erofs: update documentation

 - update new features like bloom filter and DEFLATE.

 - add documentation for the long xattr name prefixes, which was
   landed upstream since v6.4.

Signed-off-by: Jingbo Xu <[email protected]>
Reviewed-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Gao Xiang <[email protected]>

Author: lostjeffle

Documentation/filesystems/erofs.rst

@@ -58,12 +58,14 @@ Here are the main features of EROFS:
 
  - Support extended attributes as an option;
 
+ - Support a bloom filter that speeds up negative extended attribute lookups;
+
  - Support POSIX.1e ACLs by using extended attributes;
 
  - Support transparent data compression as an option:
-   LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition,
-   inplace decompression is also supported to avoid bounce compressed buffers
-   and page cache thrashing.
+   LZ4, MicroLZMA and DEFLATE algorithms can be used on a per-file basis; In
+   addition, inplace decompression is also supported to avoid bounce compressed
+   buffers and unnecessary page cache thrashing.
 
  - Support chunk-based data deduplication and rolling-hash compressed data
    deduplication;
@@ -268,6 +270,38 @@ details.)
 
 By the way, chunk-based files are all uncompressed for now.
 
+Long extended attribute name prefixes
+-------------------------------------
+There are use cases where extended attributes with different values can have
+only a few common prefixes (such as overlayfs xattrs).  The predefined prefixes
+work inefficiently in both image size and runtime performance in such cases.
+
+The long xattr name prefixes feature is introduced to address this issue.  The
+overall idea is that, apart from the existing predefined prefixes, the xattr
+entry could also refer to user-specified long xattr name prefixes, e.g.
+"trusted.overlay.".
+
+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
+represent the index of the referred long name prefix among all long name
+prefixes.  Therefore, only the trailing part of the name apart from the long
+xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if
+the full xattr name matches exactly as its long xattr name prefix.
+
+All long xattr prefixes are stored one by one in the packed inode as long as
+the packed inode is valid, or in the meta inode otherwise.  The
+xattr_prefix_count (of the on-disk superblock) indicates the total number of
+long xattr name prefixes, while (xattr_prefix_start * 4) indicates the start
+offset of long name prefixes in the packed/meta inode.  Note that, long extended
+attribute name prefixes are disabled if xattr_prefix_count is 0.
+
+Each long name prefix is stored in the format: ALIGN({__le16 len, data}, 4),
+where len represents the total size of the data part.  The data part is actually
+represented by 'struct erofs_xattr_long_prefix', where base_index represents the
+index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for
+"trusted.overlay." long name prefix, while the infix string keeps the string
+after stripping the short prefix, e.g. "overlay." for the example above.
+
 Data compression
 ----------------
 EROFS implements fixed-sized output compression which generates fixed-sized