Re: [PATCH] SUNRPC: backchannel RPC request must reference XPRT

[Trond Myklebust <trondmy@hammerspace.com>]

Hi Neil,

On Tue, 2019-10-15 at 10:36 +1100, NeilBrown wrote:
> The backchannel RPC requests - that are queued waiting
> for the reply to be sent by the "NFSv4 callback" thread -
> have a pointer to the xprt, but it is not reference counted.
> It is possible for the xprt to be freed while there are
> still queued requests.
> 
> I think this has been a problem since
> Commit fb7a0b9addbd ("nfs41: New backchannel helper routines")
> when the code was introduced, but I suspect it became more of
> a problem after
> Commit 80b14d5e61ca ("SUNRPC: Add a structure to track multiple
> transports")
> (or there abouts).
> Before this second patch, the nfs client would hold a reference to
> the xprt to keep it alive.  After multipath was introduced,
> a client holds a reference to a swtich, and the switch can have
> multiple
> xprts which can be added and removed.
> 
> I'm not sure of all the causal issues, but this patch has
> fixed a customer problem were an NFSv4.1 client would run out
> of memory with tens of thousands of backchannel rpc requests
> queued for an xprt that had been freed.  This was a 64K-page
> machine so each rpc_rqst consumed more than 128K of memory.
> 
> Fixes: 80b14d5e61ca ("SUNRPC: Add a structure to track multiple
> transports")
> cc: stable@vger.kernel.org (v4.6)
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  net/sunrpc/backchannel_rqst.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sunrpc/backchannel_rqst.c
> b/net/sunrpc/backchannel_rqst.c
> index 339e8c077c2d..c95ca39688b6 100644
> --- a/net/sunrpc/backchannel_rqst.c
> +++ b/net/sunrpc/backchannel_rqst.c
> @@ -61,6 +61,7 @@ static void xprt_free_allocation(struct rpc_rqst
> *req)
>  	free_page((unsigned long)xbufp->head[0].iov_base);
>  	xbufp = &req->rq_snd_buf;
>  	free_page((unsigned long)xbufp->head[0].iov_base);
> +	xprt_put(req->rq_xprt);
>  	kfree(req);
>  }

Would it perhaps make better sense to move the xprt_get() to
xprt_lookup_bc_request() and to release it in xprt_free_bc_rqst()? 

Otherwise as far as I can tell, we will have freed slots on the xprt-
>bc_pa_list that hold a reference to the transport itself, meaning that
the latter never gets released.

>  
> @@ -85,7 +86,7 @@ struct rpc_rqst *xprt_alloc_bc_req(struct rpc_xprt
> *xprt, gfp_t gfp_flags)
>  	if (req == NULL)
>  		return NULL;
>  
> -	req->rq_xprt = xprt;
> +	req->rq_xprt = xprt_get(xprt);
>  	INIT_LIST_HEAD(&req->rq_bc_list);
>  
>  	/* Preallocate one XDR receive buffer */
-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com