On Sunday 22 June 2008, Michael Buesch wrote:
quoted text
> [28375.893176] Unable to handle kernel paging request for data at address 0x00000008

access at offset 8 of a NULL pointer, maybe bh->b_this_page?

quoted text
> [28375.893181] Faulting instruction address: 0xc00000000012df84
> [28375.893186] Oops: Kernel access of bad area, sig: 11 [#1]
> [28375.893189] PREEMPT SMP NR_CPUS=4 NUMA PowerMac

Ok, important information: ppc64 architecture. It would be nice to mention
in the bug report, but here we can see it as well.

quoted text
> [28375.893320] TASK = c00000011636db00[4667] 'kded' THREAD: c000000116ae8000 CPU: 2

task was kded, i.e. not growisofs itself, thouh growisofs is probably the one
that has caused this problem (by exausting memory).

quoted text
> [28375.893327] GPR00: c00000000012df70 c000000116aeb580 c00000000090ff20 0000000000000000 
> [28375.893340] GPR04: 0000000000010000 0000000000000001 c00000011bfe37a0 0000000000000010 
> [28375.893352] GPR08: f00000000694d280 0000000000000000 c0000000008c0be0 0000000000000000 
> [28375.893364] GPR12: 0000000028004842 c000000000941700 0000000000000004 c000000116aeb840 
> [28375.893377] GPR16: c0000001195d8f78 c0000000008c0cb8 c0000000000bd064 0000000000000003 
> [28375.893389] GPR20: 0000000000000000 c0000001195d8d68 0000000000000004 c0000001195d8f80 
> [28375.893402] GPR24: c00000000082c700 0000000000010000 f00000000694d280 0000000000000000 
> [28375.893415] GPR28: 0000000000000000 f00000000694d280 c00000000088e640 c000000116aeb580 

Note: r9 and r3 are both NULL pointers. r3 is the value returned from alloc_page_buffers.
R9 is a copy of that, which gets accessed.

quoted text
> [28375.893428] NIP [c00000000012df84] .create_empty_buffers+0x44/0x180
> [28375.893439] LR [c00000000012df70] .create_empty_buffers+0x30/0x180
> [28375.893446] Call Trace:
> [28375.893451] [c000000116aeb580] [c00000000012df70] .create_empty_buffers+0x30/0x180 (unreliable)
> [28375.893463] [c000000116aeb620] [c0000000001331a4] .block_read_full_page+0x464/0x480
> [28375.893473] [c000000116aeb750] [c000000000137b28] .blkdev_readpage+0x28/0x50
> [28375.893483] [c000000116aeb7d0] [c0000000000bd2c8] .__do_page_cache_readahead+0x368/0x390
> [28375.893496] [c000000116aeb8e0] [c0000000000bd698] .ondemand_readahead+0x158/0x270
> [28375.893505] [c000000116aeb9a0] [c0000000000bd8fc] .page_cache_sync_readahead+0x3c/0x50
> [28375.893513] [c000000116aeba20] [c0000000000b2994] .generic_file_aio_read+0x4f4/0x630
> [28375.893523] [c000000116aebb50] [c0000000000f6fa4] .do_sync_read+0xe4/0x180
> [28375.893534] [c000000116aebcf0] [c0000000000f79c4] .vfs_read+0xf4/0x1c0
> [28375.893542] [c000000116aebd90] [c0000000000f8234] .sys_read+0x54/0xa0
> [28375.893550] [c000000116aebe30] [c0000000000076d4] syscall_exit+0x0/0x40

a simple file read, from a random process.

quoted text
> [28375.893560] Instruction dump:
> [28375.893566] f8010010 f821ff61 7cbb2b78 38a00001 7c7d1b78 7c3f0b78 4bfffe65 7c7c1b78 
> [28375.893586] 7c691b78 4800000c 60000000 7d695b78 <e9690008> e8090000 2fab0000 7c00db78 
> [28375.893607] ---[ end trace d2a7775e4472c36e ]---
> 

4800000c is the branch to alloc_page_buffers
7d695b78 copies the return value of that to r9
e9690008 dereferences r9

Evidently, alloc_page_buffers got an out of memory condition, which was not caught
by create_empty_buffers. No idea how it should be handled, but the fact that it's
not looks like a bug to me ;-).

	Arnd <><
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