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

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Message-ID: <20190411105617.GS10383@dhcp22.suse.cz>
Date:   Thu, 11 Apr 2019 12:56:17 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
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 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!

-- 
Michal Hocko
SUSE Labs