linux-kernel - Re: [PATCH RFC v3 6/9] mm: Allow to offline PageOffline() pages with a reference count of 0

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Message-ID: <20191016140958.GE317@dhcp22.suse.cz>
Date:   Wed, 16 Oct 2019 16:09:58 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtualization@...ts.linux-foundation.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Juergen Gross <jgross@...e.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Oscar Salvador <osalvador@...e.de>,
        Pingfan Liu <kernelfans@...il.com>, Qian Cai <cai@....pw>,
        Dan Williams <dan.j.williams@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Wei Yang <richardw.yang@...ux.intel.com>,
        Alexander Potapenko <glider@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yu Zhao <yuzhao@...gle.com>, Minchan Kim <minchan@...nel.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: Re: [PATCH RFC v3 6/9] mm: Allow to offline PageOffline() pages with
 a reference count of 0

On Wed 16-10-19 15:55:00, David Hildenbrand wrote:
> On 16.10.19 15:45, Michal Hocko wrote:
[...]
> > There is state stored in the struct page. In other words this shouldn't
> > be really different from HWPoison pages. I cannot find the code that is
> > doing that and maybe we don't handle that. But we cannot simply online
> > hwpoisoned page. Offlining the range will not make a broken memory OK
> > all of the sudden. And your usecase sounds similar to me.
> 
> Sorry to say, but whenever we online memory the memmap is overwritten,
> because there is no way you could tell it contains garbage or not. You have
> to assume it is garbage. (my recent patch even poisons the memmap when
> offlining, which helped to find a lot of these "garbage memmap" BUGs)
> 
> online_pages()
> 	...
> 	move_pfn_range_to_zone(zone, pfn, nr_pages, NULL);
> 	...
> 		memmap_init_zone()
> 			-> memmap initialized
> 
> So yes, offlining memory with HWPoison and re-onlining it effectively drops
> HWPoison markers. On the next access, you will trigger a new HWPoison.

Right you are! I need to sit on this much more and think about it with a
clean head.
-- 
Michal Hocko
SUSE Labs