ext4: clear DISCARD flag if device does not support discard

commit 79add3a ("ext4: notify when discard is not supported")
noted that keeping the DISCARD flag is for possibility that the underlying
device might change in future even without file system remount. However,
this scenario has rarely occurred in practice on the device side. Even if
it does occur, it can be resolved with remount. Clearing the DISCARD flag
not only prevents confusion caused by mount options but also avoids
sending unnecessary discard commands.

Signed-off-by: Diangang Li <lidiangang@bytedance.com>
Link: https://patch.msgid.link/20250311021310.669524-1-lidiangang@bytedance.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: Diangang Li

fs/ext4/super.c

@@ -5620,9 +5620,11 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
 			goto failed_mount9;
 	}
 
-	if (test_opt(sb, DISCARD) && !bdev_max_discard_sectors(sb->s_bdev))
+	if (test_opt(sb, DISCARD) && !bdev_max_discard_sectors(sb->s_bdev)) {
 		ext4_msg(sb, KERN_WARNING,
 			 "mounting with \"discard\" option, but the device does not support discard");
+		clear_opt(sb, DISCARD);
+	}
 
 	if (es->s_error_count)
 		mod_timer(&sbi->s_err_report, jiffies + 300*HZ); /* 5 minutes */