Re: [PATCH 2/5] fs/xfs: Isolate the physical DAX flag from effective

[Dave Chinner]

On Sun, Oct 20, 2019 at 08:59:32AM -0700, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> xfs_ioctl_setattr_dax_invalidate() currently checks if the DAX flag is
> changing as a quick check.
> 
> But the implementation mixes the physical (XFS_DIFLAG2_DAX) and
> effective (S_DAX) DAX flags.

More nuanced than that.

The idea was that if the mount option was set, clearing the
per-inode flag would override the mount option. i.e. the mount
option sets the S_DAX flag at inode instantiation, so using
FSSETXATTR to ensure the FS_XFLAG_DAX is not set would override the
mount option setting, giving applications a way of guranteeing they
aren't using DAX to access the data.

So if the mount option is going to live on, I suspect that we want
to keep this code as it stands.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx