On Tue, Sep 26, 2017 at 09:38:12AM +1000, Dave Chinner wrote: > On Mon, Sep 25, 2017 at 05:13:58PM -0600, Ross Zwisler wrote: > > 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 > > That's really a bug in the lsattr code, yes? If we've cleared the > S_DAX flag for the inode, then why is it being reported in lsattr? > Or if we failed to clear the S_DAX flag in the 'chattr -x' call, > then isn't that the bug that needs fixing? No, I think lsattr/chattr are working correctly. In both the examples above the DAX inode flag (which is represeted by an 'x') is never set. S_DAX is the in-memory inode flag (not the on-media inode flag) which is not manipulated directly by lsattr/chattr, but instead reflects whether the inode is actually using DAX or not. Manipulating and displaying the on-media inode flag works as expected with lsattr/chattr: # xfs_io -c "lsattr" ./a ---------------- ./a # xfs_io -c "chattr +x" ./a # xfs_io -c "lsattr" ./a --------------x- ./a - Ross
