Re: [PATCH 2/2 RESEND Rebased] btrfs-progs: add readmirror policy

[Anand Jain <anand.jain@xxxxxxxxxx>]

On 23/7/19 9:53 PM, David Sterba wrote:
> 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@xxxxxxxxxx>
> > ---
> >   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.

If its not toplevel subvolume it fails in kernel with -EINVAL.

----
+static int prop_readmirror_validate(struct inode *inode, const char *value,
+                                   size_t len)
+{
+       struct btrfs_root *root = BTRFS_I(inode)->root;
+
+       if (root->root_key.objectid != BTRFS_FS_TREE_OBJECTID)
+               return -EINVAL;
-----