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.
