Re: [PATCH 2/2] mkfs: allow setting dax flag on root directory

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Aug 11, 2020 at 02:39:01PM -0500, Eric Sandeen wrote:
> On 8/11/20 9:42 AM, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Teach mkfs to set the DAX flag on the root directory so that all new
> > files can be created in dax mode.  This is a complement to removing the
> > mount option.
> 
> So, a new -d option, "-d dax"
> 
> This is ~analogous to cowextsize, rtinherit, projinherit, and extszinherit
> so there is certainly precedence for this.  (where only rtinherit is a boolean
> like this, but they are all inheritable behaviors)
> 
> (I wonder if "daxinherit" would be more consistent, but won't bikeshed
> that (much))

/me is indifferent either way.  But I guess some day we might want to
have a dax= flag to indicate something like "set the data device
geometry to optimize for DAX?

Nah, I think if we were ever going to do that, we'd have something more
like:

	-d usage=dax
	-d usage=ssd
	-d usage=floopy

Meh.  I'll change it to daxinherit, since that /is/ what it does.

> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  man/man8/mkfs.xfs.8 |   11 +++++++++++
> >  mkfs/xfs_mkfs.c     |   14 ++++++++++++++
> >  2 files changed, 25 insertions(+)
> > 
> > 
> > diff --git a/man/man8/mkfs.xfs.8 b/man/man8/mkfs.xfs.8
> > index 9d762a43011a..4b4fdd86b2f4 100644
> > --- a/man/man8/mkfs.xfs.8
> > +++ b/man/man8/mkfs.xfs.8
> > @@ -394,6 +394,17 @@ All inodes created by
> >  will have this extent size hint applied.
> >  The value must be provided in units of filesystem blocks.
> >  Directories will pass on this hint to newly created children.
> > +.TP
> > +.BI dax= value
> > +All inodes created by
> > +.B mkfs.xfs
> > +will have the DAX flag set.
> > +This means that directories will pass the flag on to newly created files
> 
> let's call this "children" to match the other similar options?
> 
> (because technically it is passed on not only to regular files, right?)

Directories and regular files, though not to other special files.
Maybe we should fix that.

> > +and files will use the DAX IO paths when possible.
> > +This value is either 1 to enable the use or 0 to disable.
> > +By default,
> > +.B mkfs.xfs
> > +will not enable DAX mode.
> >  .RE
> >  .TP
> >  .B \-f
> > diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
> > index 2e6cd280e388..33507f6ea21c 100644
> > --- a/mkfs/xfs_mkfs.c
> > +++ b/mkfs/xfs_mkfs.c
> > @@ -60,6 +60,7 @@ enum {
> >  	D_PROJINHERIT,
> >  	D_EXTSZINHERIT,
> >  	D_COWEXTSIZE,
> > +	D_DAX,
> >  	D_MAX_OPTS,
> >  };
> >  
> > @@ -254,6 +255,7 @@ static struct opt_params dopts = {
> >  		[D_PROJINHERIT] = "projinherit",
> >  		[D_EXTSZINHERIT] = "extszinherit",
> >  		[D_COWEXTSIZE] = "cowextsize",
> > +		[D_DAX] = "dax",
> >  	},
> >  	.subopt_params = {
> >  		{ .index = D_AGCOUNT,
> > @@ -369,6 +371,12 @@ static struct opt_params dopts = {
> >  		  .maxval = UINT_MAX,
> >  		  .defaultval = SUBOPT_NEEDS_VAL,
> >  		},
> > +		{ .index = D_DAX,
> > +		  .conflicts = { { NULL, LAST_CONFLICT } },
> 
> er....  should we conflict with reflink ....  ?
> 
> > +		  .minval = 0,
> > +		  .maxval = 1,
> > +		  .defaultval = 1,
> 
> Hm, interesting that this is a little different from rtinherit:
> 
>                 { .index = D_RTINHERIT,
>                   .conflicts = { { NULL, LAST_CONFLICT } },
>                   .minval = 1,
>                   .maxval = 1,
>                   .defaultval = 1,
>                 },
> 
> I think this means that:
> 
> -d rtinherit
> -d rtinherit=1
> 
> are valid, but
> 
> -d rtinherit=0 is not, but
> 
> -d dax
> -d dax=1
> -d dax=0
> 
> are all valid?

TBH, I find it a little odd that you *can't* say "-d rtinherit=0" from a
completeness perspective, but...

> While the latter makes a bit more sense, I wonder if we should stay
> consistent w/ the rtinherit semantics.  Or do you envision some sort
> of automatic enabling of this based on device typethat we'd need to
> override in the future?

...the goal is to set this automatically once distros start shipping a
libblkid that has blkid_topology_get_dax().  At that point we'll
probably want a way to force it off.

Unless we want the ability to specify -ddax=0 the magic seekrit hook to
discover if (future) mkfs actually supports dax autodetection?  Hmm,
that alone sounds like sufficient justification.  Ok.

--D

> 
> > +		},
> >  	},
> >  };
> >  
> > @@ -1434,6 +1442,12 @@ data_opts_parser(
> >  		cli->fsx.fsx_cowextsize = getnum(value, opts, subopt);
> >  		cli->fsx.fsx_xflags |= FS_XFLAG_COWEXTSIZE;
> >  		break;
> > +	case D_DAX:
> > +		if (getnum(value, opts, subopt))
> > +			cli->fsx.fsx_xflags |= FS_XFLAG_DAX;
> > +		else
> > +			cli->fsx.fsx_xflags &= ~FS_XFLAG_DAX;
> > +		break;
> >  	default:
> >  		return -EINVAL;
> >  	}
> >