linux-kernel - Re: [RFC PATCH 18/20] famfs: Support character dax via the dev_dax

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Date: Mon, 26 Feb 2024 13:52:28 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: John Groves <John@...ves.net>
CC: John Groves <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>, "Dan
 Williams" <dan.j.williams@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>, "Alexander
 Viro" <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, "Jan
 Kara" <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
	<linux-cxl@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<nvdimm@...ts.linux.dev>, <john@...alactic.com>, Dave Chinner
	<david@...morbit.com>, Christoph Hellwig <hch@...radead.org>,
	<dave.hansen@...ux.intel.com>, <gregory.price@...verge.com>
Subject: Re: [RFC PATCH 18/20] famfs: Support character dax via the
 dev_dax_iomap patch

On Fri, 23 Feb 2024 11:42:02 -0600
John Groves <John@...ves.net> wrote:

> This commit introduces the ability to open a character /dev/dax device
> instead of a block /dev/pmem device. This rests on the dev_dax_iomap
> patches earlier in this series.

Not sure the back reference is needed given it's in the series.

> 
> Signed-off-by: John Groves <john@...ves.net>
> ---
>  fs/famfs/famfs_inode.c | 97 +++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 87 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/famfs/famfs_inode.c b/fs/famfs/famfs_inode.c
> index 0d659820e8ff..7d65ac497147 100644
> --- a/fs/famfs/famfs_inode.c
> +++ b/fs/famfs/famfs_inode.c
> @@ -215,6 +215,93 @@ static const struct super_operations famfs_ops = {
>  	.show_options	= famfs_show_options,
>  };
>  
> +/*****************************************************************************/
> +
> +#if defined(CONFIG_DEV_DAX_IOMAP)
> +
> +/*
> + * famfs dax_operations  (for char dax)
> + */
> +static int
> +famfs_dax_notify_failure(struct dax_device *dax_dev, u64 offset,
> +			u64 len, int mf_flags)
> +{
> +	pr_err("%s: offset %lld len %llu flags %x\n", __func__,
> +	       offset, len, mf_flags);
> +	return -EOPNOTSUPP;
> +}
> +
> +static const struct dax_holder_operations famfs_dax_holder_ops = {
> +	.notify_failure		= famfs_dax_notify_failure,
> +};
> +
> +/*****************************************************************************/
> +
> +/**
> + * famfs_open_char_device()
> + *
> + * Open a /dev/dax device. This only works in kernels with the dev_dax_iomap patch

That comment you definitely don't need as this won't get merged without
that patch being in place.


> + */
> +static int
> +famfs_open_char_device(
> +	struct super_block *sb,
> +	struct fs_context  *fc)
> +{
> +	struct famfs_fs_info *fsi = sb->s_fs_info;
> +	struct dax_device    *dax_devp;
> +	struct inode         *daxdev_inode;
> +
> +	int rc = 0;
set in all paths where it's used.

> +
> +	pr_notice("%s: Opening character dax device %s\n", __func__, fc->source);

pr_debug

> +
> +	fsi->dax_filp = filp_open(fc->source, O_RDWR, 0);
> +	if (IS_ERR(fsi->dax_filp)) {
> +		pr_err("%s: failed to open dax device %s\n",
> +		       __func__, fc->source);
> +		fsi->dax_filp = NULL;
Better to use a local variable

	fp = filp_open(fc->source, O_RDWR, 0);
	if (IS_ERR(fp)) {
		pr_err.
		return;
	}
	fsi->dax_filp = fp;
or similar.

> +		return PTR_ERR(fsi->dax_filp);
> +	}
> +
> +	daxdev_inode = file_inode(fsi->dax_filp);
> +	dax_devp     = inode_dax(daxdev_inode);
> +	if (IS_ERR(dax_devp)) {
> +		pr_err("%s: unable to get daxdev from inode for %s\n",
> +		       __func__, fc->source);
> +		rc = -ENODEV;
> +		goto char_err;
> +	}
> +
> +	rc = fs_dax_get(dax_devp, fsi, &famfs_dax_holder_ops);
> +	if (rc) {
> +		pr_info("%s: err attaching famfs_dax_holder_ops\n", __func__);
> +		goto char_err;
> +	}
> +
> +	fsi->bdev_handle = NULL;
> +	fsi->dax_devp = dax_devp;
> +
> +	return 0;
> +
> +char_err:
> +	filp_close(fsi->dax_filp, NULL);

You carefully set fsi->dax_filp to null in other other error paths.
Why there and not here?

> +	return rc;
> +}
> +
> +#else /* CONFIG_DEV_DAX_IOMAP */
> +static int
> +famfs_open_char_device(
> +	struct super_block *sb,
> +	struct fs_context  *fc)
> +{
> +	pr_err("%s: Root device is %s, but your kernel does not support famfs on /dev/dax\n",
> +	       __func__, fc->source);
> +	return -ENODEV;
> +}
> +
> +
> +#endif /* CONFIG_DEV_DAX_IOMAP */
> +
>  /***************************************************************************************
>   * dax_holder_operations for block dax
>   */
> @@ -236,16 +323,6 @@ const struct dax_holder_operations famfs_blk_dax_holder_ops = {
>  	.notify_failure		= famfs_blk_dax_notify_failure,
>  };
>  

Put it in right place earlier! Makes this less noisy.

> -static int
> -famfs_open_char_device(
> -	struct super_block *sb,
> -	struct fs_context  *fc)
> -{
> -	pr_err("%s: Root device is %s, but your kernel does not support famfs on /dev/dax\n",
> -	       __func__, fc->source);
> -	return -ENODEV;
> -}
> -
>  /**
>   * famfs_open_device()
>   *