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Date:   Tue, 23 Jul 2019 15:53:30 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Anand Jain <anand.jain@oracle.com>
Cc:     linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 2/2 RESEND Rebased] btrfs-progs: add readmirror policy
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References: <20190626083402.1895-1-anand.jain@oracle.com>
 <20190626083723.2094-1-anand.jain@oracle.com>
 <20190626083723.2094-2-anand.jain@oracle.com>
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On Wed, Jun 26, 2019 at 04:37:23PM +0800, Anand Jain wrote:
> This sets the readmirror=<parm> as a btrfs.<attr> extentded attribute.
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>  props.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
> 
> diff --git a/props.c b/props.c
> index 3a498bd9e904..1d1a2c7f9d14 100644
> --- a/props.c
> +++ b/props.c
> @@ -178,6 +178,53 @@ out:
>  	return ret;
>  }
>  
> +static int prop_readmirror(enum prop_object_type type, const char *object,
> +			   const char *name, const char *value)
> +{
> +	int fd;
> +	int ret;
> +	char buf[256] = {0};
> +	char *xattr_name;
> +	DIR *dirstream = NULL;
> +
> +	fd = open_file_or_dir3(object, &dirstream, value ? O_RDWR : O_RDONLY);
> +	if (fd < 0) {
> +		ret = -errno;
> +		error("failed to open %s: %m", object);
> +		return ret;
> +	}
> +
> +	xattr_name = alloc_xattr_name(name);
> +	if (IS_ERR(xattr_name)) {
> +		error("failed to alloc xattr_name %s: %m", object);
> +		return PTR_ERR(xattr_name);
> +	}
> +
> +	ret = 0;
> +	if (value) {
> +		if (fsetxattr(fd, xattr_name, value, strlen(value), 0) < 0) {
> +			ret = -errno;
> +			error("failed to set readmirror for %s: %m", object);
> +		}
> +	} else {
> +		if (fgetxattr(fd, xattr_name, buf, 256) < 0) {
> +			if (errno != ENOATTR) {
> +				ret = -errno;
> +				error("failed to get readmirror for %s: %m",
> +				      object);
> +			}
> +		} else {
> +			fprintf(stdout, "readmirror=%.*s\n", (int) strlen(buf),
> +				buf);
> +		}
> +	}
> +
> +	free(xattr_name);
> +	close_file_or_dir(fd, dirstream);
> +
> +	return ret;
> +}
> +
>  const struct prop_handler prop_handlers[] = {
>  	{"ro", "Set/get read-only flag of subvolume.", 0, prop_object_subvol,
>  	 prop_read_only},
> @@ -185,5 +232,7 @@ const struct prop_handler prop_handlers[] = {
>  	 prop_object_dev | prop_object_root, prop_label},
>  	{"compression", "Set/get compression for a file or directory", 0,
>  	 prop_object_inode, prop_compression},
> +	{"readmirror", "set/get readmirror policy for filesystem", 0,
> +	 prop_object_root, prop_readmirror},

For some unknown reason the object type for filesystem-wide props is
called prop_object_root, which is correct, but it got me confused first.

So the most reliable way to set it is

  $ btrfs prop set -t filesystem /path readmirror <VALUE>

and

  $ btrfs prop set /path readmirror <VALUE>

will auto-detect the object type by /path, but I'm not sure what exactly
does it do in case it's a mount point but not the toplevel subvolume.