[PATCH makedumpfile] Add confidential VM unaccepted free pages support on Linux 6.16 and later

[Zhiquan Li]

On 2025/6/9 13:02, Zhiquan Li wrote:
> UEFI Specification version 2.9 introduces the concept of memory
> acceptance: some Virtual Machine platforms, such as Intel TDX or AMD
> SEV-SNP, requiring memory to be accepted before it can be used by the
> guest.  Accepting happens via a protocol specific for the Virtual
> Machine platform[1].
> 
> Before unaccepted memory is accepted by guest, any access from guest
> will result in the guest failed, as well as the kexec'ed kernel.  So,
> the kexec'ed kernel will skip these pages and fill in zero data for the
> reader of vmcore.
> 
> However, it introduces a problem.  When exclude a page filled with zero,
> a pd_zero, which sizeof(page_desc_t) is 24 bytes, will be written into
> cd_header and this part is not compressed by design.  As the unaccepted
> pages are exported as zero pages, which means ~1/170 of the capacity of
> unaccepted memory will be added into vmcore additionally.  In fact, they
> should be considered as free pages and most of the time should be
> excluded.
> 
> Unaccepted memory is unusable free memory, so they are not managed by
> buddy, instead, they are added to a new list zone.unaccepted_pages only
> when the order is MAX_PAGE_ORDER each time conventionally.  The new
> page type, PGTY_unaccepted can be used to identify whether a page is
> unaccepted[2].  Therefore, add following changes to exclude them like
> free pages:
> 
> 1. Add NUMBER(PAGE_UNACCEPTED_MAPCOUNT_VALUE) to identify a page is
>    unaccepted, a kernel patch[3] to export the value of page type
>    PAGE_UNACCEPTED_MAPCOUNT_VALUE since kernel 6.16.
> 
> 2. Add a condition to exclude these unaccepted free pages.
> 
> Dumping host kernel will not be impacted by the modification, because it
> cannot enable CONFIG_UNACCEPTED_MEMORY, so the page type
> PAGE_UNACCEPTED_MAPCOUNT_VALUE cannot be found in vmcoreinfo and skip
> the step.
> 
> Here is a vmcore size statistic of a freshly booted TD VM with different
> memory sizes:
> 
>   VM.mem | Before  After
>   -------+----------------
>   512G   | ~4.9G   ~2.0G
>   256G   | ~2.0G   ~1.1G

Kindly ping.