Re: [RFC PATCH 10/20] famfs: famfs_open_device() & dax_holder_operations

[Christian Brauner]

On Fri, Feb 23, 2024 at 11:41:54AM -0600, John Groves 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@xxxxxxxxxx>
> ---
>  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(

I'm confused why that function is added here but it's completely unclear
in what wider context it's called. This is really hard to follow.

> +	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;
> +
> +	if (fsi->dax_devp) {
> +		pr_err("%s: already mounted\n", __func__);
> +		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 */

Yeah, this is not just a bit ugly but also likely wrong because:

sudo mount --bind /dev/pmem /opt/muhaha

fsconfig(fd_fs, FSCONFIG_SET_STRING, "source", "/opt/muhaha", [...])

or a simple mknod to create that device somewhere else. You likely want:

lookup_bdev(fc->source, &dev);

if (!DEVICE_NUMBER_SOMETHING_SOMETHING_SANE(dev))
	return invalfc(fc, "SOMETHING SOMETHING...

bdev_open_by_dev(dev, ....)

(This reminds me that I should get back to making it possible to specify
"source" as a file descriptor instead of a mere string with the new
mount api...)

> +		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);

Hm, I suspected that FAMFS_BLKDEV_MODE would be wrong based on:
https://lore.kernel.org/r/13556dbbd8d0f51bc31e3bdec796283fe85c6baf.1708709155.git.john@xxxxxxxxxx

It's defined as FMODE_READ | FMODE_WRITE which is wrong. But these
helpers want BLOCK_OPEN_READ | BLOCK_OPEN_WRITE.

> +	if (IS_ERR(handlep->bdev)) {

@bdev_handle will be gone as of v6.9 so you might want to wait until
then to resend.

> +		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);
> +	return 0;
> +}
> +
> +
>  
>  MODULE_LICENSE("GPL");
> -- 
> 2.43.0
>