Re: [PATCH 2/2] btrfs-progs: read fsid from the sysfs in device_is_seed

[Josef Bacik <josef@toxicpanda.com>]

On Tue, Oct 19, 2021 at 08:23:45AM +0800, Anand Jain wrote:
> The kernel patch [1] added a sysfs interface to read the device fsid from
> the kernel, which is a better way to know the fsid of the device (rather
> than reading the superblock). It also works if in case the device is
> missing. Furthermore, the sysfs interface is readable from the non-root
> user.
> 
> So use this new sysfs interface here to read the fsid.
> 
> [1]
> btrfs: sysfs add devinfo/fsid to retrieve fsid from the device
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>  cmds/filesystem-usage.c | 38 ++++++++++++++++++++++++++++++--------
>  1 file changed, 30 insertions(+), 8 deletions(-)
> 
> diff --git a/cmds/filesystem-usage.c b/cmds/filesystem-usage.c
> index 0dfc798e8dcc..f658c27b9609 100644
> --- a/cmds/filesystem-usage.c
> +++ b/cmds/filesystem-usage.c
> @@ -40,6 +40,7 @@
>  #include "common/help.h"
>  #include "common/device-utils.h"
>  #include "common/open-utils.h"
> +#include "common/path-utils.h"
>  
>  /*
>   * Add the chunk info to the chunk_info list
> @@ -706,14 +707,33 @@ out:
>  	return ret;
>  }
>  
> -static int device_is_seed(const char *dev_path, u8 *mnt_fsid)
> +static int device_is_seed(int fd, const char *dev_path, u64 devid, u8 *mnt_fsid)
>  {
> +	char fsidparse[BTRFS_UUID_UNPARSED_SIZE];
> +	char fsid_path[PATH_MAX];
> +	char devid_str[20];
>  	uuid_t fsid;
> -	int ret;
> +	int ret = -1;
> +	int sysfs_fd;
> +
> +	snprintf(devid_str, 20, "%llu", devid);
> +	/* devinfo/<devid>/fsid */
> +	path_cat3_out(fsid_path, "devinfo", devid_str, "fsid");
> +
> +	/* /sys/fs/btrfs/<fsid>/devinfo/<devid>/fsid */
> +	sysfs_fd = sysfs_open_fsid_file(fd, fsid_path);
> +	if (sysfs_fd >= 0) {
> +		sysfs_read_file(sysfs_fd, fsidparse, BTRFS_UUID_UNPARSED_SIZE);
> +		fsidparse[BTRFS_UUID_UNPARSED_SIZE - 1] = 0;
> +		ret = uuid_parse(fsidparse, fsid);
> +		close(sysfs_fd);
> +	}
>  
> -	ret = dev_to_fsid(dev_path, fsid);

Why not just have dev_to_fsid() use the sysfs thing so all callers can benefit
from it, and then have it fall back to the reading of the super block?  Thanks,

Josef