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Subjectblktrace/relay/s390: Oops in subbuf_splice_actor
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


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