linux-kernel - Re: [RFC PATCH 10/20] famfs: famfs_open_device() & dax_holder

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Date: Mon, 26 Feb 2024 12:56:42 +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 10/20] famfs: famfs_open_device() &
 dax_holder_operations

On Fri, 23 Feb 2024 11:41:54 -0600
John Groves <John@...ves.net> wrote:

> Famfs works on both /dev/pmem and /dev/dax devices. This commit introduces
> the function that opens a block (pmem) device and the struct
> dax_holder_operations that are needed for that ABI.
> 
> In this commit, support for opening character /dev/dax is stubbed. A
> later commit introduces this capability.
> 
> Signed-off-by: John Groves <john@...ves.net>

Formatting comments mostly same as previous patches, so I'll stop repeating them.

> ---
>  fs/famfs/famfs_inode.c | 83 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 83 insertions(+)
> 
> diff --git a/fs/famfs/famfs_inode.c b/fs/famfs/famfs_inode.c
> index 3329aff000d1..82c861998093 100644
> --- a/fs/famfs/famfs_inode.c
> +++ b/fs/famfs/famfs_inode.c
> @@ -68,5 +68,88 @@ static const struct super_operations famfs_ops = {
>  	.show_options	= famfs_show_options,
>  };
>  
> +/***************************************************************************************
> + * dax_holder_operations for block dax
> + */
> +
> +static int
> +famfs_blk_dax_notify_failure(
> +	struct dax_device	*dax_devp,
> +	u64			offset,
> +	u64			len,
> +	int			mf_flags)
> +{
> +
> +	pr_err("%s: dax_devp %llx offset %llx len %lld mf_flags %x\n",
> +	       __func__, (u64)dax_devp, (u64)offset, (u64)len, mf_flags);
> +	return -EOPNOTSUPP;
> +}
> +
> +const struct dax_holder_operations famfs_blk_dax_holder_ops = {
> +	.notify_failure		= famfs_blk_dax_notify_failure,
> +};
> +
> +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()
> + *
> + * Open the memory device. If it looks like /dev/dax, call famfs_open_char_device().
> + * Otherwise try to open it as a block/pmem device.
> + */
> +static int
> +famfs_open_device(
> +	struct super_block *sb,
> +	struct fs_context  *fc)
> +{
> +	struct famfs_fs_info *fsi = sb->s_fs_info;
> +	struct dax_device    *dax_devp;
> +	u64 start_off = 0;
> +	struct bdev_handle   *handlep;
Definitely don't force alignment in local parameter definitions.
Always goes wrong and makes for unreadable mess in patches!

> +
> +	if (fsi->dax_devp) {
> +		pr_err("%s: already mounted\n", __func__);
Fine to fail but worth a error message? Not sure on convention on this but seems noisy
and maybe in userspace control which isn't good.
> +		return -EALREADY;
> +	}
> +
> +	if (strstr(fc->source, "/dev/dax")) /* There is probably a better way to check this */
> +		return famfs_open_char_device(sb, fc);
> +
> +	if (!strstr(fc->source, "/dev/pmem")) { /* There is probably a better way to check this */
> +		pr_err("%s: primary backing dev (%s) is not pmem\n",
> +		       __func__, fc->source);
> +		return -EINVAL;
> +	}
> +
> +	handlep = bdev_open_by_path(fc->source, FAMFS_BLKDEV_MODE, fsi, &fs_holder_ops);
> +	if (IS_ERR(handlep->bdev)) {
> +		pr_err("%s: failed blkdev_get_by_path(%s)\n", __func__, fc->source);
> +		return PTR_ERR(handlep->bdev);
> +	}
> +
> +	dax_devp = fs_dax_get_by_bdev(handlep->bdev, &start_off,
> +				      fsi  /* holder */,
> +				      &famfs_blk_dax_holder_ops);
> +	if (IS_ERR(dax_devp)) {
> +		pr_err("%s: unable to get daxdev from handlep->bdev\n", __func__);
> +		bdev_release(handlep);
> +		return -ENODEV;
> +	}
> +	fsi->bdev_handle = handlep;
> +	fsi->dax_devp    = dax_devp;
> +
> +	pr_notice("%s: root device is block dax (%s)\n", __func__, fc->source);

pr_debug()  Kernel log is too noisy anyway! + I'd assume we can tell this succeeded
in lots of other ways.


> +	return 0;
> +}
> +
> +
>  
>  MODULE_LICENSE("GPL");