Re: [PATCH V4 6/8] fs/ext4: Make DAX mount option a tri-state

[Xiao Yang]

On 2020/5/28 17:41, Jan Kara wrote:
> On Thu 28-05-20 16:56:51, Xiao Yang wrote:
> > On 2020/5/28 7:50, Ira Weiny wrote:
> > > On Wed, May 27, 2020 at 01:54:54PM +0800, Xiao Yang wrote:
> > > > On 2020/5/22 3:13, ira.weiny@xxxxxxxxx wrote:
> > > > > From: Ira Weiny<ira.weiny@xxxxxxxxx>
> > > > >
> > > > > We add 'always', 'never', and 'inode' (default).  '-o dax' continues to
> > > > > operate the same which is equivalent to 'always'.  This new
> > > > > functionality is limited to ext4 only.
> > > > >
> > > > > Specifically we introduce a 2nd DAX mount flag EXT4_MOUNT2_DAX_NEVER and set
> > > > > it and EXT4_MOUNT_DAX_ALWAYS appropriately for the mode.
> > > > >
> > > > > We also force EXT4_MOUNT2_DAX_NEVER if !CONFIG_FS_DAX.
> > > > >
> > > > > Finally, EXT4_MOUNT2_DAX_INODE is used solely to detect if the user
> > > > > specified that option for printing.
> > > > Hi Ira,
> > > >
> > > > I have two questions when reviewing this patch:
> > > > 1) After doing mount with the same dax=inode option, ext4/xfs shows
> > > > differnt output(i.e. xfs doesn't print 'dax=inode'):
> > > > ---------------------------------------------------
> > > > # mount -o dax=inode /dev/pmem0 /mnt/xfstests/test/
> > > > # mount | grep pmem0
> > > > /dev/pmem0 on /mnt/xfstests/test type ext4 (rw,relatime,seclabel,dax=inode)
> > > >
> > > > # mount -odax=inode /dev/pmem1 /mnt/xfstests/scratch/
> > > > # mount | grep pmem1
> > > > /dev/pmem1 on /mnt/xfstests/scratch type xfs
> > > > (rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota)
> > > > ----------------------------------------------------
> > > > Is this expected output? why don't unify the output?
> > >
> > > Correct. dax=inode is the default.  xfs treats that default the same whether
> > > you specify it on the command line or not.
> > >
> > > For ext4 Jan specifically asked that if the user specified dax=inode on the
> > > command line that it be printed on the mount options.  If you don't specify
> > > anything then dax=inode is in effect but ext4 will not print anything.
> > >
> > > I had the behavior the same as XFS originally but Jan wanted it this way.  The
> > > XFS behavior is IMO better and is what the new mount infrastructure gives by
> > > default.
> >
> > Could we unify the output?  It is strange for me to use differnt output on
> > ext4 and xfs.
>
> If we'd unify the output with XFS, it would be inconsistent with all the
> other ext4 mount options. So I disagree with that. I agree it is not ideal
> to have different behavior between xfs and ext4 but such is the historical
> behavior. If we want to change that, we need to change the handling for all
> the ext4 mount options. I'm open for that discussion but it is a problem
> unrelated to this patch set.
Hi Jan,

Thanks for your quick feedback.
Of course, this doubt should not block the patch set.

Best Regards,
Xiao Yang
> 								Honza