linux-kernel - Re: [PATCH 2/5] fs/xfs: Isolate the physical DAX flag from effective

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Message-ID: <20191021002621.GC8015@dread.disaster.area>
Date:   Mon, 21 Oct 2019 11:26:21 +1100
From:   Dave Chinner <david@...morbit.com>
To:     ira.weiny@...el.com
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.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 2/5] fs/xfs: Isolate the physical DAX flag from effective

On Sun, Oct 20, 2019 at 08:59:32AM -0700, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> xfs_ioctl_setattr_dax_invalidate() currently checks if the DAX flag is
> changing as a quick check.
> 
> But the implementation mixes the physical (XFS_DIFLAG2_DAX) and
> effective (S_DAX) DAX flags.

More nuanced than that.

The idea was that if the mount option was set, clearing the
per-inode flag would override the mount option. i.e. the mount
option sets the S_DAX flag at inode instantiation, so using
FSSETXATTR to ensure the FS_XFLAG_DAX is not set would override the
mount option setting, giving applications a way of guranteeing they
aren't using DAX to access the data.

So if the mount option is going to live on, I suspect that we want
to keep this code as it stands.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com