[PATCH] powerpc/perf: Fix memory allocation for core-imc based on num_possible_cpus()

[Michael Ellerman]

Anju T Sudhakar <anju at linux.vnet.ibm.com> writes:
> On Saturday 12 May 2018 06:05 AM, Balbir Singh wrote:
>> On Fri, May 11, 2018 at 11:43 PM, Anju T Sudhakar
>> <anju at linux.vnet.ibm.com> wrote:
>>> 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>
>>> ---
>>>   arch/powerpc/perf/imc-pmu.c | 4 ++--
>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>> The changelog does not clearly call out the confusion between present
>> and possible.
>> Guarded CPUs are possible but not present, so it blows a hole when we assume the
>> max length of our allocation is driven by our max present cpus, where
>> as one of the cpus
>> might be online and be beyond the max present cpus, due to the hole..
>>
>> Reviewed-by: Balbir Singh <bsingharora at gmail.com>
>
> Thanks for the review.
> OK. I will update the commit message here.

Yeah please do. "Guarded" CPUs is also not a well understand term, so
please explain what that means for people who don't know.

cheers