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

[David Hildenbrand]

On 16.10.19 15:45, Michal Hocko wrote:
> On Wed 16-10-19 14:50:30, 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?
> >
> > 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.
>
> But we can skip those pages when onlining and keep them in the offline
> state right? We do not poison offlined pages.

https://lkml.org/lkml/2019/10/6/60

But  again, onlining will overwrite the whole memmap right now and there 
is no way to identify if a memmap contains garbage or not.

We would have to identify/remember if re-onlining, but I am not yet sure 
if re-using memmaps when onlining is such a good idea ...

--

Thanks,

David / dhildenb