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Date:   Mon, 30 Jul 2018 14:05:29 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Jan Kara <jack@suse.cz>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Souptick Joarder <jrdr.linux@gmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Oscar Salvador <osalvador@techadventures.net>,
        YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>,
        Mathieu Malaterre <malat@debian.org>,
        Mel Gorman <mgorman@suse.de>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH v1] mm: inititalize struct pages when adding a section
Message-ID: <20180730120529.GN24267@dhcp22.suse.cz>
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On Mon 30-07-18 13:53:06, David Hildenbrand wrote:
> On 30.07.2018 13:30, Michal Hocko wrote:
> > On Fri 27-07-18 18:54:54, David Hildenbrand wrote:
> >> Right now, struct pages are inititalized when memory is onlined, not
> >> when it is added (since commit d0dc12e86b31 ("mm/memory_hotplug: optimize
> >> memory hotplug")).
> >>
> >> remove_memory() will call arch_remove_memory(). Here, we usually access
> >> the struct page to get the zone of the pages.
> >>
> >> So effectively, we access stale struct pages in case we remove memory that
> >> was never onlined. So let's simply inititalize them earlier, when the
> >> memory is added. We only have to take care of updating the zone once we
> >> know it. We can use a dummy zone for that purpose.
> > 
> > I have considered something like this when I was reworking memory
> > hotplug to not associate struct pages with zone before onlining and I
> > considered this to be rather fragile. I would really not like to get
> > back to that again if possible.
> > 
> >> So effectively, all pages will already be initialized and set to
> >> reserved after memory was added but before it was onlined (and even the
> >> memblock is added). We only inititalize pages once, to not degrade
> >> performance.
> > 
> > To be honest, I would rather see d0dc12e86b31 reverted. It is late in
> > the release cycle and if the patch is buggy then it should be reverted
> > rather than worked around. I found the optimization not really
> > convincing back then and this is still the case TBH.
> > 
> 
> If I am not wrong, that's already broken in 4.17, no? What about that?

Ohh, I thought this was merged in 4.18.
$ git describe --contains d0dc12e86b31 --match="v*"
v4.17-rc1~99^2~44

proves me wrong. This means that the fix is not so urgent as I thought.
If you can figure out a reasonable fix then it should be preferable to
the revert.

Fake zone sounds too hackish to me though.
-- 
Michal Hocko
SUSE Labs