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
