Re: [PATCH RFC v3 6/9] mm: Allow to offline PageOffline() pages with a reference count of 0

[David Hildenbrand]

On 16.10.19 16:03, Michal Hocko wrote:
> On Wed 16-10-19 15:45:06, David Hildenbrand wrote:
> > On 16.10.19 13:43, Michal Hocko wrote:
> > > On Thu 19-09-19 16:22:25, David Hildenbrand wrote:
> > > > virtio-mem wants to allow to offline memory blocks of which some parts
> > > > were unplugged, especially, to later offline and remove completely
> > > > unplugged memory blocks. The important part is that PageOffline() has
> > > > to remain set until the section is offline, so these pages will never
> > > > get accessed (e.g., when dumping). The pages should not be handed
> > > > back to the buddy (which would require clearing PageOffline() and
> > > > result in issues if offlining fails and the pages are suddenly in the
> > > > buddy).
> > > >
> > > > Let's use "PageOffline() + reference count = 0" as a sign to
> > > > memory offlining code that these pages can simply be skipped when
> > > > offlining, similar to free or HWPoison pages.
> > > >
> > > > Pass flags to test_pages_isolated(), similar as already done for
> > > > has_unmovable_pages(). Use a new flag to indicate the
> > > > requirement of memory offlining to skip over these special pages.
> > > >
> > > > In has_unmovable_pages(), make sure the pages won't be detected as
> > > > movable. This is not strictly necessary, however makes e.g.,
> > > > alloc_contig_range() stop early, trying to isolate such page blocks -
> > > > compared to failing later when testing if all pages were isolated.
> > > >
> > > > Also, make sure that when a reference to a PageOffline() page is
> > > > dropped, that the page will not be returned to the buddy.
> > > >
> > > > memory devices (like virtio-mem) that want to make use of this
> > > > functionality have to make sure to synchronize against memory offlining,
> > > > using the memory hotplug notifier.
> > > >
> > > > Alternative: Allow to offline with a reference count of 1
> > > > and use some other sign in the struct page that offlining is permitted.
> > >
> > > Few questions. I do not see onlining code to take care of this special
> > > case. What should happen when offline && online?
> > > Should we allow to try_remove_memory to succeed with these pages?
> > > Do we really have hook into __put_page? Why do we even care about the
> > > reference count of those pages?
> >
> > Oh, I forgot to answer this questions. The __put_page() change is necessary
> > for the following race I identified:
> >
> > Page has a refcount of 1 (e.g., allocated by virtio-mem using
> > alloc_contig_range()).
> >
> > a) kernel: get_page_unless_zero(page): refcount = 2
> > b) virtio-mem: set page PG_offline, reduce refcount): refocunt = 1
> > c) kernel: put_page(page): refcount = 0
> >
> > The page would suddenly be given to the buddy. which is bad.
>
> But why cannot you keep the reference count at 1 (do get_page when
> offlining the page)? In other words as long as the driver knows the page
> has been returned to the host then it has ref count at 1. Once the page
> is returned to the guest for whatever reason it can free it to the
> system by clearing the offline state and put_page.

I think I explained how the reference count of 1 is problematic when 
wanting to offline the memory. After all that's the problem I try to 
solve: Keep PG_offline set until the memory is offline and make sure 
nobody will touch the page.

See below on details on how to revive such unplugged memory.

> An elevated ref count could help to detect that the memory hotremove is
> not safe until the driver removes all potential metadata it might still
> hold. You also know that memory online should skip such a page.
Again, when onlining memory you have to assume the memmap is complete 
garbage. No trusting *at all* on that as of now. This represents a BIG 
change to what we have right now. Not totally against that, but it 
sounds like a bigger rework that mainly helps to fix HWPoison.

> All in all your page is still in use by the driver and the life cycle is
> controlled by that driver.

The driver let go of all direct references. The page (or rather chunks) 
are in no list. The "knowledge" that these pages are offline are stored 
in some external metadata. This metadata is updated when notified via 
the memory notifier.

What happens in case virtio-mem wants to revive a chunk (IOW, plug 
unplugged memory)?

a) it makes sure no concurrent memory onlining/offlining can happen 
(locking via memory notifiers)
b) it grabs a reference to the page (increasing the refcount)
c) it clears PG_offline and issues free_contig_range().

> Or am I am missing something?
It's just complex stuff :) I guess the part you are missing is that the 
driver officially signals "I have no direct reference, you can offline 
this memory, I know how to deal with that". It's not like "this is a 
balloon inflated page I keep in a list".

--

Thanks,

David / dhildenb