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Subject: Re: [PATCH] mm/memory_hotplug: Drop memory device reference after
 find_memory_block()
To:     Michal Hocko <mhocko@kernel.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Wei Yang <richard.weiyang@gmail.com>, Qian Cai <cai@lca.pw>,
        Arun KS <arunks@codeaurora.org>,
        Mathieu Malaterre <malat@debian.org>
References: <20190410101455.17338-1-david@redhat.com>
 <20190411084141.GQ10383@dhcp22.suse.cz>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <711db571-ee39-eb64-4551-baaa5b562579@redhat.com>
Date:   Thu, 11 Apr 2019 13:18:07 +0200
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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
>>> which ADD THE USER VISIBLE EFFECT HERE.
>>> "
>>
>> 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.

Thanks!

-- 

Thanks,

David / dhildenb