[V6,4/8] fs/xfs: Make DAX mount option a tri-state
diff mbox series

Message ID 20200407182958.568475-5-ira.weiny@intel.com
State Superseded
Headers show
Series
  • Enable per-file/per-directory DAX operations V6
Related show

Commit Message

Ira Weiny April 7, 2020, 6:29 p.m. UTC
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(-)

Comments

Dave Chinner April 7, 2020, 11:59 p.m. UTC | #1
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
> 
>
Ira Weiny April 8, 2020, 12:09 a.m. UTC | #2
On Wed, Apr 08, 2020 at 09:59:09AM +1000, Dave Chinner wrote:
> 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:

LOL...  I was afraid someone would say that.  At first I used 2 flags with
fsparam_string, but then I realized Darrick suggested fsparam_enum:

	"It would be handy if we could have a single fsparam_enum for figuring
	out the dax mount options."

	-- https://lore.kernel.org/lkml/20200403183746.GQ80283@magnolia/

So I changed it...

Darrick?

Ira

> 
> #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
Dave Chinner April 8, 2020, 12:48 a.m. UTC | #3
On Tue, Apr 07, 2020 at 05:09:04PM -0700, Ira Weiny wrote:
> On Wed, Apr 08, 2020 at 09:59:09AM +1000, Dave Chinner wrote:
> > 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:
> 
> LOL...  I was afraid someone would say that.  At first I used 2 flags with
> fsparam_string, but then I realized Darrick suggested fsparam_enum:

Well, I'm not concerned about the fsparam enum, it's just that
encoding an integer into a flags bit field is just ... messy.
Especially when encoding that state can be done with just 2 flags.

If you want to keep the xfs_mount_dax_mode() wrapper, then:

static inline uint32_t xfs_mount_dax_mode(struct xfs_mount *mp)
{
	if (mp->m_flags & XFS_MOUNT_DAX_NEVER)
		return XFS_DAX_NEVER;
	if (mp->m_flags & XFS_MOUNT_DAX_ALWAYS)
		return XFS_DAX_ALWAYS;
	return XFS_DAX_IFLAG;
}

but once it's encoded in flags like this, the wrapper really isn't
necessary...

Also, while I think of it, can we change "iflag" to "inode". i.e.
the DAX state is held on the inode. Saying it comes from an "inode
flag" encodes the implementation into the user interface. i.e. it
could well be held in an xattr on the inode on another filesystem,
so we shouldn't mention "flag" in the user API....

Cheers,

Dave.
Ira Weiny April 9, 2020, 3:03 p.m. UTC | #4
On Wed, Apr 08, 2020 at 10:48:01AM +1000, Dave Chinner wrote:
> On Tue, Apr 07, 2020 at 05:09:04PM -0700, Ira Weiny wrote:
> > On Wed, Apr 08, 2020 at 09:59:09AM +1000, Dave Chinner wrote:
> > > 
> > > This is overly complex. Just use 2 flags:
> > 
> > LOL...  I was afraid someone would say that.  At first I used 2 flags with
> > fsparam_string, but then I realized Darrick suggested fsparam_enum:
> 
> Well, I'm not concerned about the fsparam enum, it's just that
> encoding an integer into a flags bit field is just ... messy.
> Especially when encoding that state can be done with just 2 flags.
> 
> If you want to keep the xfs_mount_dax_mode() wrapper, then:
> 
> static inline uint32_t xfs_mount_dax_mode(struct xfs_mount *mp)
> {
> 	if (mp->m_flags & XFS_MOUNT_DAX_NEVER)
> 		return XFS_DAX_NEVER;
> 	if (mp->m_flags & XFS_MOUNT_DAX_ALWAYS)
> 		return XFS_DAX_ALWAYS;
> 	return XFS_DAX_IFLAG;
> }
> 
> but once it's encoded in flags like this, the wrapper really isn't
> necessary...

Done for v7

> 
> Also, while I think of it, can we change "iflag" to "inode". i.e.
> the DAX state is held on the inode. Saying it comes from an "inode
> flag" encodes the implementation into the user interface. i.e. it
> could well be held in an xattr on the inode on another filesystem,
> so we shouldn't mention "flag" in the user API....

Sure "inode" is fine with me.  Easy change, done for v7

Ira

Patch
diff mbox series

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;
+}
 
 /*
  * Max and min values for mount-option defined I/O
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 2094386af8ac..d2fd465eeed5 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -47,6 +47,13 @@  static struct kset *xfs_kset;		/* top-level xfs sysfs dir */
 static struct xfs_kobj xfs_dbg_kobj;	/* global debug sysfs attrs */
 #endif
 
+static const struct constant_table dax_param_enums[] = {
+	{"iflag",	XFS_DAX_IFLAG },
+	{"always",	XFS_DAX_ALWAYS },
+	{"never",	XFS_DAX_NEVER },
+	{}
+};
+
 /*
  * Table driven mount option parser.
  */
@@ -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" },
 		{ 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;
+	}
+
 	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) {
 		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,