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

[Nicholas Piggin <npiggin@gmail.com>]

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?

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

Thanks,
Nick