linux-kernel - Re: Memory hotplug not increasing the total RAM

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Date:   Tue, 30 Jan 2018 15:23:45 +0530
From:   Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        pasha.tatashin@...cle.com
Subject: Re: Memory hotplug not increasing the total RAM

On Tue, Jan 30, 2018 at 10:28:15AM +0100, Michal Hocko wrote:
> On Tue 30-01-18 10:16:00, Michal Hocko wrote:
> > On Tue 30-01-18 14:00:06, Bharata B Rao wrote:
> > > Hi,
> > > 
> > > With the latest upstream, I see that memory hotplug is not working
> > > as expected. The hotplugged memory isn't seen to increase the total
> > > RAM pages. This has been observed with both x86 and Power guests.
> > > 
> > > 1. Memory hotplug code intially marks pages as PageReserved via
> > > __add_section().
> > > 2. Later the struct page gets cleared in __init_single_page().
> > > 3. Next online_pages_range() increments totalram_pages only when
> > >    PageReserved is set.
> > 
> > You are right. I have completely forgot about this late struct page
> > initialization during onlining. memory hotplug really doesn't want
> > zeroying. Let me think about a fix.
> 
> Could you test with the following please? Not an act of beauty but
> we are initializing memmap in sparse_add_one_section for memory
> hotplug. I hate how this is different from the initialization case
> but there is quite a long route to unify those two... So a quick
> fix should be as follows.

Tested on Power guest, fixes the issue. I can now see the total memory
size increasing after hotplug.

Regards,
Bharata.