[PATCH 1/7] xfs: always use DAX if mount option is used

[Ross Zwisler <ross.zwisler@linux.intel.com>]

Before support for the per-inode DAX flag was disabled the XFS the code had
an issue where the user couldn't reliably tell whether or not DAX was being
used to service page faults and I/O when the DAX mount option was used.  In
this case each inode within the mounted filesystem started with S_DAX set
due to the mount option, but it could be cleared if someone touched the
individual inode flag.

For example (v4.13 and before):

  # mount | grep dax
  /dev/pmem0 on /mnt type xfs
  (rw,relatime,seclabel,attr2,dax,inode64,sunit=4096,swidth=4096,noquota)

  # touch /mnt/a /mnt/b   # both files currently use DAX

  # xfs_io -c "lsattr" /mnt/*  # neither has the DAX inode option set
  ----------e----- /mnt/a
  ----------e----- /mnt/b

  # xfs_io -c "chattr -x" /mnt/a  # this clears S_DAX for /mnt/a

  # xfs_io -c "lsattr" /mnt/*
  ----------e----- /mnt/a
  ----------e----- /mnt/b

We end up with both /mnt/a and /mnt/b looking identical from the point of
view of the mount option and from lsattr, but one is using DAX and the
other is not.

Fix this by always doing DAX I/O when either the mount option is set or
when the DAX inode flag is set.  This means that DAX will always be used
for all inodes on a filesystem mounted with -o dax, making the usage
reliable and detectable.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 fs/xfs/xfs_ioctl.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 5049e8a..26faeb9 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1013,7 +1013,7 @@ xfs_diflags_to_linux(
 	else
 		inode->i_flags &= ~S_NOATIME;
 #if 0	/* disabled until the flag switching races are sorted out */
-	if (xflags & FS_XFLAG_DAX)
+	if ((xflags & FS_XFLAG_DAX) || (ip->i_mount->m_flags & XFS_MOUNT_DAX))
 		inode->i_flags |= S_DAX;
 	else
 		inode->i_flags &= ~S_DAX;
@@ -1104,7 +1104,14 @@ xfs_ioctl_setattr_dax_invalidate(
 			return -EINVAL;
 	}
 
-	/* If the DAX state is not changing, we have nothing to do here. */
+	/*
+	 * If the DAX state is not changing, we have nothing to do here.  If
+	 * the DAX mount option was used we will update the DAX inode flag as
+	 * the user requested but we will continue to use DAX for I/O and page
+	 * faults regardless of how the inode flag is set.
+	 */
+	if (ip->i_mount->m_flags & XFS_MOUNT_DAX)
+		return 0;
 	if ((fa->fsx_xflags & FS_XFLAG_DAX) && IS_DAX(inode))
 		return 0;
 	if (!(fa->fsx_xflags & FS_XFLAG_DAX) && !IS_DAX(inode))
-- 
2.9.5