Re: [PATCH] btrfs: apply first key check for readahead when possible

[David Sterba <dsterba@suse.cz>]

On Sun, Apr 12, 2026 at 08:31:05PM +0930, Qu Wenruo wrote:
> Currently for tree block readahead we never pass a
> btrfs_tree_parent_check with @has_first_key set.
> 
> Without @has_first_key set, btrfs will skip the following extra
> checks:
> 
> - Header generation check
>   This is a minor one.
> 
> - Empty leaf/node checks
>   This is more serious, for certain trees like the csum tree, they are
>   allowed to be empty, thus an empty leaf can pass the tree checker.
>   But if there is a parent node for such an empty leaf, it indicates
>   corruption.
> 
>   Without @has_first_key set, we can no longer detect such a problem.
> 
>   In fact there is already a fuzzed image report that a corrupted csum
>   leaf which has zero nritems but still has a parent node can trigger
>   a BUG_ON() during csum deletion.
> 
> However there are only two call sites of btrfs_readahead_tree_block():
> 
> - Inside relocate_tree_blocks()
>   At this call site we are trying to grab the first key of the tree
>   block, thus we are not able to pass a @first_key parameter.
> 
> - Inside btrfs_readahead_node_child()
>   This is the more common call site, where we have the parent node and
>   want to readahead the child tree blocks.
> 
>   In this case we can easily grab the node key and pass it for checks.
> 
> Add a new parameter @first_key to btrfs_readahead_tree_block() and pass
> the node key to it inside btrfs_readahead_node_child().
> 
> This should plug the gap in empty leaf detection during readahead.
> 
> Link: https://lore.kernel.org/linux-btrfs/20260409071255.3358044-1-gality369@gmail.com/
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Reviewed-by: David Sterba <dsterba@suse.com>

> ---
>  fs/btrfs/extent_io.c  | 16 +++++++++++++---
>  fs/btrfs/extent_io.h  |  3 ++-
>  fs/btrfs/relocation.c |  2 +-
>  3 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 3f59fb0a9ee5..4d5605aaf827 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4635,15 +4635,21 @@ int try_release_extent_buffer(struct folio *folio)
>   * to read the block we will not block on anything.
>   */
>  void btrfs_readahead_tree_block(struct btrfs_fs_info *fs_info,
> -				u64 bytenr, u64 owner_root, u64 gen, int level)
> +				u64 bytenr, u64 owner_root, u64 gen, int level,
> +				const struct btrfs_key *first_key)
>  {
>  	struct btrfs_tree_parent_check check = {
>  		.level = level,
> -		.transid = gen
> +		.transid = gen,

It's not necessary to add the "," or was there another reason?

>  	};
>  	struct extent_buffer *eb;
>  	int ret;
>  
> +	if (first_key) {
> +		memcpy(&check.first_key, first_key, sizeof(struct btrfs_key));
> +		check.has_first_key = true;
> +	}
> +
>  	eb = btrfs_find_create_tree_block(fs_info, bytenr, owner_root, level);
>  	if (IS_ERR(eb))
>  		return;