Re: [PATCH linux-next v3] SUNRPC: rpcrdma_register_default_external: Dynamically allocate ib_phys_buf

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Mar 11, 2013 at 03:15:08PM -0600, Tim Gardner wrote:
> rpcrdma_register_default_external() is several frames into the call stack which
> goes deeper yet. You run the risk of stack corruption by declaring such a large
> automatic variable, so move the array of 'struct ib_phys_buf' objects into the
> transport structure 'struct rpcrdma_xprt' (which is dynamically allocated) in
> order to silence the frame-larger-than warning. Access to each struct
> rpcrdma_xprt is serialized by XPRT_LOCKED in xprt_reserve_xprt(), so there is
> no danger of multiple accessors to the array of struct ib_phys_buf objects.
> 
> net/sunrpc/xprtrdma/verbs.c: In function 'rpcrdma_register_default_external':
> net/sunrpc/xprtrdma/verbs.c:1774:1: warning: the frame size of 1056 bytes is larger than 1024 bytes [-Wframe-larger-than=]
> 
> gcc version 4.6.3
> 
> Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
> Cc: "J. Bruce Fields" <bfields@fieldses.org>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Tom Tucker <tom@ogc.us>
> Cc: Haggai Eran <haggaie@mellanox.com>
> Cc: Or Gerlitz <ogerlitz@mellanox.com>
> Cc: Shani Michaeli <shanim@mellanox.com>
> Cc: linux-nfs@vger.kernel.org
> Cc: netdev@vger.kernel.org
> Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
> ---
> v1 - Use kmalloc() to dynamically allocate and free the array of 'struct
> ib_phys_buf' objects
> 
> v2 - Move the array of 'struct ib_phys_buf' objects into struct rpcrdma_req
> and pass this request down through rpcrdma_register_external() and
> rpcrdma_register_default_external(). This is less overhead then using
> kmalloc() and requires no extra error checking as the allocation burden is
> shifted to the transport client.
> 
> v3 - Move the array of 'struct ib_phys_buf' objects into struct rpcrdma_xprt.
> Pass a pointer to this transport structure into rpcrdma_register_default_external().
> This is less overhead then using kmalloc() and requires no extra error checking
> as the allocation burden is shifted to the transport client.

Looks good to me; wish we could get it tested....

In future if we do decide to also increase the size of that array we may
need to allocate it separately from struct rpcrdma_xprt itself, which
looks already fairly large without it; on x86_64:

	$ gdb net/sunrpc/xprtrdma/xprtrdma.ko
	...
	(gdb) p sizeof(struct rpcrdma_xprt)
	$1 = 2912

But that shouldn't be a big deal to do.

--b.

> 
>  net/sunrpc/xprtrdma/verbs.c     |   10 ++++++----
>  net/sunrpc/xprtrdma/xprt_rdma.h |    5 ++++-
>  2 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/net/sunrpc/xprtrdma/verbs.c b/net/sunrpc/xprtrdma/verbs.c
> index 93726560..c7aa3da 100644
> --- a/net/sunrpc/xprtrdma/verbs.c
> +++ b/net/sunrpc/xprtrdma/verbs.c
> @@ -1730,13 +1730,14 @@ rpcrdma_deregister_memwin_external(struct rpcrdma_mr_seg *seg,
>  }
>  
>  static int
> -rpcrdma_register_default_external(struct rpcrdma_mr_seg *seg,
> -			int *nsegs, int writing, struct rpcrdma_ia *ia)
> +rpcrdma_register_default_external(struct rpcrdma_xprt *r_xprt,
> +			struct rpcrdma_mr_seg *seg, int *nsegs, int writing,
> +			struct rpcrdma_ia *ia)
>  {
>  	int mem_priv = (writing ? IB_ACCESS_REMOTE_WRITE :
>  				  IB_ACCESS_REMOTE_READ);
>  	struct rpcrdma_mr_seg *seg1 = seg;
> -	struct ib_phys_buf ipb[RPCRDMA_MAX_DATA_SEGS];
> +	struct ib_phys_buf *ipb = r_xprt->ipb;
>  	int len, i, rc = 0;
>  
>  	if (*nsegs > RPCRDMA_MAX_DATA_SEGS)
> @@ -1827,7 +1828,8 @@ rpcrdma_register_external(struct rpcrdma_mr_seg *seg,
>  
>  	/* Default registration each time */
>  	default:
> -		rc = rpcrdma_register_default_external(seg, &nsegs, writing, ia);
> +		rc = rpcrdma_register_default_external(r_xprt, seg, &nsegs,
> +			writing, ia);
>  		break;
>  	}
>  	if (rc)
> diff --git a/net/sunrpc/xprtrdma/xprt_rdma.h b/net/sunrpc/xprtrdma/xprt_rdma.h
> index cc1445d..d7b440f 100644
> --- a/net/sunrpc/xprtrdma/xprt_rdma.h
> +++ b/net/sunrpc/xprtrdma/xprt_rdma.h
> @@ -269,7 +269,8 @@ struct rpcrdma_stats {
>   * for convenience. This structure need not be visible externally.
>   *
>   * It is allocated and initialized during mount, and released
> - * during unmount.
> + * during unmount. Access to this structure is serialized by XPRT_LOCKED
> + * in xprt_reserve_xprt().
>   */
>  struct rpcrdma_xprt {
>  	struct rpc_xprt		xprt;
> @@ -279,6 +280,8 @@ struct rpcrdma_xprt {
>  	struct rpcrdma_create_data_internal rx_data;
>  	struct delayed_work	rdma_connect;
>  	struct rpcrdma_stats	rx_stats;
> +	/* temp work array */
> +	struct ib_phys_buf	ipb[RPCRDMA_MAX_DATA_SEGS];
>  };
>  
>  #define rpcx_to_rdmax(x) container_of(x, struct rpcrdma_xprt, xprt)
> -- 
> 1.7.9.5
>