On Fri, Mar 14 2008, Christof Schmitt wrote:
> 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.
That is indeed a bug, does this work for you?
diff --git a/kernel/relay.c b/kernel/relay.c
index d080b9d..39d1fa8 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -1066,7 +1066,7 @@ static int subbuf_splice_actor(struct file *in,
unsigned int flags,
int *nonpad_ret)
{
- unsigned int pidx, poff, total_len, subbuf_pages, ret;
+ unsigned int pidx, poff, total_len, subbuf_pages, nr_pages, ret;
struct rchan_buf *rbuf = in->private_data;
unsigned int subbuf_size = rbuf->chan->subbuf_size;
uint64_t pos = (uint64_t) *ppos;
@@ -1098,7 +1098,9 @@ static int subbuf_splice_actor(struct file *in,
pidx = (read_start / PAGE_SIZE) % subbuf_pages;
poff = read_start & ~PAGE_MASK;
- for (total_len = 0; spd.nr_pages < subbuf_pages; spd.nr_pages++) {
+ nr_pages = min_t(unsigned int, subbuf_pages, PIPE_BUFFERS);
+
+ for (total_len = 0; spd.nr_pages < nr_pages; spd.nr_pages++) {
unsigned int this_len, this_end, private;
unsigned int cur_pos = read_start + total_len;
--
Jens Axboe
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