Traceback due to 'powerpc/mm: Fix kernel RAM protection...' when running ppc image in qemu

[Christophe LEROY]

Le 24/09/2017 à 18:05, Guenter Roeck a écrit :
> On 09/21/2017 11:44 AM, Christophe LEROY wrote:
>>
>>
>> Le 20/09/2017 à 05:45, Guenter Roeck a écrit :
>>> On 09/19/2017 08:05 PM, Michael Ellerman wrote:
>>>> Guenter Roeck <linux at roeck-us.net> writes:
>>>>
>>>>> Hi,
>>>>>
>>>>> I see a the following traceback when running an SMP image based on
>>>>> 85xx/mpc85xx_cds_defconfig in qemu.
>>>>>
>>>>> ------------[ cut here ]------------
>>>>> WARNING: CPU: 0 PID: 1 at kernel/smp.c:416 
>>>>> smp_call_function_many+0xcc/0x2fc
>>>>> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.14.0-rc1-00009-g0666f56 #1
>>>>> task: cf830000 task.stack: cf82e000
>>>>> NIP:  c00a93c8 LR: c00a9634 CTR: 00000001
>>>>> REGS: cf82fde0 TRAP: 0700   Not tainted  (4.14.0-rc1-00009-g0666f56)
>>>>> MSR:  00021000 <CE,ME>  CR: 24000082  XER: 00000000
>>>>>
>>>>> GPR00: c00a9634 cf82fe90 cf830000 c050ad3c c0015a54 00000000 
>>>>> 00000001 00000001
>>>>> GPR08: 00000001 00000000 00000000 cf82e000 24000084 00000000 
>>>>> c0003150 00000000
>>>>> GPR16: 00000000 00000000 00000000 00000000 00000000 00000001 
>>>>> 00000000 c0510000
>>>>> GPR24: 00000000 c0015a54 00000000 c050ad3c c051823c c050ad3c 
>>>>> 00000025 00000000
>>>>> NIP [c00a93c8] smp_call_function_many+0xcc/0x2fc
>>>>> LR [c00a9634] smp_call_function+0x3c/0x50
>>>>> Call Trace:
>>>>> [cf82fe90] [00000010] 0x10 (unreliable)
>>>>> [cf82fed0] [c00a9634] smp_call_function+0x3c/0x50
>>>>> [cf82fee0] [c0015d2c] flush_tlb_kernel_range+0x20/0x38
>>>>> [cf82fef0] [c001524c] mark_initmem_nx+0x154/0x16c
>>>>> [cf82ff20] [c001484c] free_initmem+0x20/0x4c
>>>>> [cf82ff30] [c000316c] kernel_init+0x1c/0x108
>>>>> [cf82ff40] [c000f3a8] ret_from_kernel_thread+0x5c/0x64
>>>>> Instruction dump:
>>>>> 7c0803a6 7d808120 38210040 4e800020 3d20c052 812981a0 2f890000 
>>>>> 40beffac
>>>>> 3d20c051 8929ac64 2f890000 40beff9c <0fe00000> 4bffff94 7fc3f378 
>>>>> 7f64db78
>>>>> ---[ end trace 7da7bdcf8b15ddb3 ]---
>>>>
>>>> Thanks.
>>>>
>>>> I guess the system still runs OK otherwise, you're just seeing the 
>>>> warning?
>>>>
>>> Yes, though I am not sure if that is because there is only one active 
>>> CPU (there is
>>> still only one if I say "-smp 4" on the qemu command line).
>>>
>>>>> A complete log is available at:
>>>>> http://kerneltests.org/builders/qemu-ppc-master/builds/814/steps/qemubuildcommand/logs/stdio 
>>>>>
>>>>>
>>>>> Bisect points to commit 3184cc4b6f6a1dc0 ("powerpc/mm: Fix kernel 
>>>>> RAM protection
>>>>> after freeing unused memory on PPC32"). Bisect log is attached. A 
>>>>> quick look
>>>>> suggests that mark_initmem_nx() is called with interrupts disabled, 
>>>>> which
>>>>> triggers the traceback.
>>>>
>>>> Hmm. Yes the MSR says you have interrupts disabled (EE missing).
>>>>
>>>> But I don't see why. start_kernel() did local_irq_enable(), so I don't
>>>> understand why we got to mark_initmem_nx() with them disabled. I'll 
>>>> hope
>>>> that Christophe has some idea.
>>>>
>>> Good question. I only see this with one of 9 ppc emulations, with 
>>> 85xx/mpc85xx_cds_defconfig
>>> +CONFIG_DEVTMPFS=y +CONFIG_SMP=y. Maybe there is a platform specific 
>>> init function
>>> which leaves interrupts disabled. Question is which one that might be.
>>>
>>
>> Unfortunatly no, I have no idea. My three platforms (860, 885 and 
>> 8321) are not SMPs so that warning would not appear, but I added a 
>> WARN_ON(1) just become calling mark_initmem_nx(), and I can confirm 
>> that MSR has EE set on all three at that time.
>>
> 
> You should still be able to compile and run a SMP kernel. 
> mpc85xx_cds_defconfig

SMP doesn't support the 8xx, and the 83xx has hash MMU.

> without CONFIG_SMP=y does not show the warning either.

Yes that's normal, as the smp_call_function() is not called in that 
case, hence my test with a WARN_ON(1) just before calling mark_initram_nx()

> 
> Turns out interrupts are disabled in change_page_attr(), called by 
> mark_initmem_nx().

Oops, you're right, I missed it.

> change_page_attr() calls flush_tlb_kernel_range() with interrupts disabled.
> This only happens if CONFIG_PPC_MMU_NOHASH=y.
> Given that, I would assume that this will be seen with every 32 bit ppc 
> build which has
> CONFIG_SMP=y and CONFIG_PPC_MMU_NOHASH=y.
> 
> Maybe the problem was really introduced with commit e611939fc8ec1 
> ("powerpc/mm: Ensure
> change_page_attr() doesn't invalidate pinned TLBs"). From the context it 
> appears that
> flush_tlb_kernel_range() should not be called with interrupts disabled.

Right, it looks like that warning was introduced by this commit.
However, by looking at flush_tlb_page() which was the function that was 
called instead before that commit, there was most likely also an issue 
with SMP because flush_tlb_page() called with a NULL vma results in a 
warning in the SMP NOHASH version of flush_tlb_page().

> Indeed, moving flush_tlb_kernel_range() outside the irq disabled code fixes
> the problem for me.

Yes that's likely the solution it seems.

Thanks
Christophe

> 
> Thanks,
> Guenter
> 
>> So as you suggest, there must be a platform specific stuff leaving the 
>> interrupts disabled.
>>
>> Christophe
>>
>>
>>> Guenter
>>