Re: [PATCH 4/7] erofs: introduce on-disk format for long xattr name prefixes

[Gao Xiang]

On 2023/4/7 22:17, Jingbo Xu wrote:
> Besides the predefined xattr name prefixes, introduces long xattr name
> prefixes, which work similarly as the predefined name prefixes, except
> that they are user specified.
>
> It is especially useful for use cases together with overlayfs like
> Composefs model, which introduces diverse xattr values with only a few
> common xattr names (trusted.overlay.redirect, trusted.overlay.digest,
> and maybe more in the future).  That makes the existing predefined
> prefixes ineffective in both image size and runtime performance.
>
> When a user specified long xattr name prefix is used, only the trailing
> part of the xattr name apart from the long xattr name prefix will be
> stored in erofs_xattr_entry.e_name.  e_name is empty if the xattr name
> matches exactly as the long xattr name prefix.  All long xattr prefixes
> are stored in the packed or meta inode, which depends if fragments
> feature is enabled or not.
>
> For each long xattr name prefix, the on-disk format is kept as the same
> as the unique metadata format: ALIGN({__le16 len, data}, 4), where len
> represents the total size of struct erofs_xattr_long_prefix, followed
> by data of struct erofs_xattr_long_prefix itself.
>
> Each erofs_xattr_long_prefix keeps predefined prefixes (base_index)
> and the remaining prefix string without the trailing '\0'.
>
> Two fields are introduced to the on-disk superblock, where
> xattr_prefix_count represents the total number of the long xattr name
> prefixes recorded, and xattr_prefix_start represents the start offset of
> recorded name prefixes in the packed/meta inode divided by 4.
>
> When referring to a long xattr name prefix, the highest bit (bit 7) of
> erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6)
> as a whole represents the index of the referred long name prefix among
> all long xattr name prefixes.
>
> Signed-off-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>

Reviewed-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>

Thanks,
Gao Xiang

> ---
>   fs/erofs/erofs_fs.h | 16 +++++++++++++++-
>   1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
> index 44876a97cabd..ea62f83dac40 100644
> --- a/fs/erofs/erofs_fs.h
> +++ b/fs/erofs/erofs_fs.h
> @@ -76,7 +76,8 @@ struct erofs_super_block {
>   	__le16 extra_devices;	/* # of devices besides the primary device */
>   	__le16 devt_slotoff;	/* startoff = devt_slotoff * devt_slotsize */
>   	__u8 dirblkbits;	/* directory block size in bit shift */
> -	__u8 reserved[5];
> +	__u8 xattr_prefix_count;	/* # of long xattr name prefixes */
> +	__le32 xattr_prefix_start;	/* start of long xattr prefixes */
>   	__le64 packed_nid;	/* nid of the special packed inode */
>   	__u8 reserved2[24];
>   };
> @@ -229,6 +230,13 @@ struct erofs_xattr_ibody_header {
>   #define EROFS_XATTR_INDEX_LUSTRE            5
>   #define EROFS_XATTR_INDEX_SECURITY          6
>   
> +/*
> + * bit 7 of e_name_index is set when it refers to a long xattr name prefix,
> + * while the remained lower bits represent the index of the prefix.
> + */
> +#define EROFS_XATTR_LONG_PREFIX		0x80
> +#define EROFS_XATTR_LONG_PREFIX_MASK	0x7f
> +
>   /* xattr entry (for both inline & shared xattrs) */
>   struct erofs_xattr_entry {
>   	__u8   e_name_len;      /* length of name */
> @@ -238,6 +246,12 @@ struct erofs_xattr_entry {
>   	char   e_name[];        /* attribute name */
>   };
>   
> +/* long xattr name prefix */
> +struct erofs_xattr_long_prefix {
> +	__u8   base_index;	/* short xattr name prefix index */
> +	char   infix[];		/* infix apart from short prefix */
> +};
> +
>   static inline unsigned int erofs_xattr_ibody_size(__le16 i_xattr_icount)
>   {
>   	if (!i_xattr_icount)