Re: [PATCH] blk-mq: make sure active queue usage is held for bio_integrity_prep()

Christoph Hellwig
2023-11-09T07:30:11Z

> +++ b/block/blk-mq.c
> @@ -2858,11 +2858,8 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
> };
> struct request *rq;
>
> - if (unlikely(bio_queue_enter(bio)))
> - return NULL;
> -
> if (blk_mq_attempt_bio_merge(q, bio, nsegs))
> - goto queue_exit;
> + return NULL;
>
> rq_qos_throttle(q, bio);

For clarity splitting this out might be a bit more readable.

> +static inline struct request *blk_mq_cached_req(const struct request_queue *q,
> + const struct blk_plug *plug)
> +{
> + if (plug) {
> + struct request *rq = rq_list_peek(&plug->cached_rq);
> +
> + if (rq && rq->q == q)
> + return rq;
> + }
> + return NULL;
> +}

Not sure this helper adds much value over just open coding it.

> +/* return true if this bio needs to handle by allocating new request */
> +static inline bool blk_mq_try_cached_rq(struct request *rq,
> struct blk_plug *plug, struct bio **bio, unsigned int nsegs)

The polarity here is a bit weird, I'd expect a true return if the
request can be used and a naming implying that.

> + rq = blk_mq_cached_req(q, plug);
> + if (rq) {
> + /* cached request held queue usage counter */
> + if (!bio_integrity_prep(bio))
> + return;
> +
> + need_alloc = blk_mq_try_cached_rq(rq, plug, &bio, nr_segs);
> if (!bio)
> return;

If you take the blk_mq_attempt_bio_merge merge out of the helper,
the calling convention becomes much saner.

> + /* cached request held queue usage counter */
> + percpu_ref_get(&q->q_usage_counter);

I don't think we can just do the percpu_ref_get here. While getting
the counter obviosuly can't fail, we still need to do the queue
pm only check.

Below is the untested version of how I'd do this that I hacked while
reviewing it:

---
commit 1134ce39504c30a9804ed8147027e66bf7d3157e
Author: Ming Lei <ming.lei@redhat.com>
Date: Wed Nov 8 16:05:04 2023 +0800

blk-mq: make sure active queue usage is held for bio_integrity_prep()

blk_integrity_unregister() can come if queue usage counter isn't held
for one bio with integrity prepared, so this request may be completed with
calling profile->complete_fn, then kernel panic.

Another constraint is that bio_integrity_prep() needs to be called
before bio merge.

Fix the issue by:

- call bio_integrity_prep() with one queue usage counter grabbed reliably

- call bio_integrity_prep() before bio merge

Fixes: 900e080752025f00 ("block: move queue enter logic into blk_mq_submit_bio()")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index e2d11183f62e37..b758dc8ed10957 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2858,11 +2858,8 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
};
struct request *rq;

- if (unlikely(bio_queue_enter(bio)))
- return NULL;
-
if (blk_mq_attempt_bio_merge(q, bio, nsegs))
- goto queue_exit;
+ return NULL;

rq_qos_throttle(q, bio);

@@ -2878,35 +2875,24 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
rq_qos_cleanup(q, bio);
if (bio->bi_opf & REQ_NOWAIT)
bio_wouldblock_error(bio);
-queue_exit:
- blk_queue_exit(q);
return NULL;
}

-static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
- struct blk_plug *plug, struct bio **bio, unsigned int nsegs)
+/* return true if this @rq can be used for @bio */
+static bool blk_mq_can_use_cached_rq(struct request *rq, struct blk_plug *plug,
+ struct bio *bio)
{
- struct request *rq;
- enum hctx_type type, hctx_type;
+ struct request_queue *q = rq->q;
+ enum hctx_type type = blk_mq_get_hctx_type(bio->bi_opf);
+ enum hctx_type hctx_type = rq->mq_hctx->type;

- if (!plug)
- return NULL;
- rq = rq_list_peek(&plug->cached_rq);
- if (!rq || rq->q != q)
- return NULL;
+ WARN_ON_ONCE(rq_list_peek(&plug->cached_rq) != rq);

- if (blk_mq_attempt_bio_merge(q, *bio, nsegs)) {
- *bio = NULL;
- return NULL;
- }
-
- type = blk_mq_get_hctx_type((*bio)->bi_opf);
- hctx_type = rq->mq_hctx->type;
if (type != hctx_type &&
!(type == HCTX_TYPE_READ && hctx_type == HCTX_TYPE_DEFAULT))
- return NULL;
- if (op_is_flush(rq->cmd_flags) != op_is_flush((*bio)->bi_opf))
- return NULL;
+ return false;
+ if (op_is_flush(rq->cmd_flags) != op_is_flush(bio->bi_opf))
+ return false;

/*
* If any qos ->throttle() end up blocking, we will have flushed the
@@ -2914,12 +2900,12 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
* before we throttle.
*/
plug->cached_rq = rq_list_next(rq);
- rq_qos_throttle(q, *bio);
+ rq_qos_throttle(q, bio);

blk_mq_rq_time_init(rq, 0);
- rq->cmd_flags = (*bio)->bi_opf;
+ rq->cmd_flags = bio->bi_opf;
INIT_LIST_HEAD(&rq->queuelist);
- return rq;
+ return true;
}

static void bio_set_ioprio(struct bio *bio)
@@ -2949,7 +2935,7 @@ void blk_mq_submit_bio(struct bio *bio)
struct blk_plug *plug = blk_mq_plug(bio);
const int is_sync = op_is_sync(bio->bi_opf);
struct blk_mq_hw_ctx *hctx;
- struct request *rq;
+ struct request *rq = NULL;
unsigned int nr_segs = 1;
blk_status_t ret;

@@ -2960,20 +2946,39 @@ void blk_mq_submit_bio(struct bio *bio)
return;
}

- if (!bio_integrity_prep(bio))
- return;
-
bio_set_ioprio(bio);

- rq = blk_mq_get_cached_request(q, plug, &bio, nr_segs);
- if (!rq) {
- if (!bio)
+ if (plug) {
+ rq = rq_list_peek(&plug->cached_rq);
+ if (rq && rq->q != q)
+ rq = NULL;
+ }
+ if (rq) {
+ /* rq already holds a q_usage_counter reference */
+ if (!bio_integrity_prep(bio))
return;
- rq = blk_mq_get_new_requests(q, plug, bio, nr_segs);
- if (unlikely(!rq))
+ if (blk_mq_attempt_bio_merge(q, bio, nr_segs))
+ return;
+
+ if (blk_mq_can_use_cached_rq(rq, plug, bio))
+ goto done;
+
+ percpu_ref_get(&q->q_usage_counter);
+ } else {
+ if (unlikely(bio_queue_enter(bio)))
+ return;
+
+ if (!bio_integrity_prep(bio))
return;
}

+ rq = blk_mq_get_new_requests(q, plug, bio, nr_segs);
+ if (unlikely(!rq)) {
+ blk_queue_exit(q);
+ return;
+ }
+
+done:
trace_block_getrq(bio);

rq_qos_track(q, rq, bio);