NFS over RDMA stops working for data larger than 812 Bytes

[Klemens Senn <klemens.senn@ims.co.at>]

Hi,

I started setting up NFS over RDMA on a OpenSUSE 12.3 machine using the
3.7.10-1.16-default kernel.
Mounting the NFS share works and getting a file with 812 Bytes succeeds.
Reading 813 Bytes raises a Input/output error.

The export on the server side is done with
/data
172.16.100.0/24(rw,wdelay,crossmnt,insecure,no_root_squash,no_subtree_check,fsid=0,mountpoint)

Following command is used for mounting the NFSv4 share:
  mount -t nfs -o port=20049,rdma,vers=4.0,timeo=900 172.16.100.2:/ /mnt/

Testing the client side with older kernels stated that the mount is
working properly with the 3.4.67 kernel. I continued testing with the
3.10.17 kernel and enabled the debug level and noticed the message
  "server cheating in read reply: count 813 > recvd 0"

Here is the syslog output:
NFS: read(//813_bytes, 262144@0)
NFS: nfs_readpage (ffffea001db1e898 4096@0)
NFS:     0 initiated read call (req 0:33/70, 813 bytes @ offset 0)
RPC:       new task initialized, procpid 9881
RPC:       allocated task ffff88105dacabf8
RPC:  1631 __rpc_execute flags=0x1
<-- nfs4_setup_sequence status=0
RPC:  1631 call_start nfs4 proc READ (async)
RPC:  1631 call_reserve (status 0)
RPC:  1631 reserved req ffff88085e641c00 xid 3f6a89a6
RPC:  1631 call_reserveresult (status 0)
RPC:  1631 call_refresh (status 0)
RPC:  1631 refreshing UNIX cred ffff88087c2108c0
RPC:  1631 call_refreshresult (status 0)
RPC:  1631 call_allocate (status 0)
RPC:       xprt_rdma_allocate: size 684, request 0xffff88084a342000
RPC:  1631 call_bind (status 0)
RPC:  1631 call_connect xprt ffff88085f297000 is connected
RPC:  1631 call_transmit (status 0)
RPC:  1631 xprt_prepare_transmit
RPC:  1631 xprt_cwnd_limited cong = 0 cwnd = 4096
RPC:  1631 rpc_xdr_encode (status 0)
RPC:  1631 marshaling UNIX cred ffff88087c2108c0
RPC:  1631 using AUTH_UNIX cred ffff88087c2108c0 to wrap rpc data
RPC:  1631 xprt_transmit(144)
RPC:       rpcrdma_inline_pullup: pad 0 destp 0xffff88084a3437f0 len 144
hdrlen 144
RPC:       rpcrdma_register_frmr_external: Using frmr ffff88084b612d10
to map 1 segments
RPC:       rpcrdma_create_chunks: write chunk elem
813@0x87bf95000:0x8a905 (more)
RPC:       rpcrdma_register_frmr_external: Using frmr ffff88084b612d38
to map 1 segments
RPC:       rpcrdma_create_chunks: write chunk elem
152@0x84a343974:0x8aa01 (last)
RPC:       rpcrdma_marshal_req: write chunk: hdrlen 68 rpclen 144 padlen
0 headerp 0xffff88084a343100 base 0xffff88084a343760 lkey 0x8000
RPC:       rpcrdma_event_process: event rep ffff88084b612d10 status 0
opcode 8 length 4294936584
RPC:       rpcrdma_event_process: event rep ffff88084b612d38 status 0
opcode 8 length 4294936584
RPC:  1631 xmit complete
RPC:  1631 sleep_on(queue "xprt_pending" time 4296115067)
RPC:  1631 added to queue ffff88085f297258 "xprt_pending"
RPC:  1631 setting alarm for 90000 ms
RPC:       wake_up_first(ffff88085f297190 "xprt_sending")
RPC:       rpcrdma_event_process: event rep ffff88085e8bf000 status 0
opcode 80 length 128
RPC:       rpcrdma_reply_handler: reply 0xffff88085e8bf000 completes
request 0xffff88084a342000
                   RPC request 0xffff88085e641c00 xid 0xa6896a3f
RPC:       rpcrdma_count_chunks: chunk 813@0x87bf95000:0x8a905
RPC:       rpcrdma_count_chunks: chunk 3@0x84a343974:0x8aa01
RPC:       rpcrdma_inline_fixup: srcp 0xffff88085e8bf0a4 len 60 hdrlen 60
RPC:       rpcrdma_reply_handler: xprt_complete_rqst(0xffff88085f297000,
0xffff88085e641c00, 876)
RPC:  1631 xid 3f6a89a6 complete (876 bytes received)
RPC:  1631 __rpc_wake_up_task (now 4296115067)
RPC:  1631 disabling timer
RPC:  1631 removed from queue ffff88085f297258 "xprt_pending"
RPC:       __rpc_wake_up_task done
RPC:  1631 __rpc_execute flags=0x801
RPC:  1631 call_status (status 876)
RPC:  1631 call_decode (status 876)
RPC:  1631 validating UNIX cred ffff88087c2108c0
RPC:  1631 using AUTH_UNIX cred ffff88087c2108c0 to unwrap rpc data
NFS: server cheating in read reply: count 813 > recvd 0
RPC:  1631 call_decode result 0
NFS: nfs_readpage_result:  1631, (status 0)
--> nfs4_read_done

Adding other debug prints to the kernel module told me that
buf->page_len is set to 0 in xdr_align_pages (in file net/sunrpc/xdr.c).
Comparing this piece of code with older kernel versions showed that the
comparison with buf->page_len was introduces in kernel 3.7.
By disabling this check the file transfer seems to work.

Here is the patch for kernel 3.10.17:

--- net/sunrpc/xdr.c	2013-10-18 19:44:19.000000000 +0200
+++ net/sunrpc/xdr_new.c	2013-11-08 14:06:00.241217773 +0100
@@ -747,7 +747,7 @@
 		nwords = xdr->nwords;
 		len = nwords << 2;
 	}
-	if (buf->page_len <= len)
+	if (buf->page_len && buf->page_len <= len)
 		len = buf->page_len;
 	else if (nwords < xdr->nwords) {
 		/* Truncate page data and move it into the tail */


Can you please tell me which entity sets (or does not set) the total
length (buf->page_len) after receiving the data from the RDMA_SEND
operation from the server?

Please advise

Klemens