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Date:   Thu, 13 Feb 2020 11:58:39 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Jeff Moyer <jmoyer@...hat.com>, linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 00/12] Enable per-file/directory DAX operations V3

On Thu, Feb 13, 2020 at 11:05:13AM -0800, Ira Weiny wrote:
> On Thu, Feb 13, 2020 at 11:01:57AM -0800, 'Ira Weiny' wrote:
> > On Wed, Feb 12, 2020 at 02:49:48PM -0500, Jeff Moyer wrote:
> > > Ira Weiny <ira.weiny@...el.com> writes:
> > > 
>  
> [snip]
> 
> > > Given that we document the dax mount
> > > option as "the way to get dax," it may be a good idea to allow for a
> > > user to selectively disable dax, even when -o dax is specified.  Is that
> > > possible?
> > 
> > Not with this patch set.  And I'm not sure how that would work.  The idea was
> > that -o dax was simply an override for users who were used to having their
> > entire FS be dax.  We wanted to depreciate the use of "-o dax" in general.  The
> > individual settings are saved so I don't think it makes sense to ignore the -o
> > dax in favor of those settings.  Basically that would IMO make the -o dax
> > useless.
> 
> Oh and I forgot to mention that setting 'dax' on the root of the FS basically
> provides '-o dax' functionality by default with the ability to "turn it off"
> for files.

Please don't further confuse FS_XFLAG_DAX and S_DAX.  They are two
separate flags with two separate behaviors:

FS_XFLAG_DAX is a filesystem inode metadata flag.

Setting FS_XFLAG_DAX on a directory causes all files and directories
created within that directory to inherit FS_XFLAG_DAX.

Mounting with -o dax causes all files and directories created to have
FS_XFLAG_DAX set regardless of the parent's status.

The FS_XFLAG_DAX can be get and set via the fs[g]etxattr ioctl.

-------

S_DAX is the flag that controls the IO path in the kernel for a given
inode.

Loading a file inode into the kernel (via _iget) with FS_XFLAG_DAX set
or creating a file inode that inherits FS_XFLAG_DAX causes the incore
inode to have the S_DAX flag set if the storage device supports it.

Files with S_DAX set use the dax IO paths through the kernel.

The S_DAX flag can be queried via statx.

--D

> Ira
> 
> > 
> > Ira
> > 

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