linux-kernel - Re: Memory hotplug softlock issue

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Message-ID: <6017b36f-3e29-c2ad-f2d1-2ebd77bbaef1@suse.cz>
Date:   Mon, 19 Nov 2018 17:48:35 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Baoquan He <bhe@...hat.com>, David Hildenbrand <david@...hat.com>,
        linux-mm@...ck.org, pifang@...hat.com,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        aarcange@...hat.com, Mel Gorman <mgorman@...e.de>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: Memory hotplug softlock issue

On 11/19/18 5:46 PM, Vlastimil Babka wrote:
> On 11/19/18 5:46 PM, Michal Hocko wrote:
>> On Mon 19-11-18 17:36:21, Vlastimil Babka wrote:
>>>
>>> So what protects us from locking a page whose refcount dropped to zero?
>>> and is being freed? The checks in freeing path won't be happy about a
>>> stray lock.
>>
>> Nothing really prevents that. But does it matter. The worst that might
>> happen is that we lock a freed or reused page. Who would complain?
> 
> free_pages_check() for example
> 
> PAGE_FLAGS_CHECK_AT_FREE includes PG_locked

And besides... what about the last page being offlined and then the
whole struct page's part of vmemmap destroyed as the node goes away?