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

[Michal Hocko]

On Wed 08-11-17 16:39:49, Vitaly Kuznetsov wrote:
> Michal Hocko <mhocko@xxxxxxxxxx> 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.

> >> What is not working is offlining of such partly populated blocks:
> >> check_pages_isolated_cb() callback will not pass with a sinle Reserved page
> >> and we end up with -EBUSY. However, there's no reason to fail offlining in
> >> this case: these pages are already offline, we may just skip them. Add the
> >> appropriate workaround to test_pages_isolated().
> >
> > How do you recognize pages reserved by other users. You cannot simply
> > remove them, it would just blow up.
> >
> 
> I exepcted sumothing like that, thus RFC. Is there a way to detect pages
> which were never onlined? E.g. it is Reserved and count == 0?

That would be quite tricky. But I am not convinced that the whole thing
makes any sense at all. We are in fact always creating the full section
so onlining only a part of it sounds really dubious to me.
-- 
Michal Hocko
SUSE Labs