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.
---
fs/btrfs/disk-io.c | 48 +++++++++-------------------------------------
1 file changed, 9 insertions(+), 39 deletions(-)
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index ac899fdb1414..e266949529fb 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1786,23 +1786,6 @@ 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 back up 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
@@ -1810,22 +1793,8 @@ static void init_backup_root_slot(struct btrfs_fs_info *info)
*/
static void backup_super_roots(struct btrfs_fs_info *info)
{
- int next_backup;
+ 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;
@@ -2531,6 +2500,7 @@ static int btrfs_validate_write_super(struct btrfs_fs_info *fs_info,
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;
@@ -2557,7 +2527,7 @@ int __cold init_tree_roots(struct btrfs_fs_info *fs_info)
/* we can't trust the free space cache either */
btrfs_set_opt(fs_info->mount_opt, CLEAR_CACHE);
- ret = read_backup_root(fs_info, i);
+ ret = backup_index = read_backup_root(fs_info, i);
if (ret < 0)
return ret;
}
@@ -2604,6 +2574,12 @@ 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 one being used */
+ if (backup_index < 0)
+ fs_info->backup_root_index = 0;
+ else
+ fs_info->backup_root_index = (backup_index + 1) % BTRFS_NUM_BACKUP_ROOTS;
break;
}
@@ -2898,12 +2874,6 @@ 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.
--
2.17.1
