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Subjectcrash in entry.S restore_all, 2.6.12-rc2, x86, PAGEALLOC

the crashes below happen when PAGEALLOC is enabled. It's this
instruction:

movb OLDSS(%esp), %ah

OLDSS is 0x38, esp is f4f83fc8, OLDSS(%esp) is thus f4f84000, which
correctly creates the PAGEALLOC pagefault. esp is off by 4 bytes?

it could be the ESP-16-bit-corruption patch causing this, or it could be
an already existing latent bug getting triggered now: normally only iret
accesses the OLDSS, and we fix any iret faults up, but now that we
explicitly access %esp the esp bug shows up.

so it would be nice to understand why this triggers. It seems to be a
sporadic event - first it hit hotplug, then input.agent. If i disable
PAGEALLOC the system boots up fine. In any case, the ESP-corruption
patch is not safe until this bug is understood, as it right now may read
a random byte off the next page, and possibly doing bogus calls to the
16-bit-fixup code.

Ingo

-------------

BUG: Unable to handle kernel paging request at virtual address f4f84000
printing eip:
c010287c
*pde = 00527067
*pte = 34f84000
Oops: 0000 [#1]
PREEMPT DEBUG_PAGEALLOC
Modules linked in:
CPU: 0
EIP: 0060:[<c010287c>] Not tainted VLI
EFLAGS: 00010046 (2.6.12-rc2-RT-V0.7.43-09)
EIP is at restore_all+0x4/0x18
eax: 00000206 ebx: 00000000 ecx: 00000000 edx: 00000001
esi: 00000000 edi: 009b63f9 ebp: f4f82000 esp: f4f83fc8
ds: 007b es: 007b ss: 0068 preempt: 00000001
Process 10-udev.hotplug (pid: 1264, threadinfo=f4f82000 task=f5034a10)
Stack: 00000000 bfa71dd0 009c0ffc 00000000 009b63f9 bfa71d44 000000c5 0000007b
0000007b ffffffef c01027ba 00000060 00000206 0000007b
Call Trace:
[<c01036ac>] show_stack+0x7a/0x90 (32)
[<c0103835>] show_registers+0x15a/0x1d2 (56)
[<c0103a30>] die+0xf4/0x17e (68)
[<c010f444>] do_page_fault+0x3de/0x60a (212)
[<c01032eb>] error_code+0x4f/0x54 (-8076)

---------------------

BUG: Unable to handle kernel paging request at virtual address f57bc000
printing eip:
c010287c
*pde = 00529067
*pte = 357bc000
Oops: 0000 [#1]
PREEMPT DEBUG_PAGEALLOC
Modules linked in:
CPU: 0
EIP: 0060:[<c010287c>] Not tainted VLI
EFLAGS: 00010046 (2.6.12-rc2-RT-V0.7.43-09)
EIP is at restore_all+0x4/0x18
eax: 00000206 ebx: b7f11000 ecx: 00000000 edx: 00000000
esi: 080e4f28 edi: 00000000 ebp: f57ba000 esp: f57bbfc8
ds: 007b es: 007b ss: 0068 preempt: 00000001
Process input.agent (pid: 1131, threadinfo=f57ba000 task=f57b9a10)
Stack: b7f11000 00001000 009c0ffc 080e4f28 00000000 bfc112c0 0000005b 0000007b
0000007b ffffff00 c01027ba 00000060 00000206 0000007b
Call Trace:
[<c01036ac>] show_stack+0x7a/0x90 (32)
[<c0103835>] show_registers+0x15a/0x1d2 (56)
[<c0103a30>] die+0xf4/0x17e (68)
[<c010f474>] do_page_fault+0x3de/0x60a (212)
[<c01032eb>] error_code+0x4f/0x54 (-8076)
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