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

[David Hildenbrand]

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?

Once offline, the memmap is garbage. When onlining again:

a) memmap will be re-initialized
b) online_page_callback_t will be called for every page in the section. 
The driver can mark them offline again and not give them to the buddy.
c) section will be marked online.

The driver that marked these pages to be skipped when offlining is 
responsible for registering the online_page_callback_t callback where 
these pages will get excluded.

This is exactly the same as when onling a memory block that is partially 
populated (e.g., HpyerV balloon right now).

So it's effectively "re-initializing the memmap using the driver 
knowledge" when onlining.

> Should we allow to try_remove_memory to succeed with these pages?

I think we should first properly offline them (mark sections offline and 
memory blocks, fixup numbers, shrink zones ...). The we can cleanly 
remove the memory. (see [PATCH RFC v3 8/9] mm/memory_hotplug: Introduce 
offline_and_remove_memory())

Once offline, the memmap is irrelevant and try_remove_memory() can do 
its job.

> Do we really have hook into __put_page? Why do we even care about the
> reference count of those pages? Wouldn't it be just more consistent to
> elevate the reference count (I guess this is what you suggest in the
> last paragraph) and the virtio driver would return that page to the
> buddy by regular put_page. This is also related to the above question
> about the physical memory remove.

Returning them to the buddy is problematic for various reasons. Let's 
have a look at __offline_pages():

1) start_isolate_page_range()
-> offline pages with a reference count of one will be detected as 
unmovable -> BAD, we abort right away. We could hack around that.

2) memory_notify(MEM_GOING_OFFLINE, &arg);
-> Here, we could release all pages to the buddy, clearing PG_offline
-> BAD, PF_offline must not be cleared so dumping tools will not touch
   these pages. I don't see a way to hack around that.

3) scan_movable_pages() ...

4a) memory_notify(MEM_OFFLINE, &arg);

Perfect, it worked. Sections are offline.

4b) undo_isolate_page_range(start_pfn, end_pfn, MIGRATE_MOVABLE);
    memory_notify(MEM_CANCEL_OFFLINE, &arg);

-> Offlining failed for whatever reason.
-> Pages are in the buddy, but we already un-isolated them. BAD.

By not going via the buddy we avoid these issues and can leave 
PG_offline set until the section is fully offline. Something that is 
very desirable for virtio-mem (and as far as I can tell also HyperV in 
the future).

> [...]
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index d5d7944954b3..fef74720d8b4 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -8221,6 +8221,15 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
> >   		if (!page_ref_count(page)) {
> >   			if (PageBuddy(page))
> >   				iter += (1 << page_order(page)) - 1;
> > +			/*
> > +			* Memory devices allow to offline a page if it is
> > +			* marked PG_offline and has a reference count of 0.
> > +			* However, the pages are not movable as it would be
> > +			* required e.g., for alloc_contig_range().
> > +			*/
> > +			if (PageOffline(page) && !(flags & SKIP_OFFLINE))
> > +				if (++found > count)
> > +					goto unmovable;
> >   			continue;
> >   		}
>
> Do we really need to distinguish offline and hwpoison pages? They are
> both unmovable for allocator purposes and offlineable for the hotplug,
> right? Should we just hide them behind a helper and use it rather than
> an explicit SKIP_$FOO?

Makes sense. It really boils down to "offline" vs. "allocate" use cases.

So maybe instead of "SKIP_FOO" something like "MEMORY_OFFLINE". ?


--

Thanks,

David / dhildenb