Btrfs: don't allow degraded mount if too many devices are missing

The current behavior is to allow mounting or remounting a filesystem
writeable in degraded mode if at least one writeable device is
present.
The next failed write access to a missing device which is above
the tolerance of the configured level of redundancy results in an
read-only enforcement. Even without this, the next time
barrier_all_devices() is called and more devices are missing than
tolerable, the switch to read-only mode takes place.

In order to behave predictably and to provide proper feedback to
the user at mount time, this patch compares the number of missing
devices with the number of devices that are tolerated to be missing
according to the configured RAID level. If more devices are missing
than tolerated, e.g. if two devices are missing in case of RAID1,
only a read-only mount and remount is allowed.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index bd70c2852ba050ad69347e33c457ea1be17f560c..064315990f8a816736f376b4ae6c0b7cd61b9ea6 100644 (file)
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2508,6 +2508,13 @@ retry_root_backup:
        }
        fs_info->num_tolerated_disk_barrier_failures =
                btrfs_calc_num_tolerated_disk_barrier_failures(fs_info);
+       if (fs_info->fs_devices->missing_devices >
+            fs_info->num_tolerated_disk_barrier_failures &&
+           !(sb->s_flags & MS_RDONLY)) {
+               printk(KERN_WARNING
+                      "Btrfs: too many missing devices, writeable mount is not allowed\n");
+               goto fail_block_groups;
+       }
 
        fs_info->cleaner_kthread = kthread_run(cleaner_kthread, tree_root,
                                               "btrfs-cleaner");

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 915ac14c20642ec619ec159d90d0fbed2a02b0da..acd2df85bed5cd7b81b88191baee26e1829a14a7 100644 (file)
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1226,6 +1226,15 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
                        goto restore;
                }
 
+               if (fs_info->fs_devices->missing_devices >
+                    fs_info->num_tolerated_disk_barrier_failures &&
+                   !(*flags & MS_RDONLY)) {
+                       printk(KERN_WARNING
+                              "Btrfs: too many missing devices, writeable remount is not allowed\n");
+                       ret = -EACCES;
+                       goto restore;
+               }
+
                if (btrfs_super_log_root(fs_info->super_copy) != 0) {
                        ret = -EINVAL;
                        goto restore;