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

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Date:   Fri, 17 Jan 2020 11:17:43 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
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 Fri 17-01-20 10:42:10, David Hildenbrand wrote:
> On 17.01.20 10:40, Michal Hocko wrote:
> > On Fri 17-01-20 10:25:06, David Hildenbrand wrote:
> >> 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.
> > 
> > Doesn't this path use device lock or something? If not than the new code
> > is not more racy then the existing one. Just look at
> > is_pageblock_removable_nolock and how it dereferences struct page
> > (page_zonenum in  page_zone.)
> > 
> 
> AFAIK no device lock, no device hotplug lock, no memory hotplug lock. I
> think it holds a reference to the device and to the kernelfs node. But
> AFAIK that does not block removal of offlining/memory, just when the
> objects get freed.

OK, so we are bug compatible after this patch ;)
-- 
Michal Hocko
SUSE Labs