Re: [PATCH v2] virtio_blk: implement init_hctx MQ operation

[Marek Szyprowski <m.szyprowski@samsung.com>]

Dear All,

On 08.08.2024 00:41, Max Gurtovoy wrote:
> Set the driver data of the hardware context (hctx) to point directly to
> the virtio block queue. This cleanup improves code readability and
> reduces the number of dereferences in the fast path.
>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> ---
>   drivers/block/virtio_blk.c | 42 ++++++++++++++++++++------------------
>   1 file changed, 22 insertions(+), 20 deletions(-)

This patch landed in recent linux-next as commit 8d04556131c1 
("virtio_blk: implement init_hctx MQ operation"). In my tests I found 
that it introduces a regression in system suspend/resume operation. From 
time to time system crashes during suspend/resume cycle. Reverting this 
patch on top of next-20240911 fixes this problem.

I've even managed to catch a kernel panic log of this problem on QEMU's 
ARM64 'virt' machine:

root@target:~# time rtcwake -s10 -mmem
rtcwake: wakeup from "mem" using /dev/rtc0 at Thu Sep 12 07:11:52 2024
Unable to handle kernel NULL pointer dereference at virtual address 
0000000000000090
Mem abort info:
   ESR = 0x0000000096000046
   EC = 0x25: DABT (current EL), IL = 32 bits
   SET = 0, FnV = 0
   EA = 0, S1PTW = 0
   FSC = 0x06: level 2 translation fault
Data abort info:
   ISV = 0, ISS = 0x00000046, ISS2 = 0x00000000
   CM = 0, WnR = 1, TnD = 0, TagAccess = 0
   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
user pgtable: 4k pages, 48-bit VAs, pgdp=0000000046bbb000
...
Internal error: Oops: 0000000096000046 [#1] PREEMPT SMP
Modules linked in: bluetooth ecdh_generic ecc rfkill ipv6
CPU: 0 UID: 0 PID: 9 Comm: kworker/0:0H Not tainted 6.11.0-rc6+ #9024
Hardware name: linux,dummy-virt (DT)
Workqueue: kblockd blk_mq_requeue_work
pstate: 800000c5 (Nzcv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : virtqueue_add_split+0x458/0x63c
lr : virtqueue_add_split+0x1d0/0x63c
...
Call trace:
  virtqueue_add_split+0x458/0x63c
  virtqueue_add_sgs+0xc4/0xec
  virtblk_add_req+0x8c/0xf4
  virtio_queue_rq+0x6c/0x1bc
  blk_mq_dispatch_rq_list+0x21c/0x714
  __blk_mq_sched_dispatch_requests+0xb4/0x58c
  blk_mq_sched_dispatch_requests+0x30/0x6c
  blk_mq_run_hw_queue+0x14c/0x40c
  blk_mq_run_hw_queues+0x64/0x124
  blk_mq_requeue_work+0x188/0x1bc
  process_one_work+0x20c/0x608
  worker_thread+0x238/0x370
  kthread+0x124/0x128
  ret_from_fork+0x10/0x20
Code: f9404282 79401c21 b9004a81 f94047e1 (f8206841)
---[ end trace 0000000000000000 ]---
note: kworker/0:0H[9] exited with irqs disabled
note: kworker/0:0H[9] exited with preempt_count 1


> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 2351f411fa46..35a7a586f6f5 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -129,14 +129,6 @@ static inline blk_status_t virtblk_result(u8 status)
>   	}
>   }
>   
> -static inline struct virtio_blk_vq *get_virtio_blk_vq(struct blk_mq_hw_ctx *hctx)
> -{
> -	struct virtio_blk *vblk = hctx->queue->queuedata;
> -	struct virtio_blk_vq *vq = &vblk->vqs[hctx->queue_num];
> -
> -	return vq;
> -}
> -
>   static int virtblk_add_req(struct virtqueue *vq, struct virtblk_req *vbr)
>   {
>   	struct scatterlist out_hdr, in_hdr, *sgs[3];
> @@ -377,8 +369,7 @@ static void virtblk_done(struct virtqueue *vq)
>   
>   static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
>   {
> -	struct virtio_blk *vblk = hctx->queue->queuedata;
> -	struct virtio_blk_vq *vq = &vblk->vqs[hctx->queue_num];
> +	struct virtio_blk_vq *vq = hctx->driver_data;
>   	bool kick;
>   
>   	spin_lock_irq(&vq->lock);
> @@ -428,10 +419,10 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>   			   const struct blk_mq_queue_data *bd)
>   {
>   	struct virtio_blk *vblk = hctx->queue->queuedata;
> +	struct virtio_blk_vq *vq = hctx->driver_data;
>   	struct request *req = bd->rq;
>   	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
>   	unsigned long flags;
> -	int qid = hctx->queue_num;
>   	bool notify = false;
>   	blk_status_t status;
>   	int err;
> @@ -440,26 +431,26 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>   	if (unlikely(status))
>   		return status;
>   
> -	spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
> -	err = virtblk_add_req(vblk->vqs[qid].vq, vbr);
> +	spin_lock_irqsave(&vq->lock, flags);
> +	err = virtblk_add_req(vq->vq, vbr);
>   	if (err) {
> -		virtqueue_kick(vblk->vqs[qid].vq);
> +		virtqueue_kick(vq->vq);
>   		/* Don't stop the queue if -ENOMEM: we may have failed to
>   		 * bounce the buffer due to global resource outage.
>   		 */
>   		if (err == -ENOSPC)
>   			blk_mq_stop_hw_queue(hctx);
> -		spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
> +		spin_unlock_irqrestore(&vq->lock, flags);
>   		virtblk_unmap_data(req, vbr);
>   		return virtblk_fail_to_queue(req, err);
>   	}
>   
> -	if (bd->last && virtqueue_kick_prepare(vblk->vqs[qid].vq))
> +	if (bd->last && virtqueue_kick_prepare(vq->vq))
>   		notify = true;
> -	spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
> +	spin_unlock_irqrestore(&vq->lock, flags);
>   
>   	if (notify)
> -		virtqueue_notify(vblk->vqs[qid].vq);
> +		virtqueue_notify(vq->vq);
>   	return BLK_STS_OK;
>   }
>   
> @@ -504,7 +495,7 @@ static void virtio_queue_rqs(struct request **rqlist)
>   	struct request *requeue_list = NULL;
>   
>   	rq_list_for_each_safe(rqlist, req, next) {
> -		struct virtio_blk_vq *vq = get_virtio_blk_vq(req->mq_hctx);
> +		struct virtio_blk_vq *vq = req->mq_hctx->driver_data;
>   		bool kick;
>   
>   		if (!virtblk_prep_rq_batch(req)) {
> @@ -1164,6 +1155,16 @@ static const struct attribute_group *virtblk_attr_groups[] = {
>   	NULL,
>   };
>   
> +static int virtblk_init_hctx(struct blk_mq_hw_ctx *hctx, void *data,
> +		unsigned int hctx_idx)
> +{
> +	struct virtio_blk *vblk = data;
> +	struct virtio_blk_vq *vq = &vblk->vqs[hctx_idx];
> +
> +	hctx->driver_data = vq;
> +	return 0;
> +}
> +
>   static void virtblk_map_queues(struct blk_mq_tag_set *set)
>   {
>   	struct virtio_blk *vblk = set->driver_data;
> @@ -1205,7 +1206,7 @@ static void virtblk_complete_batch(struct io_comp_batch *iob)
>   static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
>   {
>   	struct virtio_blk *vblk = hctx->queue->queuedata;
> -	struct virtio_blk_vq *vq = get_virtio_blk_vq(hctx);
> +	struct virtio_blk_vq *vq = hctx->driver_data;
>   	struct virtblk_req *vbr;
>   	unsigned long flags;
>   	unsigned int len;
> @@ -1236,6 +1237,7 @@ static const struct blk_mq_ops virtio_mq_ops = {
>   	.queue_rqs	= virtio_queue_rqs,
>   	.commit_rqs	= virtio_commit_rqs,
>   	.complete	= virtblk_request_done,
> +	.init_hctx	= virtblk_init_hctx,
>   	.map_queues	= virtblk_map_queues,
>   	.poll		= virtblk_poll,
>   };

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland