Re: [v3 PATCH 1/5] powerpc/pseries: convert rtas_log_buf to linear allocation.

[Mahesh Jagannath Salgaonkar <mahesh@linux.vnet.ibm.com>]

On 06/08/2018 07:01 AM, Nicholas Piggin wrote:
> On Thu, 07 Jun 2018 22:58:11 +0530
> Mahesh J Salgaonkar <mahesh@linux.vnet.ibm.com> wrote:
> 
>> From: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
>>
>> rtas_log_buf is a buffer to hold RTAS event data that are communicated
>> to kernel by hypervisor. This buffer is then used to pass RTAS event
>> data to user through proc fs. This buffer is allocated from vmalloc
>> (non-linear mapping) area.
>>
>> On Machine check interrupt, register r3 points to RTAS extended event
>> log passed by hypervisor that contains the MCE event. The pseries
>> machine check handler then logs this error into rtas_log_buf. The
>> rtas_log_buf is a vmalloc-ed (non-linear) buffer we end up taking up a
>> page fault (vector 0x300) while accessing it. Since machine check
>> interrupt handler runs in NMI context we can not afford to take any
>> page fault. Page faults are not honored in NMI context and causes
>> kernel panic. This patch fixes this issue by allocating rtas_log_buf
>> using kmalloc.
>>
>> Fixes: b96672dd840f ("powerpc: Machine check interrupt is a non-maskable interrupt")
>> Cc: stable@vger.kernel.org
>> Suggested-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>> Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
>> ---
>>  arch/powerpc/kernel/rtasd.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/kernel/rtasd.c b/arch/powerpc/kernel/rtasd.c
>> index f915db93cd42..3957d4ae2ba2 100644
>> --- a/arch/powerpc/kernel/rtasd.c
>> +++ b/arch/powerpc/kernel/rtasd.c
>> @@ -559,7 +559,7 @@ static int __init rtas_event_scan_init(void)
>>  	rtas_error_log_max = rtas_get_error_log_max();
>>  	rtas_error_log_buffer_max = rtas_error_log_max + sizeof(int);
>>  
>> -	rtas_log_buf = vmalloc(rtas_error_log_buffer_max*LOG_NUMBER);
>> +	rtas_log_buf = kmalloc(rtas_error_log_buffer_max*LOG_NUMBER, GFP_KERNEL);
> 
> Does this have to be in the RMA region if it's to be accessed with
> relocation off in the guest?

Nope not required. It never gets accessed with relocation off.

> 
> A comment about it being accessed with relocation off might be helpful
> too.

Sure.

Thanks,
-Mahesh.