block: don't update BLK_FEAT_POLL in __blk_mq_update_nr_hw_queues

JIRA: https://issues.redhat.com/browse/RHEL-71345
Upstream Status: for-6.4/block

commit d432c81
Author: Christoph Hellwig <hch@lst.de>
Date:   Fri Jan 10 06:47:12 2025 +0100

    block: don't update BLK_FEAT_POLL in __blk_mq_update_nr_hw_queues

    When __blk_mq_update_nr_hw_queues changes the number of tag sets, it
    might have to disable poll queues.  Currently it does so by adjusting
    the BLK_FEAT_POLL, which is a bit against the intent of features that
    describe hardware / driver capabilities, but more importantly causes
    nasty lock order problems with the broadly held freeze when updating the
    number of hardware queues and the limits lock.  Fix this by leaving
    BLK_FEAT_POLL alone, and instead check for the number of poll queues in
    the bio submission and poll handlers.  While this adds extra work to the
    fast path, the variables are in cache lines used by these operations
    anyway, so it should be cheap enough.

    Fixes: 8023e14 ("block: move the poll flag to queue_limits")
    Signed-off-by: Christoph Hellwig <hch@lst.de>
    Reviewed-by: Ming Lei <ming.lei@redhat.com>
    Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
    Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
    Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
    Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
    Link: https://lore.kernel.org/r/20250110054726.1499538-5-hch@lst.de
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

block/blk-core.c

@@ -934,14 +934,13 @@ int bio_poll(struct bio *bio, struct io_comp_batch *iob, unsigned int flags)
 	 */
 	if (!percpu_ref_tryget(&q->q_usage_counter))
 		return 0;
-	if (!(q->limits.features & BLK_FEAT_POLL)) {
-		ret = 0;
-	} else if (queue_is_mq(q)) {
+	if (queue_is_mq(q)) {
 		ret = blk_mq_poll(q, cookie, iob, flags);
 	} else {
 		struct gendisk *disk = q->disk;
 
-		if (disk && disk->fops->poll_bio)
+		if ((q->limits.features & BLK_FEAT_POLL) && disk &&
+		    disk->fops->poll_bio)
 			ret = disk->fops->poll_bio(bio, iob, flags);
 	}
 	blk_queue_exit(q);

block/blk-mq.c

@@ -2985,8 +2985,7 @@ void blk_mq_submit_bio(struct bio *bio)
 		goto queue_exit;
 	}
 
-	if ((bio->bi_opf & REQ_POLLED) &&
-	    !(q->limits.features & BLK_FEAT_POLL)) {
+	if ((bio->bi_opf & REQ_POLLED) && !blk_mq_can_poll(q)) {
 		bio->bi_status = BLK_STS_NOTSUPP;
 		bio_endio(bio);
 		goto queue_exit;
@@ -4138,12 +4137,6 @@ void blk_mq_release(struct request_queue *q)
 	blk_mq_sysfs_deinit(q);
 }
 
-static bool blk_mq_can_poll(struct blk_mq_tag_set *set)
-{
-	return set->nr_maps > HCTX_TYPE_POLL &&
-		set->map[HCTX_TYPE_POLL].nr_queues;
-}
-
 struct request_queue *blk_mq_alloc_queue(struct blk_mq_tag_set *set,
 		struct queue_limits *lim, void *queuedata)
 {
@@ -4154,7 +4147,7 @@ struct request_queue *blk_mq_alloc_queue(struct blk_mq_tag_set *set,
 	if (!lim)
 		lim = &default_lim;
 	lim->features |= BLK_FEAT_IO_STAT | BLK_FEAT_NOWAIT;
-	if (blk_mq_can_poll(set))
+	if (set->nr_maps > HCTX_TYPE_POLL)
 		lim->features |= BLK_FEAT_POLL;
 
 	q = blk_alloc_queue(lim, set->numa_node);
@@ -4827,8 +4820,6 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 fallback:
 	blk_mq_update_queue_map(set);
 	list_for_each_entry(q, &set->tag_list, tag_set_list) {
-		struct queue_limits lim;
-
 		blk_mq_realloc_hw_ctxs(set, q);
 
 		if (q->nr_hw_queues != set->nr_hw_queues) {
@@ -4842,13 +4833,6 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 			set->nr_hw_queues = prev_nr_hw_queues;
 			goto fallback;
 		}
-		lim = queue_limits_start_update(q);
-		if (blk_mq_can_poll(set))
-			lim.features |= BLK_FEAT_POLL;
-		else
-			lim.features &= ~BLK_FEAT_POLL;
-		if (queue_limits_commit_update(q, &lim) < 0)
-			pr_warn("updating the poll flag failed\n");
 		blk_mq_map_swqueue(q);
 	}
 
@@ -4908,9 +4892,9 @@ static int blk_hctx_poll(struct request_queue *q, struct blk_mq_hw_ctx *hctx,
 int blk_mq_poll(struct request_queue *q, blk_qc_t cookie,
 		struct io_comp_batch *iob, unsigned int flags)
 {
-	struct blk_mq_hw_ctx *hctx = xa_load(&q->hctx_table, cookie);
-
-	return blk_hctx_poll(q, hctx, iob, flags);
+	if (!blk_mq_can_poll(q))
+		return 0;
+	return blk_hctx_poll(q, xa_load(&q->hctx_table, cookie), iob, flags);
 }
 
 int blk_rq_poll(struct request *rq, struct io_comp_batch *iob,

block/blk-mq.h

@@ -438,4 +438,10 @@ do {								\
 #define blk_mq_run_dispatch_ops(q, dispatch_ops)		\
 	__blk_mq_run_dispatch_ops(q, true, dispatch_ops)	\
 
+static inline bool blk_mq_can_poll(struct request_queue *q)
+{
+	return (q->limits.features & BLK_FEAT_POLL) &&
+		q->tag_set->map[HCTX_TYPE_POLL].nr_queues;
+}
+
 #endif

block/blk-sysfs.c

@@ -256,10 +256,17 @@ static ssize_t queue_##_name##_show(struct gendisk *disk, char *page)	\
 		!!(disk->queue->limits.features & _feature));		\
 }
 
-QUEUE_SYSFS_FEATURE_SHOW(poll, BLK_FEAT_POLL);
 QUEUE_SYSFS_FEATURE_SHOW(fua, BLK_FEAT_FUA);
 QUEUE_SYSFS_FEATURE_SHOW(dax, BLK_FEAT_DAX);
 
+static ssize_t queue_poll_show(struct gendisk *disk, char *page)
+{
+	if (queue_is_mq(disk->queue))
+		return sysfs_emit(page, "%u\n", blk_mq_can_poll(disk->queue));
+	return sysfs_emit(page, "%u\n",
+		!!(disk->queue->limits.features & BLK_FEAT_POLL));
+}
+
 static ssize_t queue_zoned_show(struct gendisk *disk, char *page)
 {
 	if (blk_queue_is_zoned(disk->queue))