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Date:   Wed, 8 Apr 2020 09:59:09 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     ira.weiny@intel.com
Cc:     linux-kernel@vger.kernel.org,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Christoph Hellwig <hch@lst.de>,
        "Theodore Y. Ts'o" <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
        Jeff Moyer <jmoyer@redhat.com>, linux-ext4@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH V6 4/8] fs/xfs: Make DAX mount option a tri-state
Message-ID: <20200407235909.GF24067@dread.disaster.area>
References: <20200407182958.568475-1-ira.weiny@intel.com>
 <20200407182958.568475-5-ira.weiny@intel.com>
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On Tue, Apr 07, 2020 at 11:29:54AM -0700, ira.weiny@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.com>
> 
> As agreed upon[1].  We make the dax mount option a tri-state.  '-o dax'
> continues to operate the same.  We add 'always', 'never', and 'iflag'
> (default).
> 
> [1] https://lore.kernel.org/lkml/20200405061945.GA94792@iweiny-DESK2.sc.intel.com/
> 
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> 
> ---
> Changes from v5:
> 	New Patch
> ---
>  fs/xfs/xfs_iops.c  |  2 +-
>  fs/xfs/xfs_mount.h | 26 +++++++++++++++++++++++++-
>  fs/xfs/xfs_super.c | 34 +++++++++++++++++++++++++++++-----
>  3 files changed, 55 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 81f2f93caec0..1ec4a36917bd 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -1248,7 +1248,7 @@ xfs_inode_supports_dax(
>  		return false;
>  
>  	/* DAX mount option or DAX iflag must be set. */
> -	if (!(mp->m_flags & XFS_MOUNT_DAX) &&
> +	if (xfs_mount_dax_mode(mp) != XFS_DAX_ALWAYS &&
>  	    !(ip->i_d.di_flags2 & XFS_DIFLAG2_DAX))
>  		return false;
>  
> diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> index 88ab09ed29e7..ce027ee06692 100644
> --- a/fs/xfs/xfs_mount.h
> +++ b/fs/xfs/xfs_mount.h
> @@ -233,7 +233,31 @@ typedef struct xfs_mount {
>  						   allocator */
>  #define XFS_MOUNT_NOATTR2	(1ULL << 25)	/* disable use of attr2 format */
>  
> -#define XFS_MOUNT_DAX		(1ULL << 62)	/* TEST ONLY! */
> +/* DAX flag is a 2 bit field representing a tri-state for dax
> + *      iflag, always, never
> + * We reserve/document the 2 bits using dax field/field2
> + */
> +#define XFS_DAX_FIELD_MASK 0x3ULL
> +#define XFS_DAX_FIELD_SHIFT 62
> +#define XFS_MOUNT_DAX_FIELD	(1ULL << 62)
> +#define XFS_MOUNT_DAX_FIELD2	(1ULL << 63)
> +
> +enum {
> +	XFS_DAX_IFLAG = 0,
> +	XFS_DAX_ALWAYS = 1,
> +	XFS_DAX_NEVER = 2,
> +};
> +
> +static inline void xfs_mount_set_dax(struct xfs_mount *mp, u32 val)
> +{
> +	mp->m_flags &= ~(XFS_DAX_FIELD_MASK << XFS_DAX_FIELD_SHIFT);
> +	mp->m_flags |= ((val & XFS_DAX_FIELD_MASK) << XFS_DAX_FIELD_SHIFT);
> +}
> +
> +static inline u32 xfs_mount_dax_mode(struct xfs_mount *mp)
> +{
> +	return (mp->m_flags >> XFS_DAX_FIELD_SHIFT) & XFS_DAX_FIELD_MASK;
> +}

This is overly complex. Just use 2 flags:

#define XFS_MOUNT_DAX_ALWAYS	(1ULL << 26)
#define XFS_MOUNT_DAX_NEVER	(1ULL << 27)

and if no mount flag is set, we use the inode flag....

