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

[Dan Williams]

On Sun, Feb 4, 2018 at 7:46 PM, Haozhong Zhang <haozhong.zhang@xxxxxxxxx> wrote:
> 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@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(-)
>>
>> 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);
>
> vmas is not used subsequently if this branch is taken, so can we use
> NULL here?

I'd rather go the other way and refactor this a bit further to skip
the find_vma_intersection() below since get_user_pages() already does
that work.