block: Catch possible entries missing from alloc_policy_name[]

Make BLK_TAG_ALLOC_x an enum and add a "max" entry.

Add a BUILD_BUG_ON() call to ensure that we are not missing entries in
hctx_flag_name[].
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20240719112912.3830443-12-john.g.garry@oracle.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-debugfs.c

@@ -198,6 +198,7 @@ static int hctx_flags_show(void *data, struct seq_file *m)
 
 	BUILD_BUG_ON(ARRAY_SIZE(hctx_flag_name) !=
 			BLK_MQ_F_ALLOC_POLICY_START_BIT);
+	BUILD_BUG_ON(ARRAY_SIZE(alloc_policy_name) != BLK_TAG_ALLOC_MAX);
 
 	seq_puts(m, "alloc_policy=");
 	if (alloc_policy < ARRAY_SIZE(alloc_policy_name) &&

include/linux/blk-mq.h

@@ -278,8 +278,12 @@ enum blk_eh_timer_return {
 	BLK_EH_RESET_TIMER,
 };
 
-#define BLK_TAG_ALLOC_FIFO 0 /* allocate starting from 0 */
-#define BLK_TAG_ALLOC_RR 1 /* allocate starting from last allocated tag */
+/* Keep alloc_policy_name[] in sync with the definitions below */
+enum {
+	BLK_TAG_ALLOC_FIFO,	/* allocate starting from 0 */
+	BLK_TAG_ALLOC_RR,	/* allocate starting from last allocated tag */
+	BLK_TAG_ALLOC_MAX
+};
 
 /**
  * struct blk_mq_hw_ctx - State for a hardware queue facing the hardware