linux-kernel - Re: [RFC PATCH 02/20] dev_dax_iomap: Add fs_dax_get() func to prepare dax for fs-dax usage

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Date: Mon, 26 Feb 2024 12:05:35 +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 02/20] dev_dax_iomap: Add fs_dax_get() func to
 prepare dax for fs-dax usage

On Fri, 23 Feb 2024 11:41:46 -0600
John Groves <John@...ves.net> wrote:

> This function should be called by fs-dax file systems after opening the
> devdax device. This adds holder_operations.
> 
> This function serves the same role as fs_dax_get_by_bdev(), which dax
> file systems call after opening the pmem block device.
> 
> Signed-off-by: John Groves <john@...ves.net>

A few trivial comments form a first read to get my head around this.

Yeah, it is only an RFC, but who doesn't like tidy code? :)


> ---
>  drivers/dax/super.c | 38 ++++++++++++++++++++++++++++++++++++++
>  include/linux/dax.h |  5 +++++
>  2 files changed, 43 insertions(+)
> 
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index f4b635526345..fc96362de237 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -121,6 +121,44 @@ void fs_put_dax(struct dax_device *dax_dev, void *holder)
>  EXPORT_SYMBOL_GPL(fs_put_dax);
>  #endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
>  
> +#if IS_ENABLED(CONFIG_DEV_DAX_IOMAP)
> +
> +/**
> + * fs_dax_get()

Smells like kernel doc but fairly sure it needs a short description.
Have you sanity checked for warnings when running scripts/kerneldoc on it?

> + *
> + * fs-dax file systems call this function to prepare to use a devdax device for fsdax.
Trivial but lines too long. Keep under 80 chars unless there is a strong
readability arguement for not doing so.


> + * This is like fs_dax_get_by_bdev(), but the caller already has struct dev_dax (and there
> + * is no bdev). The holder makes this exclusive.

Not familiar with this area: what does exclusive mean here?

> + *
> + * @dax_dev: dev to be prepared for fs-dax usage
> + * @holder: filesystem or mapped device inside the dax_device
> + * @hops: operations for the inner holder
> + *
> + * Returns: 0 on success, -1 on failure

Why not return < 0 and use somewhat useful return values?

> + */
> +int fs_dax_get(
> +	struct dax_device *dax_dev,
> +	void *holder,
> +	const struct dax_holder_operations *hops)

Match local style for indents - it's a bit inconsistent but probably...

int fs_dax_get(struct dad_device *dev_dax, void *holder,
	       const struct dax_holder_operations *hops)

> +{
> +	/* dax_dev->ops should have been populated by devm_create_dev_dax() */
> +	if (WARN_ON(!dax_dev->ops))
> +		return -1;
> +
> +	if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode))

You dereferenced dax_dev on the line above so check is too late or
unnecessary

> +		return -1;
> +
> +	if (cmpxchg(&dax_dev->holder_data, NULL, holder)) {
> +		pr_warn("%s: holder_data already set\n", __func__);

Perhaps nicer to use a pr_fmt() deal with the func name if you need it.
or make it pr_debug and let dynamic debug control formatting if anyone
wants the function name.

> +		return -1;
> +	}
> +	dax_dev->holder_ops = hops;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(fs_dax_get);
> +#endif /* DEV_DAX_IOMAP */
> +
>  enum dax_device_flags {
>  	/* !alive + rcu grace period == no new operations / mappings */
>  	DAXDEV_ALIVE,
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index b463502b16e1..e973289bfde3 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -57,7 +57,12 @@ struct dax_holder_operations {
>  
>  #if IS_ENABLED(CONFIG_DAX)
>  struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
> +
> +#if IS_ENABLED(CONFIG_DEV_DAX_IOMAP)
> +int fs_dax_get(struct dax_device *dax_dev, void *holder, const struct dax_holder_operations *hops);
line wrap < 80 chars

> +#endif
>  void *dax_holder(struct dax_device *dax_dev);
> +struct dax_device *inode_dax(struct inode *inode);

Unrelated change?

>  void put_dax(struct dax_device *dax_dev);
>  void kill_dax(struct dax_device *dax_dev);
>  void dax_write_cache(struct dax_device *dax_dev, bool wc);