linux-kernel - Re: [PATCH -next v4] mm/hotplug: silence a lockdep splat with printk()

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Message-ID: <b8aba013-16a8-8407-9330-8884d17b9594@redhat.com>
Date:   Fri, 17 Jan 2020 10:25:06 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Qian Cai <cai@....pw>, akpm@...ux-foundation.org,
        sergey.senozhatsky.work@...il.com, pmladek@...e.com,
        rostedt@...dmis.org, peterz@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v4] mm/hotplug: silence a lockdep splat with
 printk()

On 17.01.20 09:59, Michal Hocko wrote:
> On Fri 17-01-20 09:51:05, David Hildenbrand wrote:
>> On 17.01.20 03:21, Qian Cai wrote:
> [...]
>>> Even though has_unmovable_pages doesn't hold any reference to the
>>> returned page this should be reasonably safe for the purpose of
>>> reporting the page (dump_page) because it cannot be hotremoved. The
>>
>> This is only true in the context of memory unplug, but not in the
>> context of is_mem_section_removable()-> is_pageblock_removable_nolock().
> 
> Well, the above should hold for that path as well AFAICS. If the page is
> unmovable then a racing hotplug cannot remove it, right? Or do you
> consider a temporary unmovability to be a problem?

Somebody could test /sys/devices/system/memory/memoryX/removable. While
returning the unmovable page, it could become movable and
offlining+removing could succeed. Just a matter of where you put a
longer sleep after is_mem_section_removable().

Very unlikely, and it's racy as hell already (e.g., offlining
concurrently while testing for removability etc.).

-- 
Thanks,

David / dhildenb