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From:   Jeff Moyer <jmoyer@redhat.com>
To:     Ira Weiny <ira.weiny@intel.com>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Christoph Hellwig <hch@lst.de>,
        "Theodore Y. Ts'o" <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH v3 00/12] Enable per-file/directory DAX operations V3
References: <20200208193445.27421-1-ira.weiny@intel.com>
        <x49imke1nj0.fsf@segfault.boston.devel.redhat.com>
        <20200211201718.GF12866@iweiny-DESK2.sc.intel.com>
        <x49sgjf1t7n.fsf@segfault.boston.devel.redhat.com>
        <20200213190156.GA22854@iweiny-DESK2.sc.intel.com>
        <20200213190513.GB22854@iweiny-DESK2.sc.intel.com>
        <20200213195839.GG6870@magnolia>
        <20200213232923.GC22854@iweiny-DESK2.sc.intel.com>
        <CAPcyv4hkWoC+xCqicH1DWzmU2DcpY0at_A6HaBsrdLbZ6qzWow@mail.gmail.com>
        <20200214200607.GA18593@iweiny-DESK2.sc.intel.com>
Date:   Fri, 14 Feb 2020 16:23:19 -0500
In-Reply-To: <20200214200607.GA18593@iweiny-DESK2.sc.intel.com> (Ira Weiny's
        message of "Fri, 14 Feb 2020 12:06:07 -0800")
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Ira Weiny <ira.weiny@intel.com> writes:

> [disclaimer: the following assumes the underlying 'device' (superblock)
> supports DAX]
>
> ... which results in S_DAX == false when the file is opened without the mount
> option.  The key would be that all directories/files created under a root with
> XFS_DIFLAG2_DAX == true would inherit their flag and be XFS_DIFLAG2_DAX == true
> all the way down the tree.  Any file not wanting DAX would need to set
> XFS_DIFLAG2_DAX == false.  And setting false could be used on a directory to
> allow a user or group to not use dax on files in that sub-tree.
>
> Then without '-o dax' (XFS_MOUNT_DAX == false) all files when opened set S_DAX
> equal to XFS_DIFLAG2_DAX value.  (Directories, as of V4, never get S_DAX set.)
>
> If '-o dax' (XFS_MOUNT_DAX == true) then S_DAX is set on all files.

One more clarifying question.  Let's say I set XFS_DIFLAG2_DAX on an
inode.  I then open the file, and perform mmap/load/store/etc.  I close
the file, and I unset XFS_DIFLAG2_DAX.  Will the next open treat the
file as S_DAX or not?  My guess is the inode won't be evicted, and so
S_DAX will remain set.

The reason I ask is I've had requests from application developers to do
just this.  They want to be able to switch back and forth between dax
modes.

Thanks,
Jeff

> [1] I'm beginning to think that if I type dax one more time I'm going to go
> crazy...  :-P

dax dax dax!