Williams <dan.j.williams@xxxxxxxxx> 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@xxxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxx>
>> Cc: kvm@xxxxxxxxxxxxxxx
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Reported-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
>> Fixes: d475c6346a38 ("dax,ext2: replace XIP read and write with DAX I/O")
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>>  drivers/vfio/vfio_iommu_type1.c |   18 +++++++++++++++---
>>  1 file changed, 15 insertions(+), 3 deletions(-)
>
> This isn't without some expense, a vfio mapping and un-mapping unit
> test incurs ~1.5% increase in system time losing access to gup_fast().
> Also, I think tce_iommu_use_page() is going to have the same problem, it
> provides the same sort of functionality for a different vfio IOMMU
> backend.  Please take this through your tree and I'll add a todo list
> item to see how we might improve this.
>
> Acked-by: Alex Williamson <alex.williamson@xxxxxxxxxx>

Thanks Alex.