Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page

[David Hildenbrand]

On 20.11.20 10:39, Michal Hocko wrote:
> On Fri 20-11-20 10:27:05, David Hildenbrand wrote:
> > On 20.11.20 09:42, Michal Hocko wrote:
> > > On Fri 20-11-20 14:43:04, Muchun Song wrote:
> > > [...]
> > >
> > > Thanks for improving the cover letter and providing some numbers. I have
> > > only glanced through the patchset because I didn't really have more time
> > > to dive depply into them.
> > >
> > > Overall it looks promissing. To summarize. I would prefer to not have
> > > the feature enablement controlled by compile time option and the kernel
> > > command line option should be opt-in. I also do not like that freeing
> > > the pool can trigger the oom killer or even shut the system down if no
> > > oom victim is eligible.
> > >
> > > One thing that I didn't really get to think hard about is what is the
> > > effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be
> > > invalid when racing with the split. How do we enforce that this won't
> > > blow up?
> >
> > I have the same concerns - the sections are online the whole time and
> > anybody with pfn_to_online_page() can grab them
> >
> > I think we have similar issues with memory offlining when removing the
> > vmemmap, it's just very hard to trigger and we can easily protect by
> > grabbing the memhotplug lock.
>
> I am not sure we can/want to span memory hotplug locking out to all pfn
> walkers. But you are right that the underlying problem is similar but
> much harder to trigger because vmemmaps are only removed when the
> physical memory is hotremoved and that happens very seldom. Maybe it
> will happen more with virtualization usecases. But this work makes it
> even more tricky. If a pfn walker races with a hotremove then it would
> just blow up when accessing the unmapped physical address space. For
> this feature a pfn walker would just grab a real struct page re-used for
> some unpredictable use under its feet. Any failure would be silent and
> hard to debug.

Right, we don't want the memory hotplug locking, thus discussions 
regarding rcu. Luckily, for now I never saw a BUG report regarding this 
- maybe because the time between memory offlining (offline_pages()) and 
memory/vmemmap getting removed (try_remove_memory()) is just too long. 
Someone would have to sleep after pfn_to_online_page() for quite a while 
to trigger it.

> [...]
> > To keep things easy, maybe simply never allow to free these hugetlb pages
> > again for now? If they were reserved during boot and the vmemmap condensed,
> > then just let them stick around for all eternity.
>
> Not sure I understand. Do you propose to only free those vmemmap pages
> when the pool is initialized during boot time and never allow to free
> them up? That would certainly make it safer and maybe even simpler wrt
> implementation.

Exactly, let's keep it simple for now. I guess most use cases of this 
(virtualization, databases, ...) will allocate hugepages during boot and 
never free them.

--
Thanks,

David / dhildenb