[RFC 1/2] erofs: update on-disk format for xattr bloom filter

[Jingbo Xu]

The xattr bloom filter feature is going to be introduced to speed up the
negative xattr lookup, e.g. system.posix_acl_[access|default] lookup
when running "ls -lR" workload.

The number of common used xattr (n) is approximately 8, including
system.[posix_acl_access|posix_acl_default], security.[capability|selinux]
and security.[SMACK64|SMACK64TRANSMUTE|SMACK64EXEC|SMACK64MMAP].  Given the
number of bits of the bloom filter (m) is 32, the optimal value for the
number of the hash functions (k) is 2 (ln2 * m/n = 2.7).

Signed-off-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>
---
 fs/erofs/erofs_fs.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
index 2c7b16e340fe..9daea86cdb52 100644
--- a/fs/erofs/erofs_fs.h
+++ b/fs/erofs/erofs_fs.h
@@ -13,6 +13,7 @@
 
 #define EROFS_FEATURE_COMPAT_SB_CHKSUM          0x00000001
 #define EROFS_FEATURE_COMPAT_MTIME              0x00000002
+#define EROFS_FEATURE_COMPAT_XATTR_BLOOM	0x00000003
 
 /*
  * Any bits that aren't in EROFS_ALL_FEATURE_INCOMPAT should
@@ -200,7 +201,7 @@ struct erofs_inode_extended {
  * for read-only fs, no need to introduce h_refcount
  */
 struct erofs_xattr_ibody_header {
-	__le32 h_reserved;
+	__le32 h_map;	/* bloom filter, bit value 1 indicates not-present */
 	__u8   h_shared_count;
 	__u8   h_reserved2[7];
 	__le32 h_shared_xattrs[];       /* shared xattr id array */
@@ -221,6 +222,11 @@ struct erofs_xattr_ibody_header {
 #define EROFS_XATTR_LONG_PREFIX		0x80
 #define EROFS_XATTR_LONG_PREFIX_MASK	0x7f
 
+#define EROFS_XATTR_BLOOM_BITS		32
+#define EROFS_XATTR_BLOOM_MASK		(EROFS_XATTR_BLOOM_BITS - 1)
+#define EROFS_XATTR_BLOOM_DEFAULT	UINT32_MAX
+#define EROFS_XATTR_BLOOM_COUNTS	2
+
 /* xattr entry (for both inline & shared xattrs) */
 struct erofs_xattr_entry {
 	__u8   e_name_len;      /* length of name */
-- 
2.19.1.6.gb485710b