Re: Memory hotplug not increasing the total RAM

[Michal Hocko]

[Cc Andrew - thread starts here
 http://lkml.kernel.org/r/20180130083006.GB1245@xxxxxxxxxx]

On Tue 30-01-18 15:23:45, Bharata B Rao wrote:
> 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.

Thanks for your quick testing. Here we go with the fix.