Hi Changyuan,
On Wed, Mar 19 2025, Changyuan Lyu wrote:
> From: "Mike Rapoport (Microsoft)" <r...@kernel.org>
>
> Introduce APIs allowing KHO users to preserve memory across kexec and
> get access to that memory after boot of the kexeced kernel
>
> kho_preserve_folio() - record a folio to be preserved over kexec
> kho_restore_folio() - recreates the folio from the preserved memory
> kho_preserve_phys() - record physically contiguous range to be
> preserved over kexec.
> kho_restore_phys() - recreates order-0 pages corresponding to the
> preserved physical range
>
> The memory preservations are tracked by two levels of xarrays to manage
> chunks of per-order 512 byte bitmaps. For instance the entire 1G order
> of a 1TB x86 system would fit inside a single 512 byte bitmap. For
> order 0 allocations each bitmap will cover 16M of address space. Thus,
> for 16G of memory at most 512K of bitmap memory will be needed for order 0.
>
> At serialization time all bitmaps are recorded in a linked list of pages
> for the next kernel to process and the physical address of the list is
> recorded in KHO FDT.
>
> The next kernel then processes that list, reserves the memory ranges and
> later, when a user requests a folio or a physical range, KHO restores
> corresponding memory map entries.
>
> Suggested-by: Jason Gunthorpe <j...@nvidia.com>
> Signed-off-by: Mike Rapoport (Microsoft) <r...@kernel.org>
> Co-developed-by: Changyuan Lyu <changyu...@google.com>
> Signed-off-by: Changyuan Lyu <changyu...@google.com>
> ---
> include/linux/kexec_handover.h | 38 +++
> kernel/kexec_handover.c | 486 ++++++++++++++++++++++++++++++++-
> 2 files changed, 522 insertions(+), 2 deletions(-)
[...]
> +int kho_preserve_phys(phys_addr_t phys, size_t size)
> +{
> + unsigned long pfn = PHYS_PFN(phys), end_pfn = PHYS_PFN(phys + size);
> + unsigned int order = ilog2(end_pfn - pfn);
This caught my eye when playing around with the code. It does not put
any limit on the order, so it can exceed NR_PAGE_ORDERS. Also, when
initializing the page after KHO, we pass the order directly to
prep_compound_page() without sanity checking it. The next kernel might
not support all the orders the current one supports. Perhaps something
to fix?
> + unsigned long failed_pfn;
> + int err = 0;
> +
> + if (!kho_enable)
> + return -EOPNOTSUPP;
> +
> + down_read(&kho_out.tree_lock);
> + if (kho_out.fdt) {
> + err = -EBUSY;
> + goto unlock;
> + }
> +
> + for (; pfn < end_pfn;
> + pfn += (1 << order), order = ilog2(end_pfn - pfn)) {
> + err = __kho_preserve(&kho_mem_track, pfn, order);
> + if (err) {
> + failed_pfn = pfn;
> + break;
> + }
> + }
[...
> +struct folio *kho_restore_folio(phys_addr_t phys)
> +{
> + struct page *page = pfn_to_online_page(PHYS_PFN(phys));
> + unsigned long order = page->private;
> +
> + if (!page)
> + return NULL;
> +
> + order = page->private;
> + if (order)
> + prep_compound_page(page, order);
> + else
> + kho_restore_page(page);
> +
> + return page_folio(page);
> +}
[...]
--
Regards,
Pratyush Yadav
