Re: [PATCH] xfs: dax: facilitate EXPERIMENTAL warning for dax=inode case

On Thu, Aug 26, 2021 at 06:43:44PM -0500, Eric Sandeen wrote:

On 8/26/21 5:08 PM, Bill O'Donnell wrote:

quoted

On Thu, Aug 26, 2021 at 01:16:22PM -0500, Eric Sandeen wrote:

quoted

On 8/26/21 1:09 PM, Darrick J. Wong wrote:

quoted

On Thu, Aug 26, 2021 at 12:30:12PM -0500, Bill O'Donnell wrote:

quoted

quoted

@@ -1584,7 +1586,7 @@ xfs_fs_fill_super(
   	if (xfs_has_crc(mp))
   		sb->s_flags |= SB_I_VERSION;
-	if (xfs_has_dax_always(mp)) {
+	if (xfs_has_dax_always(mp) || xfs_has_dax_inode(mp)) {

Er... can't this be done without burning another feature bit by:

	if (xfs_has_dax_always(mp) || (!xfs_has_dax_always(mp) &&
				       !xfs_has_dax_never(mp))) {
		...
		xfs_warn(mp, "DAX IS EXPERIMENTAL");
	}

changing this conditional in this way will also fail dax=inode mounts on
reflink-capable (i.e. default) filesystems, no?

Correct. My original patch tests fine, and still handles the reflink and dax
incompatibility. The new suggested logic is problematic.
-Bill

I think that both your proposed patch and Darrick's suggestion have this problem.

"mount -o dax=inode" makes your new xfs_has_dax_inode(mp) true, and in that
conditional, if the filesystem has reflink enabled, mount fails:

# mkfs.xfs -f /dev/pmem0p1
meta-data=/dev/pmem0p1           isize=512    agcount=4, agsize=4194304 blks
         =                       sectsz=4096  attr=2, projid32bit=1
         =                       crc=1        finobt=1, sparse=1, rmapbt=0
         =                       reflink=1    bigtime=0 inobtcount=0
data     =                       bsize=4096   blocks=16777216, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
log      =internal log           bsize=4096   blocks=8192, version=2
         =                       sectsz=4096  sunit=1 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

# mount -o dax=inode /dev/pmem0p1 /mnt/test
mount: wrong fs type, bad option, bad superblock on /dev/pmem0p1,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.

# dmesg | tail -n 2
[  192.691733] XFS (pmem0p1): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
[  192.700300] XFS (pmem0p1): DAX and reflink cannot be used together!

So, the "DAX enabled" is a misnomer in this case. However the incompatibility of DAX and reflink is
reflected in the next message, and indeed the mount fails. Is it now a matter of fixing
the message output so as not to indicate "DAX enabled..."?

Thanks-
Bill