ext4: add EXT4_FLAGS_EMERGENCY_RO bit

EXT4_FLAGS_EMERGENCY_RO Indicates that the current file system has become
read-only due to some error. Compared to SB_RDONLY, setting it does not
require a lock because we won't clear it, which avoids over-coupling with
vfs freeze. Also, add a helper function ext4_emergency_ro() to check if
the bit is set.

Signed-off-by: Baokun Li <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Zhang Yi <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

Author: LiBaokun96

fs/ext4/ext4.h

@@ -2239,13 +2239,19 @@ enum {
 	EXT4_FLAGS_RESIZING,	/* Avoid superblock update and resize race */
 	EXT4_FLAGS_SHUTDOWN,	/* Prevent access to the file system */
 	EXT4_FLAGS_BDEV_IS_DAX,	/* Current block device support DAX */
+	EXT4_FLAGS_EMERGENCY_RO,/* Emergency read-only due to fs errors */
 };
 
 static inline int ext4_forced_shutdown(struct super_block *sb)
 {
 	return test_bit(EXT4_FLAGS_SHUTDOWN, &EXT4_SB(sb)->s_ext4_flags);
 }
 
+static inline int ext4_emergency_ro(struct super_block *sb)
+{
+	return test_bit(EXT4_FLAGS_EMERGENCY_RO, &EXT4_SB(sb)->s_ext4_flags);
+}
+
 /*
  * Default values for user and/or group using reserved blocks
  */