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

[Dan Williams]

On Thu, Sep 15, 2016 at 4:07 PM, Dave Chinner <david@xxxxxxxxxxxxx> 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?
>
> No, that's an admin flag, not a runtime hint for applications. Just
> because that flag is set on an inode, it does not mean that DAX is
> actually in use - it will be ignored if the backing dev is not dax
> capable.
>

What's the point of an admin flag if an admin can't do cat /proc/<pid
of interest>/smaps, or some other mechanism, to validate that the
setting the admin cares about is in effect?