> @@ -59,7 +66,7 @@ enum {
>  	Opt_filestreams, Opt_quota, Opt_noquota, Opt_usrquota, Opt_grpquota,
>  	Opt_prjquota, Opt_uquota, Opt_gquota, Opt_pquota,
>  	Opt_uqnoenforce, Opt_gqnoenforce, Opt_pqnoenforce, Opt_qnoenforce,
> -	Opt_discard, Opt_nodiscard, Opt_dax,
> +	Opt_discard, Opt_nodiscard, Opt_dax, Opt_dax_enum,
>  };
>  
>  static const struct fs_parameter_spec xfs_fs_parameters[] = {
> @@ -103,6 +110,7 @@ static const struct fs_parameter_spec xfs_fs_parameters[] = {
>  	fsparam_flag("discard",		Opt_discard),
>  	fsparam_flag("nodiscard",	Opt_nodiscard),
>  	fsparam_flag("dax",		Opt_dax),
> +	fsparam_enum("dax",		Opt_dax_enum, dax_param_enums),
>  	{}
>  };
>  
> @@ -129,7 +137,6 @@ xfs_fs_show_options(
>  		{ XFS_MOUNT_GRPID,		",grpid" },
>  		{ XFS_MOUNT_DISCARD,		",discard" },
>  		{ XFS_MOUNT_LARGEIO,		",largeio" },
> -		{ XFS_MOUNT_DAX,		",dax" },
+		{ XFS_MOUNT_DAX_ALWAYS,		",dax=always" },
+		{ XFS_MOUNT_DAX_NEVER,		",dax=never" },

>  		{ 0, NULL }
>  	};
>  	struct xfs_mount	*mp = XFS_M(root->d_sb);
> @@ -185,6 +192,20 @@ xfs_fs_show_options(
>  	if (!(mp->m_qflags & XFS_ALL_QUOTA_ACCT))
>  		seq_puts(m, ",noquota");
>  
> +	switch (xfs_mount_dax_mode(mp)) {
> +		case XFS_DAX_IFLAG:
> +			seq_puts(m, ",dax=iflag");
> +			break;
> +		case XFS_DAX_ALWAYS:
> +			seq_puts(m, ",dax=always");
> +			break;
> +		case XFS_DAX_NEVER:
> +			seq_puts(m, ",dax=never");
> +			break;
> +		default:
> +			break;
> +	}

	if (!(mp->m_flags & (XFS_MOUNT_DAX_ALWAYS | XFS_MOUNT_DAX_NEVER))
		seq_puts(m, ",dax=iflag");

> +
>  	return 0;
>  }
>  
> @@ -1244,7 +1265,10 @@ xfs_fc_parse_param(
>  		return 0;
>  #ifdef CONFIG_FS_DAX
>  	case Opt_dax:
> -		mp->m_flags |= XFS_MOUNT_DAX;
> +		xfs_mount_set_dax(mp, XFS_DAX_ALWAYS);
> +		return 0;
> +	case Opt_dax_enum:
> +		xfs_mount_set_dax(mp, result.uint_32);
>  		return 0;
>  #endif
>  	default:
> @@ -1437,7 +1461,7 @@ xfs_fc_fill_super(
>  	if (XFS_SB_VERSION_NUM(&mp->m_sb) == XFS_SB_VERSION_5)
>  		sb->s_flags |= SB_I_VERSION;
>  
> -	if (mp->m_flags & XFS_MOUNT_DAX) {
> +	if (xfs_mount_dax_mode(mp) == XFS_DAX_ALWAYS) {

	if (mp->m_flags & XFS_MOUNT_DAX_ALWAYS) {

>  		bool rtdev_is_dax = false, datadev_is_dax;
>  
>  		xfs_warn(mp,
> @@ -1451,7 +1475,7 @@ xfs_fc_fill_super(
>  		if (!rtdev_is_dax && !datadev_is_dax) {
>  			xfs_alert(mp,
>  			"DAX unsupported by block device. Turning off DAX.");
> -			mp->m_flags &= ~XFS_MOUNT_DAX;
> +			xfs_mount_set_dax(mp, XFS_DAX_NEVER);
>  		}
>  		if (xfs_sb_version_hasreflink(&mp->m_sb)) {
>  			xfs_alert(mp,
> -- 
> 2.25.1
> 
> 

-- 
Dave Chinner
david@fromorbit.com