Subject: Re: [PATCH 4/4] sunrpc: use SKB fragment destructors to delay completion until page is released by network stack.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 11 Nov 2011 14:38:24 +0200

On Wed, Nov 09, 2011 at 03:02:07PM +0000, Ian Campbell wrote:
> This prevents an issue where an ACK is delayed, a retransmit is queued (either
> at the RPC or TCP level) and the ACK arrives before the retransmission hits the
> wire. If this happens to an NFS WRITE RPC then the write() system call
> completes and the userspace process can continue, potentially modifying data
> referenced by the retransmission before the retransmission occurs.
> 
> Signed-off-by: Ian Campbell <[email-masked]>
> Acked-by: Trond Myklebust <[email-masked]>
> Cc: "David S. Miller" <davem-fT/PcQaiUtIeIZ0/[email-masked]>
> Cc: Neil Brown <[email-masked]>
> Cc: "J. Bruce Fields" <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]

So this blocks the system call until all page references
are gone, right?
But, there's no upper limit on how long the
page is referenced, correct? consider a bridged setup
with an skb queued at a tap device - this cause one process
to block another one by virtue of not consuming a cloned skb?

> ---
>  include/linux/sunrpc/xdr.h  |    2 ++
>  include/linux/sunrpc/xprt.h |    5 ++++-
>  net/sunrpc/clnt.c           |   27 ++++++++++++++++++++++-----
>  net/sunrpc/svcsock.c        |    3 ++-
>  net/sunrpc/xprt.c           |   13 +++++++++++++
>  net/sunrpc/xprtsock.c       |    3 ++-
>  6 files changed, 45 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/sunrpc/xdr.h b/include/linux/sunrpc/xdr.h
> index a20970e..172f81e 100644
> --- a/include/linux/sunrpc/xdr.h
> +++ b/include/linux/sunrpc/xdr.h
> @@ -16,6 +16,7 @@
>  #include <asm/byteorder.h>
>  #include <asm/unaligned.h>
>  #include <linux/scatterlist.h>
> +#include <linux/skbuff.h>
>  
>  /*
>   * Buffer adjustment
> @@ -57,6 +58,7 @@ struct xdr_buf {
>  			tail[1];	/* Appended after page data */
>  
>  	struct page **	pages;		/* Array of contiguous pages */
> +	struct skb_frag_destructor *destructor;
>  	unsigned int	page_base,	/* Start of page data */
>  			page_len,	/* Length of page data */
>  			flags;		/* Flags for data disposition */
> diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
> index 15518a1..75131eb 100644
> --- a/include/linux/sunrpc/xprt.h
> +++ b/include/linux/sunrpc/xprt.h
> @@ -92,7 +92,10 @@ struct rpc_rqst {
>  						/* A cookie used to track the
>  						   state of the transport
>  						   connection */
> -	
> +	struct skb_frag_destructor destructor;	/* SKB paged fragment
> +						 * destructor for
> +						 * transmitted pages*/
> +
>  	/*
>  	 * Partial send handling
>  	 */
> diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
> index c5347d2..919538d 100644
> --- a/net/sunrpc/clnt.c
> +++ b/net/sunrpc/clnt.c
> @@ -61,6 +61,7 @@ static void	call_reserve(struct rpc_task *task);
>  static void	call_reserveresult(struct rpc_task *task);
>  static void	call_allocate(struct rpc_task *task);
>  static void	call_decode(struct rpc_task *task);
> +static void	call_complete(struct rpc_task *task);
>  static void	call_bind(struct rpc_task *task);
>  static void	call_bind_status(struct rpc_task *task);
>  static void	call_transmit(struct rpc_task *task);
> @@ -1113,6 +1114,8 @@ rpc_xdr_encode(struct rpc_task *task)
>  			 (char *)req->rq_buffer + req->rq_callsize,
>  			 req->rq_rcvsize);
>  
> +	req->rq_snd_buf.destructor = &req->destructor;
> +
>  	p = rpc_encode_header(task);
>  	if (p == NULL) {
>  		printk(KERN_INFO "RPC: couldn't encode RPC header, exit EIO\n");
> @@ -1276,6 +1279,7 @@ call_connect_status(struct rpc_task *task)
>  static void
>  call_transmit(struct rpc_task *task)
>  {
> +	struct rpc_rqst *req = task->tk_rqstp;
>  	dprint_status(task);
>  
>  	task->tk_action = call_status;
> @@ -1309,8 +1313,8 @@ call_transmit(struct rpc_task *task)
>  	call_transmit_status(task);
