Re: [PATCH] nvme-tcp: fix usage of page_frag_cache

[Chris Leech <cleech@redhat.com>]

On Mon, Sep 29, 2025 at 02:19:51PM +0300, Dmitry Bogdanov wrote:
> nvme uses page_frag_cache to preallocate PDU for each preallocated request
> of block device. Block devices are created in parallel threads,
> consequently page_frag_cache is used in not thread-safe manner.
> That leads to incorrect refcounting of backstore pages and premature free.
> 
> That can be catched by !sendpage_ok inside network stack:
> 
> WARNING: CPU: 7 PID: 467 at ../net/core/skbuff.c:6931 skb_splice_from_iter+0xfa/0x310.
> 	tcp_sendmsg_locked+0x782/0xce0
> 	tcp_sendmsg+0x27/0x40
> 	sock_sendmsg+0x8b/0xa0
> 	nvme_tcp_try_send_cmd_pdu+0x149/0x2a0
> Then random panic may occur.
> 
> Fix that by serializing the usage of page_frag_cache.

Thank you for reporting this. I think we can fix it without blocking the
async namespace scanning with a mutex, by switching from a per-queue
page_frag_cache to per-cpu. There shouldn't be a need to keep the
page_frag allocations isolated by queue anyway.

It would be great if you could test the patch which I'll send after
this.

- Chris
 
> Cc: stable@vger.kernel.org # 6.12
> Fixes: 4e893ca81170 ("nvme_core: scan namespaces asynchronously")
> Signed-off-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
> ---
>  drivers/nvme/host/tcp.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/nvme/host/tcp.c b/drivers/nvme/host/tcp.c
> index 1413788ca7d52..823e07759e0d3 100644
> --- a/drivers/nvme/host/tcp.c
> +++ b/drivers/nvme/host/tcp.c
> @@ -145,6 +145,7 @@ struct nvme_tcp_queue {
>  
>  	struct mutex		queue_lock;
>  	struct mutex		send_mutex;
> +	struct mutex		pf_cache_lock;
>  	struct llist_head	req_list;
>  	struct list_head	send_list;
>  
> @@ -556,9 +557,11 @@ static int nvme_tcp_init_request(struct blk_mq_tag_set *set,
>  	struct nvme_tcp_queue *queue = &ctrl->queues[queue_idx];
>  	u8 hdgst = nvme_tcp_hdgst_len(queue);
>  
> +	mutex_lock(&queue->pf_cache_lock);
>  	req->pdu = page_frag_alloc(&queue->pf_cache,
>  		sizeof(struct nvme_tcp_cmd_pdu) + hdgst,
>  		GFP_KERNEL | __GFP_ZERO);
> +	mutex_unlock(&queue->pf_cache_lock);
>  	if (!req->pdu)
>  		return -ENOMEM;
>  
> @@ -1420,9 +1423,11 @@ static int nvme_tcp_alloc_async_req(struct nvme_tcp_ctrl *ctrl)
>  	struct nvme_tcp_request *async = &ctrl->async_req;
>  	u8 hdgst = nvme_tcp_hdgst_len(queue);
>  
> +	mutex_lock(&queue->pf_cache_lock);
>  	async->pdu = page_frag_alloc(&queue->pf_cache,
>  		sizeof(struct nvme_tcp_cmd_pdu) + hdgst,
>  		GFP_KERNEL | __GFP_ZERO);
> +	mutex_unlock(&queue->pf_cache_lock);
>  	if (!async->pdu)
>  		return -ENOMEM;
>  
> @@ -1450,6 +1455,7 @@ static void nvme_tcp_free_queue(struct nvme_ctrl *nctrl, int qid)
>  	kfree(queue->pdu);
>  	mutex_destroy(&queue->send_mutex);
>  	mutex_destroy(&queue->queue_lock);
> +	mutex_destroy(&queue->pf_cache_lock);
>  }
>  
>  static int nvme_tcp_init_connection(struct nvme_tcp_queue *queue)
> @@ -1772,6 +1778,7 @@ static int nvme_tcp_alloc_queue(struct nvme_ctrl *nctrl, int qid,
>  	INIT_LIST_HEAD(&queue->send_list);
>  	mutex_init(&queue->send_mutex);
>  	INIT_WORK(&queue->io_work, nvme_tcp_io_work);
> +	mutex_init(&queue->pf_cache_lock);
>  
>  	if (qid > 0)
>  		queue->cmnd_capsule_len = nctrl->ioccsz * 16;
> @@ -1903,6 +1910,7 @@ static int nvme_tcp_alloc_queue(struct nvme_ctrl *nctrl, int qid,
>  err_destroy_mutex:
>  	mutex_destroy(&queue->send_mutex);
>  	mutex_destroy(&queue->queue_lock);
> +	mutex_destroy(&queue->pf_cache_lock);
>  	return ret;
>  }
>  
> -- 
> 2.25.1
> 
>