erofs: rename ctime to mtime

EROFS images should inherit modification time rather than change time,
since users and host tooling have no easy way to control change time.

To reflect the new timestamp meaning, i_ctime and i_ctime_nsec are
renamed to i_mtime and i_mtime_nsec.

Link: https://lore.kernel.org/r/20220311041829.3109511-1-dvander@google.com # v1
Signed-off-by: David Anderson <dvander@google.com>
[ Gao Xiang: update document as well. ]
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220317114959.106787-1-hsiangkao@linux.alibaba.com # v2
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Documentation/filesystems/erofs.rst

@@ -40,7 +40,7 @@ Here is the main features of EROFS:
    Inode metadata size    32 bytes      64 bytes
    Max file size          4 GB          16 EB (also limited by max. vol size)
    Max uids/gids          65536         4294967296
-   File change time       no            yes (64 + 32-bit timestamp)
+   Per-inode timestamp    no            yes (64 + 32-bit timestamp)
    Max hardlinks          65536         4294967296
    Metadata reserved      4 bytes       14 bytes
    =====================  ============  =====================================

fs/erofs/erofs_fs.h

@@ -12,6 +12,7 @@
 #define EROFS_SUPER_OFFSET      1024
 
 #define EROFS_FEATURE_COMPAT_SB_CHKSUM          0x00000001
+#define EROFS_FEATURE_COMPAT_MTIME              0x00000002
 
 /*
  * Any bits that aren't in EROFS_ALL_FEATURE_INCOMPAT should
@@ -186,8 +187,8 @@ struct erofs_inode_extended {
 
 	__le32 i_uid;
 	__le32 i_gid;
-	__le64 i_ctime;
-	__le32 i_ctime_nsec;
+	__le64 i_mtime;
+	__le32 i_mtime_nsec;
 	__le32 i_nlink;
 	__u8   i_reserved2[16];
 };

fs/erofs/inode.c

@@ -113,8 +113,8 @@ static void *erofs_read_inode(struct erofs_buf *buf,
 		set_nlink(inode, le32_to_cpu(die->i_nlink));
 
 		/* extended inode has its own timestamp */
-		inode->i_ctime.tv_sec = le64_to_cpu(die->i_ctime);
-		inode->i_ctime.tv_nsec = le32_to_cpu(die->i_ctime_nsec);
+		inode->i_ctime.tv_sec = le64_to_cpu(die->i_mtime);
+		inode->i_ctime.tv_nsec = le32_to_cpu(die->i_mtime_nsec);
 
 		inode->i_size = le64_to_cpu(die->i_size);