>  	if (rpc_reply_expected(task))
>  		return;
> -	task->tk_action = rpc_exit_task;
> -	rpc_wake_up_queued_task(&task->tk_xprt->pending, task);
> +	task->tk_action = call_complete;
> +	skb_frag_destructor_unref(&req->destructor);
>  }
>  
>  /*
> @@ -1383,7 +1387,8 @@ call_bc_transmit(struct rpc_task *task)
>  		return;
>  	}
>  
> -	task->tk_action = rpc_exit_task;
> +	task->tk_action = call_complete;
> +	skb_frag_destructor_unref(&req->destructor);
>  	if (task->tk_status < 0) {
>  		printk(KERN_NOTICE "RPC: Could not send backchannel reply "
>  			"error: %d\n", task->tk_status);
> @@ -1423,7 +1428,6 @@ call_bc_transmit(struct rpc_task *task)
>  			"error: %d\n", task->tk_status);
>  		break;
>  	}
> -	rpc_wake_up_queued_task(&req->rq_xprt->pending, task);
>  }
>  #endif /* CONFIG_SUNRPC_BACKCHANNEL */
>  
> @@ -1589,12 +1593,14 @@ call_decode(struct rpc_task *task)
>  		return;
>  	}
>  
> -	task->tk_action = rpc_exit_task;
> +	task->tk_action = call_complete;
>  
>  	if (decode) {
>  		task->tk_status = rpcauth_unwrap_resp(task, decode, req, p,
>  						      task->tk_msg.rpc_resp);
>  	}
> +	rpc_sleep_on(&req->rq_xprt->pending, task, NULL);
> +	skb_frag_destructor_unref(&req->destructor);
>  	dprintk("RPC: %5u call_decode result %d\n", task->tk_pid,
>  			task->tk_status);
>  	return;
> @@ -1609,6 +1615,17 @@ out_retry:
>  	}
>  }
>  
> +/*
> + * 8.	Wait for pages to be released by the network stack.
> + */
> +static void
> +call_complete(struct rpc_task *task)
> +{
> +	dprintk("RPC: %5u call_complete result %d\n",
> +		task->tk_pid, task->tk_status);
> +	task->tk_action = rpc_exit_task;
> +}
> +
>  static __be32 *
>  rpc_encode_header(struct rpc_task *task)
>  {
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index 852a258..3685cad 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -196,7 +196,8 @@ int svc_send_common(struct socket *sock, struct xdr_buf *xdr,
>  	while (pglen > 0) {
>  		if (slen == size)
>  			flags = 0;
> -		result = kernel_sendpage(sock, *ppage, NULL, base, size, flags);
> +		result = kernel_sendpage(sock, *ppage, xdr->destructor,
> +					 base, size, flags);
>  		if (result > 0)
>  			len += result;
>  		if (result != size)
> diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
> index f4385e4..925aa0c 100644
> --- a/net/sunrpc/xprt.c
> +++ b/net/sunrpc/xprt.c
> @@ -1103,6 +1103,16 @@ static inline void xprt_init_xid(struct rpc_xprt *xprt)
>  	xprt->xid = net_random();
>  }
>  
> +static int xprt_complete_skb_pages(void *calldata)
> +{
> +	struct rpc_task *task = calldata;
> +	struct rpc_rqst	*req = task->tk_rqstp;
> +
> +	dprintk("RPC: %5u completing skb pages\n", task->tk_pid);
> +	rpc_wake_up_queued_task(&req->rq_xprt->pending, task);
> +	return 0;
> +}
> +
>  static void xprt_request_init(struct rpc_task *task, struct rpc_xprt *xprt)
>  {
>  	struct rpc_rqst	*req = task->tk_rqstp;
> @@ -1115,6 +1125,9 @@ static void xprt_request_init(struct rpc_task *task, struct rpc_xprt *xprt)
>  	req->rq_xid     = xprt_alloc_xid(xprt);
>  	req->rq_release_snd_buf = NULL;
>  	xprt_reset_majortimeo(req);
> +	atomic_set(&req->destructor.ref, 1);
> +	req->destructor.destroy = &xprt_complete_skb_pages;
> +	req->destructor.data = task;
>  	dprintk("RPC: %5u reserved req %p xid %08x\n", task->tk_pid,
>  			req, ntohl(req->rq_xid));
>  }
> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
> index f79e40e9..af3a106 100644
> --- a/net/sunrpc/xprtsock.c
> +++ b/net/sunrpc/xprtsock.c
> @@ -408,7 +408,8 @@ static int xs_send_pagedata(struct socket *sock, struct xdr_buf *xdr, unsigned i
>  		remainder -= len;
>  		if (remainder != 0 || more)
>  			flags |= MSG_MORE;
> -		err = sock->ops->sendpage(sock, *ppage, NULL, base, len, flags);
> +		err = sock->ops->sendpage(sock, *ppage, xdr->destructor,
> +					  base, len, flags);
>  		if (remainder == 0 || err != len)
>  			break;
>  		sent += err;
> -- 
> 1.7.2.5
> 
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