ACK: [SRU][Bionic][PATCH 1/1] UBUNTU: SAUCE: powerpc/perf: Fix memory allocation for core-imc based on num_possible_cpus()

[Colin Ian King]

On 18/05/18 15:48, Joseph Salisbury wrote:
> From: Anju T Sudhakar <anju at linux.vnet.ibm.com>
> 
> BugLink: http://bugs.launchpad.net/bugs/1770849
> 
> Currently memory is allocated for core-imc based on cpu_present_mask, which
>  has bit 'cpu' set iff cpu is populated. We use  (cpu number / threads per core)
>  as as array index to access the memory.  So in a system with guarded cores,
>  since allocation happens based on cpu_present_mask, (cpu number / threads per core)
>  bounds the index and leads to memory overflow.
> 
> The issue is exposed in a guard test.  The guard test will make some CPU's
>  as un-available to the system during boot time as well as at runtime. So
>  when the cpu is unavailable to the system during boot time, the memory
>  allocation happens depending on the number of available cpus. And when we
>  access the memory using (cpu number / threads per core) as the index the
>  system crashes due to memory overflow.
> 
> Allocating memory for core-imc based on cpu_possible_mask, which has bit
>  'cpu' set iff cpu is populatable, will fix this issue.
> 
> Reported-by: Pridhiviraj Paidipeddi <ppaidipe at linux.vnet.ibm.com>
> Signed-off-by: Anju T Sudhakar <anju at linux.vnet.ibm.com>
> Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
> ---
>  arch/powerpc/perf/imc-pmu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/perf/imc-pmu.c b/arch/powerpc/perf/imc-pmu.c
> index be4e7f8..dfb5864 100644
> --- a/arch/powerpc/perf/imc-pmu.c
> +++ b/arch/powerpc/perf/imc-pmu.c
> @@ -1132,7 +1132,7 @@ static int init_nest_pmu_ref(void)
>  
>  static void cleanup_all_core_imc_memory(void)
>  {
> -	int i, nr_cores = DIV_ROUND_UP(num_present_cpus(), threads_per_core);
> +	int i, nr_cores = DIV_ROUND_UP(num_possible_cpus(), threads_per_core);
>  	struct imc_mem_info *ptr = core_imc_pmu->mem_info;
>  	int size = core_imc_pmu->counter_mem_size;
>  
> @@ -1248,7 +1248,7 @@ static int imc_mem_init(struct imc_pmu *pmu_ptr, struct device_node *parent,
>  		if (!pmu_ptr->pmu.name)
>  			return -ENOMEM;
>  
> -		nr_cores = DIV_ROUND_UP(num_present_cpus(), threads_per_core);
> +		nr_cores = DIV_ROUND_UP(num_possible_cpus(), threads_per_core);
>  		pmu_ptr->mem_info = kcalloc(nr_cores, sizeof(struct imc_mem_info),
>  								GFP_KERNEL);
>  
> 
Fixes the issue, positive test results, limited to specific arch so
regression potential is also limited.  Seems like a sensible fix to me.

Acked-by: Colin Ian King <colin.king at canonical.com>