Commit 6ef108dd authored by Nikolay Borisov's avatar Nikolay Borisov Committed by David Sterba
Browse files

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: default avatarNikolay Borisov <nborisov@suse.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 4ac039ad

fs/btrfs/disk-io.c

+12 −39
Original line number Diff line number Diff line
@@ -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.

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