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btrfs: Streamline btrfs_fs_info::backup_root_index semantics

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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>
This commit is contained in:
Nikolay Borisov 2019-10-15 18:42:24 +03:00 committed by David Sterba
parent 4ac039ad75
commit 6ef108dd0c
1 changed files with 12 additions and 39 deletions
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51
fs/btrfs/disk-io.c
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@ -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.