Re: [PATCH] mm/memory_hotplug: Drop memory device reference after find_memory_block()

From: David Hildenbrand
Date: Thu Apr 11 2019 - 07:18:13 EST

On 11.04.19 12:56, Michal Hocko wrote:
> On Thu 11-04-19 11:11:05, David Hildenbrand wrote:
>> On 11.04.19 10:41, Michal Hocko wrote:
>>> On Wed 10-04-19 12:14:55, David Hildenbrand wrote:
>>>> While current node handling is probably terribly broken for memory block
>>>> devices that span several nodes (only possible when added during boot,
>>>> and something like that should be blocked completely), properly put the
>>>> device reference we obtained via find_memory_block() to get the nid.
>>> The changelog could see some improvements I believe. (Half) stating
>>> broken status of multinode memblock is not really useful without a wider
>>> context so I would simply remove it. More to the point, it would be much
>>> better to actually describe the actual problem and the user visible
>>> effect.
>>> "
>>> d0dc12e86b31 ("mm/memory_hotplug: optimize memory hotplug") has started
>>> using find_memory_block to get a nodeid for the beginnig of the onlined
>>> pfn range. The commit has missed that the memblock contains a reference
>>> counted object and a missing put_device will leak the kobject behind
>>> "
>> I don't think mentioning the commit a second time is really needed.
>> "
>> Right now we are using find_memory_block() to get the node id for the
>> pfn range to online. We are missing to drop a reference to the memory
>> block device. While the device still gets unregistered via
>> device_unregister(), resulting in no user visible problem, the device is
>> never released via device_release(), resulting in a memory leak. Fix
>> that by properly using a put_device().
>> "
> OK, sounds good to me. I was not sure about all the sysfs machinery
> and the kobj dependencies but if there are no sysfs files leaking and
> crashing upon a later access then a leak of a small amount of memory
> that is not user controlable then this is not super urgent.
> Thanks!

I think it can be triggered by onlining/offlining memory in a loop. But
as you said, only leaks of small amount of memory.




David / dhildenb