[RFCv2 2/3] block: add helper for setting and clearing S_DAX on inode

[Elliott, Robert (Persistent Memory)]

> -----Original Message-----
> From: linux-block-owner at vger.kernel.org [mailto:linux-block-
> owner at vger.kernel.org] On Behalf Of Jon Derrick
> Sent: Friday, May 13, 2016 7:03 PM
...
> Subject: [RFCv2 2/3] block: add helper for setting and clearing S_DAX on
> inode
> 
> inode->i_flags locking rules suggest using the i_mutex lock. This patch
> adds a helper to do the locking and setting of S_DAX on the block device
> inode.
...
> +static void bd_set_dax(struct block_device *bdev, bool enabled)
> +{
> +	struct inode *inode = bdev->bd_inode;
> +
> +	inode_lock(inode);
> +	if (enabled)
> +		inode->i_flags = S_DAX;
> +	else
> +		inode->i_flags &= ~S_DAX;
> +	inode_unlock(inode);
> +}

This is not symmetric - setting wipes out any other bits, but
clearing only clears the S_DAX bit. That seems confusing for
a helper function.

Using |= would be symmetric, but wouldn't replace what 
__blkdev_get does (if what it does is appropriate).

> @@ -1206,9 +1218,9 @@ static int __blkdev_get(struct block_device *bdev,
> fmode_t mode, int for_part)
>  		bdev->bd_queue = disk->queue;
>  		bdev->bd_contains = bdev;
>  		if (IS_ENABLED(CONFIG_BLK_DEV_DAX) && disk->fops-
> >direct_access)
> -			bdev->bd_inode->i_flags = S_DAX;
> +			bd_set_dax(bdev, 1);
>  		else
> -			bdev->bd_inode->i_flags &= ~S_DAX;
> +			bd_set_dax(bdev, 0);