On 14.05.25 15:45, Zi Yan wrote:
On 14 May 2025, at 7:15, David Hildenbrand wrote:
This is a requirement for making PageOffline pages not have a refcount
in the long future ("frozen"), and for reworking non-folio page migration
in the near future.
I have patches mostly ready to go to handle the latter. For turning all
PageOffline() pages frozen, the non-folio page migration and memory
ballooning drivers will have to be reworked first, to no longer rely on
the refcount of PageOffline pages.
Introduce PG_offline_skippable that only applies to PageOffline() pages --
of course, reusing one of the existing PG_ flags for now -- and convert
virtio-mem to make use of the new way: to allow for skipping PageOffline
pages during memory offlining, treating them as if they would not be
allocated.
Thanks for taking a look!
IIUC, based on Documentation/admin-guide/mm/memory-hotplug.rst,
to offline a page, the page first needs to be set PageOffline() to be
PageOffline is not mentioned in there. :)
Note that PageOffline() is a bit confusing because it's "Memory block
online but page is logically offline (e.g., has a memmap that can be
touched, but the page content should not be touched)".
(memory block offline -> all pages offline and have effectively no state
because the memmap is stale)
removed from page allocator.
Usually, all pages are freed back to the buddy (isolated pageblock ->
put onto the isolated list). Memory offlining code can then simply grab
these "free" pages from the buddy -- no PageOffline involved.
If something fails during memory offlining, these isolated pages are
simply put back on the appropriate migratetype list and become ordinary
free pages that can be allocated immediately.
Some PageOffline pages can be migrated using the non-folio migration:
this is done for memory ballooning (memory comapction). As they get
migrated, they are freed back to the buddy, PageOffline() is cleared --
they become PageBuddy() -- and the above applies.
Other PageOffline pages can be skipped during memory offlining
(virtio-mem use case, what we are doing her). We don't want them to ever
go through the buddy, especially because if memory offlining fails they
must definitely not be treated like free pages that can be allocated
immediately.
Next, the page is removed from its memory
block. When will PG_offline_skippable be used? The second phase when
the page is being removed from its memory block?
PG_offline_skippable is used during memory offlining, while we look for
any pages that are not PageBuddy (... or hwpoisoned ...), to migrate
them off the memory so they get converted to PageBuddy.
PageOffline + PageOfflineSkippable are checked on that phase, such that
they don't require any migration.
--
Cheers,
David / dhildenb
