Re: [PATCH v2 3/3] btrfs: fix check_chunk_block_group_mappings() to actually iterate all chunks

[ZhengYuan Huang]

On Tue, Mar 24, 2026 at 1:52 AM David Sterba <dsterba@xxxxxxx> wrote:
> This is called during mount indirectly from open_ctree() and this is
> single threaded (partially), so the lock may not be needed. It would be
> needed if there's eg. caching thread possibly accessing the same
> structures, I haven't looked closely.
>
> > +     for (node = rb_first_cached(&fs_info->mapping_tree); node;
> > +          node = rb_next(node)) {
> >               struct btrfs_chunk_map *map;
> >               struct btrfs_block_group *bg;
> >
> > -             /*
> > -              * btrfs_find_chunk_map() will return the first chunk map
> > -              * intersecting the range, so setting @length to 1 is enough to
> > -              * get the first chunk.
> > -              */
> > -             map = btrfs_find_chunk_map(fs_info, start, 1);
> > -             if (!map)
> > -                     break;
> > -
> > +             map = rb_entry(node, struct btrfs_chunk_map, rb_node);
> >               bg = btrfs_lookup_block_group(fs_info, map->start);
>
> What concerns me is this lookup. Previously the references avoided
> taking the big lock. The time the lock is held may add up significanly
> for all block groups but as said before it might not be necessary due to
> the mount context.

Thanks for the suggestion, I’ll take a closer look at the locking here.
If the lock turns out to be unnecessary in this context, I’ll drop it
and include the change in v3.

Thanks,
ZhengYuan Huang