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Date:   Tue, 30 Jan 2018 19:29:47 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Pavel Tatashin <pasha.tatashin@oracle.com>
Cc:     Bharata B Rao <bharata@linux.vnet.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: Memory hotplug not increasing the total RAM
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On Tue 30-01-18 13:11:06, Pavel Tatashin wrote:
> Hi Michal,
> 
> Thank you for taking care of the problem. The patch may introduce a
> small performance regression during normal boot, as we add a branch
> into a hot initialization path. But, it fixes a current problem, so:
> 
> Reviewed-by: Pavel Tatashin <pasha.tatashin@oracle.com>

Thanks!

> However, I think we should change the hotplug code to also not to
> touch the map area until struct pages are initialized.
> 
> Currently, we loop through "struct page"s several times during memory hotplug:
> 
> 1. memset(0) in sparse_add_one_section()
> 2. loop in __add_section() to set do: set_page_node(page, nid); and
> SetPageReserved(page);
> 3. loop in pages_correctly_reserved() to check that SetPageReserved is set.
> 4. loop in memmap_init_zone() to call __init_single_pfn()

You might be very well correct but the hotplug code is quite subtle and
we do depend on PageReserved at some unexpected places so it is not that
easy I am afraid. My TODO list in the hotplug is quite long. If you feel
like you want to work on that I would be more than happy.
-- 
Michal Hocko
SUSE Labs