ext4: store checksum seed in superblock

Allow the filesystem to store the metadata checksum seed in the
superblock and add an incompat feature to say that we're using it.
This enables tune2fs to change the UUID on a mounted metadata_csum
FS without having to (racy!) rewrite all disk metadata.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -1184,7 +1184,8 @@ struct ext4_super_block {
 	__u8	s_encrypt_pw_salt[16];	/* Salt used for string2key algorithm */
 	__le32	s_lpf_ino;		/* Location of the lost+found inode */
 	__le32	s_prj_quota_inum;	/* inode for tracking project quota */
-	__le32	s_reserved[99];		/* Padding to the end of the block */
+	__le32	s_checksum_seed;	/* crc32c(uuid) if csum_seed set */
+	__le32	s_reserved[98];		/* Padding to the end of the block */
 	__le32	s_checksum;		/* crc32c(superblock) */
 };
 
@@ -1584,7 +1585,7 @@ static inline int ext4_encrypted_inode(struct inode *inode)
 #define EXT4_FEATURE_INCOMPAT_FLEX_BG		0x0200
 #define EXT4_FEATURE_INCOMPAT_EA_INODE		0x0400 /* EA in inode */
 #define EXT4_FEATURE_INCOMPAT_DIRDATA		0x1000 /* data in dirent */
-#define EXT4_FEATURE_INCOMPAT_BG_USE_META_CSUM	0x2000 /* use crc32c for bg */
+#define EXT4_FEATURE_INCOMPAT_CSUM_SEED		0x2000
 #define EXT4_FEATURE_INCOMPAT_LARGEDIR		0x4000 /* >2GB or 3-lvl htree */
 #define EXT4_FEATURE_INCOMPAT_INLINE_DATA	0x8000 /* data in inode */
 #define EXT4_FEATURE_INCOMPAT_ENCRYPT		0x10000
@@ -1613,7 +1614,8 @@ static inline int ext4_encrypted_inode(struct inode *inode)
 					 EXT4_FEATURE_INCOMPAT_FLEX_BG| \
 					 EXT4_FEATURE_INCOMPAT_MMP | \
 					 EXT4_FEATURE_INCOMPAT_INLINE_DATA | \
-					 EXT4_FEATURE_INCOMPAT_ENCRYPT)
+					 EXT4_FEATURE_INCOMPAT_ENCRYPT | \
+					 EXT4_FEATURE_INCOMPAT_CSUM_SEED)
 #define EXT4_FEATURE_RO_COMPAT_SUPP	(EXT4_FEATURE_RO_COMPAT_SPARSE_SUPER| \
 					 EXT4_FEATURE_RO_COMPAT_LARGE_FILE| \
 					 EXT4_FEATURE_RO_COMPAT_GDT_CSUM| \

fs/ext4/super.c

@@ -3196,7 +3196,9 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	}
 
 	/* Precompute checksum seed for all metadata */
-	if (ext4_has_metadata_csum(sb))
+	if (EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_CSUM_SEED))
+		sbi->s_csum_seed = le32_to_cpu(es->s_checksum_seed);
+	else if (ext4_has_metadata_csum(sb))
 		sbi->s_csum_seed = ext4_chksum(sbi, ~0, es->s_uuid,
 					       sizeof(es->s_uuid));
 

fs/ext4/sysfs.c

@@ -224,12 +224,14 @@ EXT4_ATTR_FEATURE(lazy_itable_init);
 EXT4_ATTR_FEATURE(batched_discard);
 EXT4_ATTR_FEATURE(meta_bg_resize);
 EXT4_ATTR_FEATURE(encryption);
+EXT4_ATTR_FEATURE(metadata_csum_seed);
 
 static struct attribute *ext4_feat_attrs[] = {
 	ATTR_LIST(lazy_itable_init),
 	ATTR_LIST(batched_discard),
 	ATTR_LIST(meta_bg_resize),
 	ATTR_LIST(encryption),
+	ATTR_LIST(metadata_csum_seed),
 	NULL,
 };