Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 6/8/22 11:40 AM, Ying Huang wrote:
> On Wed, 2022-06-08 at 10:07 +0530, Aneesh Kumar K V wrote:
>> On 6/8/22 12:13 AM, Tim Chen wrote:
>> ...
>>
>>>>
>>>> +
>>>> +static void memory_tier_device_release(struct device *dev)
>>>> +{
>>>> +	struct memory_tier *tier = to_memory_tier(dev);
>>>> +
>>>
>>> Do we need some ref counts on memory_tier?
>>> If there is another device still using the same memtier,
>>> free below could cause problem.
>>>
>>>> +	kfree(tier);
>>>> +}
>>>> +
>>>>
>>> ...
>>
>> The lifecycle of the memory_tier struct is tied to the sysfs device life
>> time. ie, memory_tier_device_relese get called only after the last
>> reference on that sysfs dev object is released. Hence we can be sure
>> there is no userspace that is keeping one of the memtier related sysfs
>> file open.
>>
>> W.r.t other memory device sharing the same memtier, we unregister the
>> sysfs device only when the memory tier nodelist is empty. That is no
>> memory device is present in this memory tier.
> 
> memory_tier isn't only used by user space.  It is used inside kernel
> too.  If some kernel code get a pointer to struct memory_tier, we need
> to guarantee the pointer will not be freed under us. 

As mentioned above current patchset avoid doing that.

> And as Tim pointed
> out, we need to use it in hot path (for statistics), so some kind of rcu
> lock may be good.
> 

Sure when those statistics code get added, we can add the relevant kref 
and locking details.

-aneesh