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Date:   Tue, 11 Feb 2020 16:47:48 +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 v3 03/12] fs/xfs: Separate functionality of
 xfs_inode_supports_dax()

On Sat, Feb 08, 2020 at 11:34:36AM -0800, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> xfs_inode_supports_dax() should reflect if the inode can support DAX not
> that it is enabled for DAX.  Leave that to other helper functions.
> 
> Change the caller of xfs_inode_supports_dax() to call
> xfs_inode_use_dax() which reflects new logic to override the effective
> DAX flag with either the mount option or the physical DAX flag.
> 
> To make the logic clear create 2 helper functions for the mount and
> physical flag.
> 
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> ---
>  fs/xfs/xfs_iops.c | 32 ++++++++++++++++++++++++++------
>  1 file changed, 26 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 81f2f93caec0..a7db50d923d4 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -1236,6 +1236,15 @@ static const struct inode_operations xfs_inline_symlink_inode_operations = {
>  	.update_time		= xfs_vn_update_time,
>  };
>  
> +static bool
> +xfs_inode_mount_is_dax(
> +	struct xfs_inode *ip)
> +{
> +	struct xfs_mount	*mp = ip->i_mount;
> +
> +	return (mp->m_flags & XFS_MOUNT_DAX) == XFS_MOUNT_DAX;
> +}
> +
>  /* Figure out if this file actually supports DAX. */
>  static bool
>  xfs_inode_supports_dax(
> @@ -1247,11 +1256,6 @@ xfs_inode_supports_dax(
>  	if (!S_ISREG(VFS_I(ip)->i_mode) || xfs_is_reflink_inode(ip))
>  		return false;
>  
> -	/* DAX mount option or DAX iflag must be set. */
> -	if (!(mp->m_flags & XFS_MOUNT_DAX) &&
> -	    !(ip->i_d.di_flags2 & XFS_DIFLAG2_DAX))
> -		return false;
> -
>  	/* Block size must match page size */
>  	if (mp->m_sb.sb_blocksize != PAGE_SIZE)
>  		return false;
> @@ -1260,6 +1264,22 @@ xfs_inode_supports_dax(
>  	return xfs_inode_buftarg(ip)->bt_daxdev != NULL;
>  }
>  
> +static bool
> +xfs_inode_is_dax(
> +	struct xfs_inode *ip)
> +{
> +	return (ip->i_d.di_flags2 & XFS_DIFLAG2_DAX) == XFS_DIFLAG2_DAX;
> +}

I don't think these wrappers add any value at all - the naming of
them is entirely confusing, too. e.g. "inode is dax" doesn't tell me
that it is checking the on disk flags - it doesn't tell me how it is
different to IS_DAX, or why I'd use one versus the other. And then
xfs_inode_mount_is_dax() is just... worse.

Naming is hard. :)

> +
> +static bool
> +xfs_inode_use_dax(
> +	struct xfs_inode *ip)
> +{
> +	return xfs_inode_supports_dax(ip) &&
> +		(xfs_inode_mount_is_dax(ip) ||
> +		 xfs_inode_is_dax(ip));
> +}

Urk. Naming - we're not "using dax" here, we are checkign to see if
we should enable DAX on this inode. IOWs:

static bool
xfs_inode_enable_dax(
	struct xfs_inode *ip)
{
	if (!xfs_inode_supports_dax(ip))
		return false;

	if (ip->i_d.di_flags2 & XFS_DIFLAG2_DAX)
		return true;
	if (ip->i_mount->m_flags & XFS_MOUNT_DAX)
		return true;
	return false;
}

Cheers,

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

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