Re: [PATCH RFC] mm/memory_hotplug: make it possible to offline blocks with reserved pages

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Michal Hocko <mhocko@kernel.org> writes:

> On Wed 08-11-17 17:16:19, Vitaly Kuznetsov wrote:
>> Michal Hocko <mhocko@kernel.org> writes:
>> 
>> > On Wed 08-11-17 16:39:49, Vitaly Kuznetsov wrote:
>> >> Michal Hocko <mhocko@kernel.org> writes:
>> >> 
>> >> > On Wed 08-11-17 14:01:55, Vitaly Kuznetsov wrote:
>> >> >> Hyper-V balloon driver needs to hotplug memory in smaller chunks and to
>> >> >> workaround Linux's 128Mb allignment requirement so it does a trick: partly
>> >> >> populated 128Mb blocks are added and then a custom online_page_callback
>> >> >> hook checks if the particular page is 'backed' during onlining, in case it
>> >> >> is not backed it is left in Reserved state. When the host adds more pages
>> >> >> to the block we bring them online from the driver (see
>> >> >> hv_bring_pgs_online()/hv_page_online_one() in drivers/hv/hv_balloon.c).
>> >> >> Eventually the whole block becomes fully populated and we hotplug the next
>> >> >> 128Mb. This all works for quite some time already.
>> >> >
>> >> > Why does HyperV needs to workaround the section size limit in the first
>> >> > place? We are allocation memmap for the whole section anyway so it won't
>> >> > save any memory. So the whole thing sounds rather dubious to me.
>> >> >
>> >> 
>> >> Memory hotplug requirements in Windows are different, they have 2Mb
>> >> granularity, not 128Mb like we have in Linux x86.
>> >> 
>> >> Imagine there's a request to add 32Mb of memory comming from the
>> >> Hyper-V host. What can we do? Don't add anything at all and wait till
>> >> we're suggested to add > 128Mb and then add a section or the current
>> >> approach.
>> >
>> > Use a different approach than memory hotplug. E.g. memory balloning.
>> >
>> 
>> But how? When we boot we may have very little memory and later on we
>> hotplug a lot so we may not even be able to ballon all possible memory
>> without running out of memory.
>
> Just add more memory and make part of it unusable and return it back to
> the host via standard ballooning means.

We don't have control over how much memory host gives us and we have no
way to return anything to the host.

>
> How realistic is that the host gives only such a small amount of memory
> btw?

It happens all the time, Hyper-V host will gradually increase guest's
memory when Dynamic Memory is enabled. Moreover, there's a manual
interface when host's admin can hotplug memory to guests (starting
WS2016) with 2M granularity.

-- 
  Vitaly