blktrace/relay/s390: Oops in subbuf_splice_actor

[Christof Schmitt <christof.schmitt@de.ibm.com>]

When i first setup blktrace on a s390 z/VM guest to trace to another
system and then put some load on the disk traced, the system oopses in
subbuf_splice_actor. The setup is as simple as

# blktrace -h tracehost -d /dev/sda
# dd if=/dev/sda of=/dev/null

This is the stack trace from the current 2.6.25-rc5, i added
noinline to subbuf_splice_actor, otherwise it will be inlined:

Unable to handle kernel pointer dereference at virtual kernel address 0000000000000000 
Oops: 0004 [#1] PREEMPT SMP DEBUG_PAGEALLOC 
Modules linked in: binfmt_misc vmur 
CPU: 1 Not tainted 2.6.25-rc5 #10 
Process blktrace (pid: 2655, task: 000000002bc38238, ksp: 000000002b0d79a8) 
Krnl PSW : 0704100180000000 00000000000874e2 (subbuf_splice_actor+0x212/0x364) 
           R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:0 CC:1 PM:0 EA:3 
Krnl GPRS: 0a00000000000001 000000002b2bb000 0000000000001000 00000000000000c8 
           0000000000001000 0000000000001000 0000000000000000 0000000000000200 
           0000000000019000 0000000000000019 0000000000066fd8 000000002b0d79e8 
           000003e040ed7938 0000000000000000 000000000008749e 000000002b0d79e8 
Krnl Code: 00000000000874d4: e31050b00004       lg      %r1,176(%r5) 
           00000000000874da: 1854               lr      %r5,%r4 
           00000000000874dc: e3cc10000004       lg      %r12,0(%r12,%r1) 
          >00000000000874e2: e3c320000024       stg    >%r12,0(%r3,%r2) 
           00000000000874e8: e330b2700014       lgf     %r3,624(%r11) 
           00000000000874ee: eb330004000d       sllg    %r3,%r3,4 
           00000000000874f4: e320b2680004       lg      %r2,616(%r11) 
           00000000000874fa: 1814               lr      %r1,%r4 
Call Trace: 
([<000000000008749e>] subbuf_splice_actor+0x1ce/0x364) 
 [<00000000000876a2>] relay_file_splice_read+0x6e/0xfc 
 [<00000000000e4f90>] do_splice_to+0x9c/0xb4 
 [<00000000000e545c>] splice_direct_to_actor+0xd8/0x21c 
 [<00000000000e55ec>] do_splice_direct+0x4c/0x70 
 [<00000000000bc2be>] do_sendfile+0x1b6/0x228 
 [<00000000000bc382>] sys_sendfile64+0x52/0xe4 
 [<00000000000241c0>] sysc_noemu+0x10/0x16 
 [<00000200001304da>] 0x200001304da 

Some debug printks show that subbuf_pages in this case is 512 and the
for loop goes until spd.nr_pages is 25, before hitting the problem. I
am wondering if the numbers make sense here, since spd.pages has only
16 pages allocated (with PIPE_BUFFERS). But i did not yet understand
how much data this loop is supposed to assign.

Does anybody have an idea what is happening here, or how to continue
debugging this problem?

Christof