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

[Darrick J. Wong]

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...)

--D

> The other problem with hiding the DAX property is that it turns out to
> not be a transparent acceleration feature.  See xfs/086 xfs/088
> xfs/089 xfs/091 which fail with DAX and, as far as I understand, it is
> due to the fact that DAX disallows delayed allocation behavior.
> 
> If behavior changes I think we should indicate that to userspace and
> VM_DAX is certainly more useful to userspace than some of the other vm
> internals we already export in those flags.
> 
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