On Mon, Apr 27, 2020 at 04:04:11PM +0800, robbieko wrote:
> From: Robbie Ko <robbieko@xxxxxxxxxxxx>
>
> When mounting, we handle deleted subvol and orphan items.
> First, find add orphan roots, then add them to fs_root radix tree.
> Second, in tree-root, process each orphan item, skip if it is dead root.
>
> The original algorithm is based on the list of dead_roots,
> one by one to visit and check whether the objectid is consistent,
> the time complexity is O (n ^ 2).
> When processing 50000 deleted subvols, it takes about 120s.
>
> We can quickly check whether the orphan item is dead root
> through the fs_roots radix tree.
>
> Signed-off-by: Robbie Ko <robbieko@xxxxxxxxxxxx>
> ---
> fs/btrfs/inode.c | 20 +++++++++-----------
> 1 file changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 320d1062068d..1becf5c63e5a 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -3000,18 +3000,16 @@ int btrfs_orphan_cleanup(struct btrfs_root *root)
> * orphan must not get deleted.
> * find_dead_roots already ran before us, so if this
> * is a snapshot deletion, we should find the root
> - * in the dead_roots list
> + * in the fs_roots radix tree.
> */
> - spin_lock(&fs_info->trans_lock);
> - list_for_each_entry(dead_root, &fs_info->dead_roots,
> - root_list) {
> - if (dead_root->root_key.objectid ==
> - found_key.objectid) {
> - is_dead_root = 1;
> - break;
> - }
> - }
> - spin_unlock(&fs_info->trans_lock);
> +
> + spin_lock(&fs_info->fs_roots_radix_lock);
> + dead_root = radix_tree_lookup(&fs_info->fs_roots_radix,
> + (unsigned long)found_key.objectid);
> + if (dead_root && btrfs_root_refs(&dead_root->root_item) == 0)
> + is_dead_root = 1;
> + spin_unlock(&fs_info->fs_roots_radix_lock);
The list uses fs_info::trans_lock and the radix uses
fs_roots_radix_lock. I'd like to know why you think it's safe.
The trans_lock is used for a lot of things, fs_roots_radix_lock is for
the radix tree insertion/deletion/update/lookup so it does not seem like
an equivalent change. It could be functionally equivalent due to some
other constraint, like that the number of references is 0 and the tree
won't be ever touched outside of the orphan cleanup process.
btrfs_orphan_cleanup can be called during the whole filesystem mount
lifetime, so we can't rely on the mount time where nothing can iterfere.
