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From:   Pavel Tatashin <pasha.tatashin@oracle.com>
Date:   Tue, 30 Jan 2018 14:12:41 -0500
Message-ID: <CAOAebxuf_GWZFPzQZs0r5Mgoq-kcDiqd0kpX6yYqC_XqLG+0PA@mail.gmail.com>
Subject: Re: Memory hotplug not increasing the total RAM
To:     Michal Hocko <mhocko@kernel.org>
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>
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> 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.

You are correct, PageReserved might be tested in offlined memory, if
we go with the proposed solution, we might even need to add "struct
page" poisoning instead of memset(0) in  sparse_add_one_section when
debugging is enabled. Similar to what we do during boot in
memblock_virt_alloc_raw()

The fix would imply to ensure that PageReserved is never tested and
page_to_nid is never executed for offlined memory. I will study for
possible solutions.

Thank you,
Pavel