ext4: correct behavior under errors=remount-ro mode

And after commit 9525798 ("ext4: drop EXT4_MF_FS_ABORTED flag") in
v6.6-rc1, the EXT4_FLAGS_SHUTDOWN bit is set in ext4_handle_error() under
errors=remount-ro mode. This causes the read to fail even when the error
is triggered in errors=remount-ro mode.

To correct the behavior under errors=remount-ro, EXT4_FLAGS_SHUTDOWN is
replaced by the newly introduced EXT4_FLAGS_EMERGENCY_RO. This new flag
only prevents writes, matching the previous behavior with SB_RDONLY.

Fixes: 9525798 ("ext4: drop EXT4_MF_FS_ABORTED flag")
Closes: https://lore.kernel.org/all/[email protected]/
Suggested-by: Jan Kara <[email protected]>
Signed-off-by: Baokun Li <[email protected]>
Reviewed-by: Zhang Yi <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

Author: LiBaokun96

fs/ext4/super.c

@@ -693,11 +693,8 @@ static void ext4_handle_error(struct super_block *sb, bool force_ro, int error,
 	if (test_opt(sb, WARN_ON_ERROR))
 		WARN_ON_ONCE(1);
 
-	if (!continue_fs && !sb_rdonly(sb)) {
-		set_bit(EXT4_FLAGS_SHUTDOWN, &EXT4_SB(sb)->s_ext4_flags);
-		if (journal)
-			jbd2_journal_abort(journal, -EIO);
-	}
+	if (!continue_fs && !ext4_emergency_ro(sb) && journal)
+		jbd2_journal_abort(journal, -EIO);
 
 	if (!bdev_read_only(sb->s_bdev)) {
 		save_error_info(sb, error, ino, block, func, line);
@@ -723,17 +720,17 @@ static void ext4_handle_error(struct super_block *sb, bool force_ro, int error,
 			sb->s_id);
 	}
 
-	if (sb_rdonly(sb) || continue_fs)
+	if (ext4_emergency_ro(sb) || continue_fs)
 		return;
 
 	ext4_msg(sb, KERN_CRIT, "Remounting filesystem read-only");
 	/*
-	 * EXT4_FLAGS_SHUTDOWN was set which stops all filesystem
-	 * modifications. We don't set SB_RDONLY because that requires
-	 * sb->s_umount semaphore and setting it without proper remount
-	 * procedure is confusing code such as freeze_super() leading to
-	 * deadlocks and other problems.
+	 * We don't set SB_RDONLY because that requires sb->s_umount
+	 * semaphore and setting it without proper remount procedure is
+	 * confusing code such as freeze_super() leading to deadlocks
+	 * and other problems.
 	 */
+	set_bit(EXT4_FLAGS_EMERGENCY_RO, &EXT4_SB(sb)->s_ext4_flags);
 }
 
 static void update_super_work(struct work_struct *work)