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

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Date:   Tue, 2 Jun 2020 09:16:33 +0800
From:   Xiao Yang <yangx.jy@...fujitsu.com>
To:     Jan Kara <jack@...e.cz>
CC:     Ira Weiny <ira.weiny@...el.com>, <linux-ext4@...r.kernel.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Eric Biggers <ebiggers@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>, Jeff Moyer <jmoyer@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V4 6/8] fs/ext4: Make DAX mount option a tri-state

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@...el.com wrote:
>>>>> From: Ira Weiny<ira.weiny@...el.com>
>>>>>
>>>>> 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