Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 17 Jan 2020 11:17:43 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Qian Cai <cai@lca.pw>, akpm@linux-foundation.org,
        sergey.senozhatsky.work@gmail.com, pmladek@suse.com,
        rostedt@goodmis.org, peterz@infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next v4] mm/hotplug: silence a lockdep splat with
 printk()
Message-ID: <20200117101743.GS19428@dhcp22.suse.cz>
References: <20200117022111.18807-1-cai@lca.pw>
 <d7068679-e28a-98a9-f5b8-49ea47f7c092@redhat.com>
 <20200117085932.GK19428@dhcp22.suse.cz>
 <b8aba013-16a8-8407-9330-8884d17b9594@redhat.com>
 <20200117094009.GP19428@dhcp22.suse.cz>
 <521da382-d9b2-8556-d603-5537b030d8fd@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <521da382-d9b2-8556-d603-5537b030d8fd@redhat.com>
User-Agent: Mutt/1.12.2 (2019-09-21)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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