btrfs: Streamline btrfs_fs_info::backup_root_index semantics

The backup_root_index member stores the index at which the backup root
should be saved upon next transaction commit. However, there is a
small deviation from this behavior in the form of a check in
backup_super_roots which checks if current root generation equals to the
generation of the previous root. This can trigger in the following
scenario:

slot0: gen-2
slot1: gen-1
slot2: gen
slot3: unused

Now suppose slot3 (which is also the root specified in the super block)
is corrupted hence init_tree_roots chooses to use the backup root at
slot2, meaning read_backup_root will read slot2 and assign the
superblock generation to gen-1. Despite this backup_root_index will
point at slot3 because its init happens in init_backup_root_slot, long
before any parsing of the backup roots occur. Then on next transaction
start, gen-1 will be incremented by 1 making the root's generation
equal gen. Subsequently, on transaction commit the following check
triggers:

  if (btrfs_backup_tree_root_gen(root_backup) ==
           btrfs_header_generation(info->tree_root->node))

This causes the 'next_backup', which is the index at which the backup is
going to be written to, to set to last_backup, which will be slot2.

All of this is a very confusing way of expressing the following
invariant:

 Always write a backup root at the index following the last used backup
 root.

This commit streamlines this logic by setting backup_root_index to the
next index after the one used for mount.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 6c44d1781f21df5b73bbbdb322c96b3a5ba6ad67..ddb39f76f71a69da76bcfde074150201a66445ee 100644 (file)
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1815,23 +1815,6 @@ static int find_newest_super_backup(struct btrfs_fs_info *info)
        return -EINVAL;
 }
 
-/*
- * Initialize backup_root_index with the next available slot, where subsequent
- * transaction commit will store the backup root
- */
-static void init_backup_root_slot(struct btrfs_fs_info *info)
-{
-       int newest_index;
-
-       newest_index = find_newest_super_backup(info);
-       /* if there was garbage in there, just move along */
-       if (newest_index == -EINVAL) {
-               info->backup_root_index = 0;
-       } else {
-               info->backup_root_index = (newest_index + 1) % BTRFS_NUM_BACKUP_ROOTS;
-       }
-}
-
 /*
  * copy all the root pointers into the super backup array.
  * this will bump the backup pointer by one when it is
@@ -1839,22 +1822,8 @@ static void init_backup_root_slot(struct btrfs_fs_info *info)
  */
 static void backup_super_roots(struct btrfs_fs_info *info)
 {
-       int next_backup;
+       const int next_backup = info->backup_root_index;
        struct btrfs_root_backup *root_backup;
-       int last_backup;
-
-       next_backup = info->backup_root_index;
-       last_backup = (next_backup + BTRFS_NUM_BACKUP_ROOTS - 1) %
-               BTRFS_NUM_BACKUP_ROOTS;
-
-       /*
-        * just overwrite the last backup if we're at the same generation
-        * this happens only at umount
-        */
-       root_backup = info->super_for_commit->super_roots + last_backup;
-       if (btrfs_backup_tree_root_gen(root_backup) ==
-           btrfs_header_generation(info->tree_root->node))
-               next_backup = last_backup;
 
        root_backup = info->super_for_commit->super_roots + next_backup;
 
@@ -2558,8 +2527,9 @@ out:
        return ret;
 }
 
-int __cold init_tree_roots(struct btrfs_fs_info *fs_info)
+static int __cold init_tree_roots(struct btrfs_fs_info *fs_info)
 {
+       int backup_index = find_newest_super_backup(fs_info);
        struct btrfs_super_block *sb = fs_info->super_copy;
        struct btrfs_root *tree_root = fs_info->tree_root;
        bool handle_error = false;
@@ -2590,6 +2560,7 @@ int __cold init_tree_roots(struct btrfs_fs_info *fs_info)
                        btrfs_set_opt(fs_info->mount_opt, CLEAR_CACHE);
 
                        ret = read_backup_root(fs_info, i);
+                       backup_index = ret;
                        if (ret < 0)
                                return ret;
                }
@@ -2636,6 +2607,14 @@ int __cold init_tree_roots(struct btrfs_fs_info *fs_info)
                /* All successful */
                fs_info->generation = generation;
                fs_info->last_trans_committed = generation;
+
+               /* Always begin writing backup roots after the one being used */
+               if (backup_index < 0) {
+                       fs_info->backup_root_index = 0;
+               } else {
+                       fs_info->backup_root_index = backup_index + 1;
+                       fs_info->backup_root_index %= BTRFS_NUM_BACKUP_ROOTS;
+               }
                break;
        }
 
@@ -2929,12 +2908,6 @@ int __cold open_ctree(struct super_block *sb,
        if (btrfs_super_flags(disk_super) & BTRFS_SUPER_FLAG_ERROR)
                set_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state);
 
-       /*
-        * run through our array of backup supers and setup
-        * our ring pointer to the oldest one
-        */
-       init_backup_root_slot(fs_info);
-
        /*
         * In the long term, we'll store the compression type in the super
         * block, and it'll be used for per file compression control.