linux-kernel - Re: [PATCH] nvme-tcp: switch to per-cpu page_frag

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Message-ID: <20251001162459.GA4234@yadro.com>
Date: Wed, 1 Oct 2025 19:24:59 +0300
From: Dmitry Bogdanov <d.bogdanov@...ro.com>
To: Chris Leech <cleech@...hat.com>
CC: Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>, "Christoph
 Hellwig" <hch@....de>, Sagi Grimberg <sagi@...mberg.me>, Stuart Hayes
	<stuart.w.hayes@...il.com>, <linux-nvme@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <linux@...ro.com>, <stable@...r.kernel.org>
Subject: Re: [PATCH] nvme-tcp: switch to per-cpu page_frag_cache

On Tue, Sep 30, 2025 at 11:34:14PM -0700, Chris Leech wrote:
> 
> nvme-tcp 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 switching from having a per-queue page_frag_cache to a
> per-cpu page_frag_cache.
> 
> Cc: stable@...r.kernel.org # 6.12
> Fixes: 4e893ca81170 ("nvme_core: scan namespaces asynchronously")
> Reported-by: Dmitry Bogdanov <d.bogdanov@...ro.com>
> Signed-off-by: Chris Leech <cleech@...hat.com>
> ---
>  drivers/nvme/host/tcp.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/nvme/host/tcp.c b/drivers/nvme/host/tcp.c
> index 1413788ca7d52..a4c4ace5be0f4 100644
> --- a/drivers/nvme/host/tcp.c
> +++ b/drivers/nvme/host/tcp.c
> @@ -174,7 +174,6 @@ struct nvme_tcp_queue {
>         __le32                  recv_ddgst;
>         struct completion       tls_complete;
>         int                     tls_err;
> -       struct page_frag_cache  pf_cache;
> 
>         void (*state_change)(struct sock *);
>         void (*data_ready)(struct sock *);
> @@ -201,6 +200,7 @@ struct nvme_tcp_ctrl {
> 
>  static LIST_HEAD(nvme_tcp_ctrl_list);
>  static DEFINE_MUTEX(nvme_tcp_ctrl_mutex);
> +static DEFINE_PER_CPU(struct page_frag_cache, pf_cache);
>  static struct workqueue_struct *nvme_tcp_wq;
>  static const struct blk_mq_ops nvme_tcp_mq_ops;
>  static const struct blk_mq_ops nvme_tcp_admin_mq_ops;
> @@ -556,7 +556,7 @@ 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);
> 
> -       req->pdu = page_frag_alloc(&queue->pf_cache,
> +       req->pdu = page_frag_alloc(this_cpu_ptr(&pf_cache),
>                 sizeof(struct nvme_tcp_cmd_pdu) + hdgst,
>                 GFP_KERNEL | __GFP_ZERO);

I am not good at scheduler subsystem, but as far as I understand,
workqueues may execute its work items in parallel up to max_active work
items on the same CPU. It means that this solution does not fix the
issue of parallel usage of the same variable.
Can anyone comment on this?

>         if (!req->pdu)
> @@ -1420,7 +1420,7 @@ 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);
> 
> -       async->pdu = page_frag_alloc(&queue->pf_cache,
> +       async->pdu = page_frag_alloc(this_cpu_ptr(&pf_cache),
>                 sizeof(struct nvme_tcp_cmd_pdu) + hdgst,
>                 GFP_KERNEL | __GFP_ZERO);

This line is executed in a different(parallel) context comparing to
nvme_tcp_init_request.

>         if (!async->pdu)
> @@ -1439,7 +1439,7 @@ static void nvme_tcp_free_queue(struct nvme_ctrl *nctrl, int qid)
>         if (!test_and_clear_bit(NVME_TCP_Q_ALLOCATED, &queue->flags))
>                 return;
> 
> -       page_frag_cache_drain(&queue->pf_cache);
> +       page_frag_cache_drain(this_cpu_ptr(&pf_cache));

This line is also definitely processed in other(parallel) context comparing
to nvme_tcp_init_request. And frees the still used pages by other queues.

>         noreclaim_flag = memalloc_noreclaim_save();
>         /* ->sock will be released by fput() */
> --
> 2.50.1
> 

In total, my patch with a mutex looks more appropriate and more error-proof.

BR,
 Dmitry