fs: introduce a wrapper uuid_to_fsid()

Some filesystem's use a digest of their uuid for f_fsid.
Create a simple wrapper for this open coded folding.

Filesystems that have a non null uuid but use the block device
number for f_fsid may also consider using this helper.

[JK: Added missing asm/byteorder.h include]
Link: https://lore.kernel.org/r/20210322173944.449469-2-amir73il@gmail.com
Acked-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext2/super.c

@@ -1399,7 +1399,6 @@ static int ext2_statfs (struct dentry * dentry, struct kstatfs * buf)
 	struct super_block *sb = dentry->d_sb;
 	struct ext2_sb_info *sbi = EXT2_SB(sb);
 	struct ext2_super_block *es = sbi->s_es;
-	u64 fsid;
 
 	spin_lock(&sbi->s_lock);
 
@@ -1453,9 +1452,7 @@ static int ext2_statfs (struct dentry * dentry, struct kstatfs * buf)
 	buf->f_ffree = ext2_count_free_inodes(sb);
 	es->s_free_inodes_count = cpu_to_le32(buf->f_ffree);
 	buf->f_namelen = EXT2_NAME_LEN;
-	fsid = le64_to_cpup((void *)es->s_uuid) ^
-	       le64_to_cpup((void *)es->s_uuid + sizeof(u64));
-	buf->f_fsid = u64_to_fsid(fsid);
+	buf->f_fsid = uuid_to_fsid(es->s_uuid);
 	spin_unlock(&sbi->s_lock);
 	return 0;
 }

fs/ext4/super.c

@@ -6148,7 +6148,6 @@ static int ext4_statfs(struct dentry *dentry, struct kstatfs *buf)
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct ext4_super_block *es = sbi->s_es;
 	ext4_fsblk_t overhead = 0, resv_blocks;
-	u64 fsid;
 	s64 bfree;
 	resv_blocks = EXT4_C2B(sbi, atomic64_read(&sbi->s_resv_clusters));
 
@@ -6169,9 +6168,7 @@ static int ext4_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_files = le32_to_cpu(es->s_inodes_count);
 	buf->f_ffree = percpu_counter_sum_positive(&sbi->s_freeinodes_counter);
 	buf->f_namelen = EXT4_NAME_LEN;
-	fsid = le64_to_cpup((void *)es->s_uuid) ^
-	       le64_to_cpup((void *)es->s_uuid + sizeof(u64));
-	buf->f_fsid = u64_to_fsid(fsid);
+	buf->f_fsid = uuid_to_fsid(es->s_uuid);
 
 #ifdef CONFIG_QUOTA
 	if (ext4_test_inode_flag(dentry->d_inode, EXT4_INODE_PROJINHERIT) &&

fs/zonefs/super.c

@@ -1104,7 +1104,6 @@ static int zonefs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	struct super_block *sb = dentry->d_sb;
 	struct zonefs_sb_info *sbi = ZONEFS_SB(sb);
 	enum zonefs_ztype t;
-	u64 fsid;
 
 	buf->f_type = ZONEFS_MAGIC;
 	buf->f_bsize = sb->s_blocksize;
@@ -1127,9 +1126,7 @@ static int zonefs_statfs(struct dentry *dentry, struct kstatfs *buf)
 
 	spin_unlock(&sbi->s_lock);
 
-	fsid = le64_to_cpup((void *)sbi->s_uuid.b) ^
-		le64_to_cpup((void *)sbi->s_uuid.b + sizeof(u64));
-	buf->f_fsid = u64_to_fsid(fsid);
+	buf->f_fsid = uuid_to_fsid(sbi->s_uuid.b);
 
 	return 0;
 }

include/linux/statfs.h

@@ -4,6 +4,7 @@
 
 #include <linux/types.h>
 #include <asm/statfs.h>
+#include <asm/byteorder.h>
 
 struct kstatfs {
 	long f_type;
@@ -50,4 +51,11 @@ static inline __kernel_fsid_t u64_to_fsid(u64 v)
 	return (__kernel_fsid_t){.val = {(u32)v, (u32)(v>>32)}};
 }
 
+/* Fold 16 bytes uuid to 64 bit fsid */
+static inline __kernel_fsid_t uuid_to_fsid(__u8 *uuid)
+{
+	return u64_to_fsid(le64_to_cpup((void *)uuid) ^
+		le64_to_cpup((void *)(uuid + sizeof(u64))));
+}
+
 #endif