Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs

[Dan Williams <dan.j.williams@xxxxxxxxx>]

On Thu, Sep 15, 2016 at 10:09 AM, Darrick J. Wong
<darrick.wong@xxxxxxxxxx> wrote:
> On Thu, Sep 15, 2016 at 10:01:03AM -0700, Dan Williams wrote:
>> On Thu, Sep 15, 2016 at 1:26 AM, Christoph Hellwig <hch@xxxxxx> wrote:
>> > On Wed, Sep 14, 2016 at 11:54:38PM -0700, Dan Williams wrote:
>> >> The DAX property, page cache bypass, of a VMA is only detectable via the
>> >> vma_is_dax() helper to check the S_DAX inode flag.  However, this is
>> >> only available internal to the kernel and is a property that userspace
>> >> applications would like to interrogate.
>> >
>> > They have absolutely no business knowing such an implementation detail.
>>
>> Hasn't that train already left the station with FS_XFLAG_DAX?
>
> Seeing as FS_IOC_FSGETXATTR is a "generic" ioctl now, why not just
> implement it for all the DAX fses and block devices?  Aside from xflags,
> the other fields are probably all zero for non-xfs (aside from project
> quota id I guess).
>
> (Yeah, sort of awkward, I know...)

It would solve the problem at hand, I'll take a look.