Re: [PATCH v2 0/5] Make btrfs-progs really compatible with any kernel version

[Anand Jain <anand.jain@oracle.com>]

David,

> the possibility of unloaded module that would remove the access to
> sysfs, as you point out.

  Kindly note, the patch below made /dev/btrfs-control a static node,

-----
commit 578454ff7eab61d13a26b568f99a89a2c9edc881
Author: Kay Sievers <kay.sievers@vrfy.org>
Date: Thu May 20 18:07:20 2010 +0200

driver core: add devname module aliases to allow module on-demand 
auto-loading
------

  And here the function, check_or_load_btrfs_ko(), in the PATCH v2 2/5,
  will take care of this problem.

--------
+
+int check_or_load_btrfs_ko()
+{
+	int fd;
+
+	/*
+	 * open will load btrfs kernel module if its not loaded,
+	 * and if the kernel has CONFIG auto load set?
+	 */
+	fd = open("/dev/btrfs-control", O_RDONLY);
+	if (fd < 0)
+		return -errno;
+
+	close(fd);
+	return 0;
+}
+
--------

  Since now static minor number for /dev/btrfs-control is mapped to
  the btrfs kernel module, it will ensure btrfs is loaded when
  /dev/btrfs-control is accessed.

  Further, /dev/btrfs-control node is created by udevd, by reading
  the modules.devname which is either supplied/updated by the distro
  or compilation.

  For systems without udev, IMO should run mknod ..btrfs-control
  in their install script which I guess is a must.

--------
# ls -li /dev/btrfs-control
7338 crw-rw---- 1 root disk 10, 234 Dec  5 10:45 /dev/btrfs-control

# cat modules.devname | egrep btrfs
btrfs btrfs-control c10:234

# cat ./include/linux/miscdevice.h | egrep BTRFS
#define BTRFS_MINOR 234
--------

  So IMO this is not a real problem.

Thanks, Anand