Re: [PATCH 2/3] populate: ensure btree directories are created reliably

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2023/1/11 06:49, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> The population function creates an XFS btree format directory by
> polling the extent count of the inode and creating new dirents until
> the extent count goes over the limit that pushes it into btree
> format.
> 
> It then removes every second dirent to create empty space in the
> directory data to ensure that operations like metadump with
> obfuscation can check that they don't leak stale data from deleted
> dirents.
> 
> Whilst this does not result in directory data blocks being freed, it
> does not take into account the fact that the dabtree index has half
> the entries removed from it and that can result in btree nodes
> merging and extents being freed. This causes the extent count to go
> down, and the inode is converted back into extent form. The
> population checks then fail because it should be in btree form.
> 
> Fix this by counting the number of directory data extents rather than
> the total number of extents in the data fork. We can do this simply
> by using xfs_bmap and counting the number of extents returned as it
> does not report extents beyond EOF (which is where the dabtree is
> located). As the number of data blocks does not change with the
> dirent removal algorithm used, this will ensure that the inode data
> fork remains in btree format.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Thanks,
Gao Xiang

> ---
>   common/populate | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/common/populate b/common/populate
> index 9b60fa5c1..7b5b16fb8 100644
> --- a/common/populate
> +++ b/common/populate
> @@ -80,8 +80,11 @@ __populate_create_nfiles() {
>   			continue
>   		fi
>   
> -		local nextents="$(_xfs_get_fsxattr nextents $name)"
> -		if [ "${nextents}" -gt "${max_nextents}" ]; then
> +		# Extent count checks use data blocks only to avoid the removal
> +		# step from removing dabtree index blocks and reducing the
> +		# number of extents below the required threshold.
> +		local nextents="$(xfs_bmap ${name} |grep -v hole | wc -l)"
> +		if [ "$((nextents - 1))" -gt "${max_nextents}" ]; then
>   			echo ${d}
>   			break
>   		fi