linux-kernel - Re: [PATCH] mm/memory_hotplug: Drop memory device reference after find_memory

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Message-ID: <711db571-ee39-eb64-4551-baaa5b562579@redhat.com>
Date:   Thu, 11 Apr 2019 13:18:07 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Wei Yang <richard.weiyang@...il.com>, Qian Cai <cai@....pw>,
        Arun KS <arunks@...eaurora.org>,
        Mathieu Malaterre <malat@...ian.org>
Subject: Re: [PATCH] mm/memory_hotplug: Drop memory device reference after
 find_memory_block()

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